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INTERNATIONAL   CHEMICAL   SERIES 
H.  P   TALBOT,  PH.D.,  Sc.D..  CONSULTING  EDITOR 


QUANTITATIVE  ANALYSIS 


Trie  Qmw'3/ill  Book  Ca  1m 

PUBLISHERS     OF     fc  O  O  K.  S     FO  R^ 

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Chemical  6  Metallurgical  Engineering 
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QUANTITATIVE 
ANALYSIS 


BY 
EDWARD  G.  MAHIN,  PH.  D. 

Professor  of  Analytical  Chemistry  in  Purdue   University 


SECOND  EDITION 

REVISED  AND  ENLARGED 

FIFTH  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    370  SEVENTH  AVENUE 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1919 


COPYRIGHT,  1914,  1919,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC: 


1'KKSS     YORK     PA 


PREFACE  TO  THE  SECOND  EDITION 

Since  the  publication  of  the  first  edition  of  this  book  numerous 
changes  have  been  made  in  the  standardized  methods  of  analysis 
of  certain  industrial  materials,  as  adopted  by  official  committees 
of  various  scientific  societies.  In  the  present  edition  the  dis- 
cussions and  detailed  procedure  have  been  modified  to  conform  to 
the  revised  methods,  wherever  possible.  This  statement  applies 
to  the  analysis  of  coal,  water,  fertilizers,  dairy  products  and 
insecticides. 

A  number  of  other  analytical  methods  are  now  described, 
of  which  may  be  mentioned  the  gravimetric  determination 
of  the  chloride,  sulphate  and  phosphate  radicals  and  the  in- 
direct determination  of  the  halogens;  the  perchlorate  method  for 
potassium;  the  determination  of  chromium  and  vanadium  in 
steel  and  the  glyoxime  method  for  nickel  in  steel;  the  analysis 
of  brass  and  of  soft  bearing  metals;  and  the  volumetric  determina- 
tion of  zinc. 

A  part  of  the  discussion  of  metallography  and  treatment  of 
steel  has  been  rewritten  and  several  new  sections  have  been 
added  to  this  portion  of  the  book.  New  photomicrographs  have 
been  substituted  for  the  old  ones  and  about  fifteen  new  figures 
have  been  used  at  various  points  throughout  the  book,  the  latter 
being  the  work  of  the  author's  students  in  Chemical  Engineering, 
to  whom  grateful  acknowledgment  is  due.  Finally,  a  new  system 
of  chapter  division  has  been  introduced  in  order  to  give  greater 
emphasis  to  the  sections  dealing  with  industrial  analysis. 

In  the  process  of  making  these  changes  a  considerable  poition 
of  the  book  has  been  rewritten  and  the  discussions  have  been 
amplified,  in  the  interest  of  added  clearness.  It  is  hoped  that 
these  changes  will  serve  to  enhance  the  usefulness  of  the  book  as  a 
college  text  and  that  the  favor  with  which  the  first  edition  was  so 
generously  received  will  not  be  undeserved  by  the  present 
edition. 

December,  1918. 


PREFACE  TO  THE  FIRST  EDITION 

This  book  cannot  be  classed  as  a  complete  reference  work  on 
quantitative  analysis,  neither  is  it  a  bare  outline  of  laboratory 
exercises.  The  author  has  felt  a  desire  that  has  probably  been 
felt  by  every  teacher  of  quantitative  analysis,  to  produce  a  book 
that  would  cover  the  ground  that  he  wishes  to  cover  in  the  college 
courses,  providing  a  reasonable  degree  of  latitude  in  the  selection 
of  exercises  for  other  possible  users  of  the  book,  and  at  the  same 
time  to  present  a  theoretical  and  practical  discussion  of  the  sub- 
ject, sufficiently  simple  to  be  comprehended  by  the  average  stu- 
dent but  not  so  elementary  as  to  destroy  his  self-respect. 

One  of  the  most  difficult  tasks  connected  with  the  teaching  of 
quantitative  analysis  is  to  produce  in  the  mind  of  the  student  a 
clear  comprehension  of  the  scientific  development  of  quantitative 
methods.  There  seems  to  be  a  more  or  less  unconscious  tendency 
toward  the  acceptance  of  the  present  laboratory  method  of  pro- 
cedure as  a  gift  of  Providence.  How  well  this  situation  has  been 
met  in  the  present  volume  must  be  shown  by  the  test  of  experience. 
The  general  discussions  have  been  given  a  large  share  of  attention 
although  elaborate  or  involved  theoretical  discussions  have  been, 
so  far  as  possible,  avoided.  References  to  original  papers  have 
been  carefully  selected  with  a  view  to  actual  reading  by  the  stu- 
dent and  such  references  are,  in  nearly  all  cases,  to  discussions 
that  will  serve  either  to  impress  more  clearly  upon  the  reader's 
mind  the  principles  mentioned  in  the  text  or  to  bring  to  his  mind  a 
realization  of  the  labor  involved  in  the  development  of  the  finished 
method.  It  is  believed  that  careful  and  systematic  reading  and 
discussion  of  such  original  papers  by  the  student  with  his  in- 
structor is  a  most  valuable  aid  in  the  understanding  of  quanti- 
tative analysis  as  a  truly  scientific  study. 

The  mathematical  development  of  quantitative  calculations  in 
this  book  is  somewhat  unusual,  in  that  the  "rule  of  three"  has 
been  carefully  excluded.  It  is  the  firm  belief  of  the  author,  after 
several  years  of  experience  in  teaching  quantitative  analysis,  that 
the  use  of  this  rule  of  proportion  has  produced  much  harm  and 
has  been  the  greatest  of  all  obstacles  to  the  student  in  his  attempt 
to  grasp  the  principles  of  quantitative  calculations,  and  par- 

vii 


viii  PREFACE 

ticularly  those  of  volumetric  analysis.  The  ideas  involved  in  the 
solution  of  proportions  are  so  labored  and  so  unnecessary  and 
require  such  cumbersome  solutions  of  problems  involving  them 
that  it  is  difficult  to  see  why  emphasis  has  so  generally  been  placed 
upon  this  rule  in  chemical  calculations.  Other  teachers  will,  no 
doubt,  differ  with  the  author  upon  this  point.  It  is  desired  only 
that  the  method  of  presentation  involved  in  these  pages  be  tested, 
not  in  part  but  in  whole,  before  final  judgment  is  given.  Most 
of  the  calculations  involved  in  the  laboratory  exercises  have  been 
left  to  the  student.  Principles  of  such  calculations  are  first 
fully  explained  but  ready-made  calculations  that  leave  nothing 
to  the  ingenuity  of  the  student  furnish  poor  preparation  for 
scientific  analysts. 

Acknowledgment  is  here  gladly  expressed  to  Mr.  H.  C.  Mahin 
for  all  of  the  original  drawings  in  this  book,  also  to  Wm.  Ainsworth 
and  Sons,  Bausch  and  Lomb  Optical  Company,  the  Bureau  of 
Standards,  Eimer  and  Amend  and  The  Scientific  Materials  Com- 
pany for  several  cuts  which  they  have  loaned. 

EDWARD  G.  MAHIN. 
LAFAYETTE,  IND., 
December,  1913. 


CONTENTS 

PAGE 

PREFACE  TO  THE  SECOND  EDITION v 

PREFACE  TO  THE  FIRST  EDITION vii 

INTRODUCTION  .          xiii 


PART  I 
GENERAL  QUANTITATIVE  ANALYSIS 

CHAPTER  I 

GENERAL  PRINCIPLES 

Cleanliness  and  care — Limit  of  accuracy — Classes  of  methods. 


CHAPTER  II 

GRAVIMETRIC  ANALYSIS 5 

Factors — Factor  weights. 

General  operations:  Preparation  of  samples — Solution — Precipita- 
tion— Colloids — Enlargement  of  particles — Filtration — Washing 
— Drying  of  precipitates — Ignition — Fusion — Weighing — Cali- 
bration of  weights. 
Reagents — Glassware — Records. 

CHAPTER  III 

EXPERIMENTAL  GRAVIMETRIC  ANALYSIS 76 

Calcium — Silver — Chlorides,  bromides  and  iodides — Aluminium — 
Barium — Sulphates — Strontium — Potassium  and  sodium — Re- 
covery of  platinum  from  waste — Magnesium — Phosphates — 
Manganese — Halogen  compounds — Carbonates  and  caibon 
dioxide. 

CHAPTER  IV 

ELECTRO -ANALYSIS 138 

Nature  of  electrolyte — Solvent — Temperature — Electrolytic  pres- 
sure— Current  density — Nature  of  electrodes — Other  apparatus. 
Copper — Silver — Iron — Lead — Nickel — Separations — Moving  elec- 
trodes— Mercury  cathode. 

CHAPTER  V 

VOLUMETRIC  ANALYSIS 168 

Apparatus — Units  of  volume — Calibration  by  weighing — Calibra- 
tion by  standardized  bulbs — Calculation  of  results — Weight  of 


X  CONTENTS 

PAGE 

one  substance  equivalent  to  a  stated  weight  of  another — Stand- 
ard solution  for  titration  of  but  one  substance — Burette  reading  a 
percentage  reading — No  system — Normal  system — Decimal 
system — Choice  of  system — Temperature  correction  for  standard 
solutions — Adjustment  to  exact  concentration. 

CHAPTER  VI 

COLOR  CHANGE  OP  INDICATORS 207 

lonization  theory — Theory  of  chromophors — Classification  of 
indicators — Description  of  indicators. 

CHAPTER  VII 

STANDARDIZATION 216 

Direct  weighing — Weighing  a  substance  produced  by  a  measured 
volume  of  solution — Measuring  the  volume  of  solution  required 
to  react  with  a  known  weight  of  a  substance  of  known  purity — 
Titration  against  another  standard  solution — Primary  standards. 

CHAPTER  VIII 
EXPERIMENTAL  VOLUMETRIC  ANALYSIS 221 

Standard  acids:  Materials  for  standardization — Standardization 
by  direct  weighing — Preparation  of  pure  sodium  carbonate. 

Standard  hydrochloric  acid:  Soda  ash — Mixtures  of  carbonates 
and  bases — Mixtures  of  carbonates  and  bicarbonates — Hardness 
and  alkalinity  of  water. 

Standard  bases:  Selection  of  base  for  standard  solutions — Stand- 
ardization. 

Standard  sodium  hydroxide:  Concentration  of  the  laboratory 
acids — Citric  acid — Vinegar — Boric  acid. 

Use  of  two  standards :  Limestone  for  agricultural  purposes. 

CHAPTER  IX 
OXIDATION  AND  REDUCTION 240 

Apparent  valence. 

Potassium  permanganate:  Iron — Reduction  of  permanganate  by 
chlorides — Primary  standards — Reduction  of  iron  solutions — 
Calcium — Manganese — Available  oxygen. 

Potassium  dichromate:  Iron — Chromium. 

Iodine  and  sodium  thiosulphate :  Oxidizing  power  of  peroxides — 
Copper — Bleaching  powder — Standard  iodine  solution — Arsen- 
ical insecticides — Total  arsenic  and  copper  in  Paris  green. 

CHAPTER  X 

TITRATIONS  INVOLVING  FORMATION  OF  PRECIPITATES 276 

Silver — Halides  and  cyanides — Zinc. 


CONTENTS  xi 

PART  II 

ANALYSIS  OF  INDUSTRIAL  PRODUCTS  AND 
RAW  MATERIALS 

CHAPTER  XI 

PAGE 

ROCK  ANALYSIS 284 

Carbonate  minerals :  Carbon  dioxide — Silica — Iron  and  aluminium 

— Calcium — Magnesium — Sodium  and  potassium. 
Silicate  minerals:   Moisture — Silica — Iron  and  aluminium — Man- 
ganese— Calcium — Magnesium — Sodium  and  potassium. 

CHAPTER  XII 

FUELS 297 

Coal:  Proximate  analysis — Fusing  point  of  ash — Ultimate  analysis 

— Calorimetry. 

Gas  mixtures:  Apparatus — Absorbents — Illuminating  gas — Chim- 
ney gases. 

CHAPTER  XIII 

OILS,  FATS  AND  WAXES 345 

Burning    oils:    Specific    gravity — Flash   point — Burning    point — 

Fractional  distillation. 
Lubricating     oils:     Viscosity — Specific     gravity — Separation     of 

saponifiable  from  mineral  oils — Chill  test — Cold  test. 
Edible  fats  and  oils :  Composition — Specific  gravity — Melting  point 
of  fats — Iodine  absorption  number — Acid  value — Saponification 
number — Insoluble  and  soluble  acids — Reichert-Meissl  number 
— Polenske  value — Acetyl  value — Maumene"  number — Specific 
temperature  reaction — Qualitative  reactions — Fish  and  marine 
animal  oils  in  mixtures  with  vegetable  oils — Examination  of  an 
unknown  oil — Hardened  oils. 

CHAPTER  XIV 
WATER 391 

Industrial  analysis:  Corrosives — Incrustants — Foam  producers — 
Hypothetical  compounds — Treatment. 

Sanitary  examination :  Potability — Collection  of  samples — Physical 
examination — Chlorine — Organic  nitrogen — Nitrogen  in  am- 
monia— Nitrites — Nitrates — Required  oxygen. 

CHAPTER  XV 
STEEL  AND  ALLOYS 432 

Steel  and  cast  iron:  Sampling — Standard  methods — Solution  and 
evaporation — Standard  samples — Carbon — Silicon — Sulphur — 


xii  CONTENTS 

PAGE 

Phosphorus — Titanium — Manganese — Tungsten — Chromium  — 
Nickel — Vanadium — Oxygen. 

Treatment  of  steel:  Thermal  changes — Allotropism — Proximate 
constituents  of  slowly  cooled  steel — Ferrite — Cementite — 
Pearlite — Relation  between  structure  and  carbon  percent — 
Austenite — Relation  of  proximate  constituents  to  critical  points 
of  steel — Steel  as  a  solid  solution — Martensite— Critical  points 
related  to  rate  of  cooling  or  heating — Hardening  and  annealing — 
Troostite — Sorbite — Quenching  media — Tempering — Granula- 
tion— Overheating — Uneven  carbon  distribution — Streaks — 
Sulphur  prints — Case  hardening — Effect  of  working — Fatigue — 
Slag — Apparatus  for  metallographic  work — Experiments. 

Brass  and  bronze:  Tin — Lead — Copper — Zinc. 

Anti-friction  metals :  Tin — Antimony — Lead — Copper. 

CHAPTER  XVI 
AGRICULTURAL  MATERIALS 510 

Fertilizers:  Moisture — Nitrogen — Availability  of  nitrogen — Phos- 
phorus— Potassium. 

Milk:  Adulteration  of  milk — Specific  gravity — Total  solids — Ash — 
Total  nitrogen — Casein — Albumin — Lactose — Fat. 

Cream:  Ash— Fat. 

Condensed  milk:  Total  solids — Ash — Nitrogen — Lactose — Fat — 
Sucrose. 

Butter  and  substitutes:  Moisture — Fat — Casein — Salt — Examina- 
tion of  fat — Coloring  matter. 

CHAPTER  XVII 

THE  FIRE  ASSAY 564 

Gold  and  silver  ores:  Sampling — Weighing — Crucible  process — 
Fluxes — Reducing  agents — Oxidizing  agents — Crucible  charge — 
Reducing  ores — Ores  containing  copper,  arsenic  or  antimony — 
Cupellation — Inqu  art  ation — Parting — Annealing — Scorific  at  ion . 

TABLE  OF  LOGARITHMS  AND  ANTILOGARITHMS 586 

INDEX ...  .  593 


INTRODUCTION 

In  the  study  of  such  divisions  of  chemistry  as  naturally  pre- 
cede quantitative  analysis  the  work  has  been  mainly  descriptive. 
The  chemical  and  physical  properties  of  elements  and  compounds 
have  been  determined.  Laws  of  chemical  action  have  been  devel- 
oped and  theories  have  been  evolved  for  the  explanation  of  such 
action  and  as  generalizations  upon  which  to  plan  further  studies 
and  investigations.  In  the  courses  in  qualitative  analysis  an 
effort  was  made  to  detect  and  recognize  elements  and  their 
compounds  by  certain  characteristic  reactions  of  these  sub- 
stances and  to  separate  more  or  less  complicated  mixtures  into 
their  constituents. 

Quantitative  analysis  is  the  next  logical  step  in  the  study  of 
the  composition  of  matter.  The  qualitative  analysis  should  pre- 
cede the  quantitative,  unless  the  nature  of  the  substance  is 
already  known,  because  in  nearly  all  cases  the  presence  of  sub- 
stances other  than  those  whose  percent  is  being  determined  will 
make  necessary  certain  modifications  in  the  method  to  be  em- 
ployed. Whether  or  not  a  qualitative  analysis  is  made  it  is 
a  fact  to  be  constantly  kept  in  mind  that  an  intelligent  understand- 
ing of  quantitative  processes  can  be  obtained  only  by  a  continued 
application  of  the  facts  and  laws  earlier  learned  to  the  newer 
processes  which  are  being  studied.  The  industrial  development 
of  the  world,  as  well  as  the  evolution  of  chemistry  as  a  science, 
would  be  a  more  rapid  and  substantial  change  were  it  not  for 
the  numbers  of  inadequately  trained  chemists  who  have  found  a 
place  in  industrial  work  and  who  have  been  content  to  allow 
their  study  to  go  no  farther  than  the  routine  of  following  direc- 
tions without  understanding.  This  is  the  inevitable  end  of  the 
student  who  does  not  begin  his  scientific  study  with  the  definite 
determination  patiently  and  persistently  to  think  out  each  prob- 
lem to  its  logical  conclusion  as  it  presents  itself,  and  who  does  not 
continue  this  process  in  his  work  in  quantitative  analysis,  re- 
viewing his  earlier  work  until  the  principles  that  were  imperfectly 
understood  expand  and  illuminate  the  newer  problems. 

xiii 


QUANTITATIVE  ANALYSIS 


PART  I 
GENERAL  QUANTITATIVE  ANALYSIS 


CHAPTER  I 
GENERAL  PRINCIPLES 

Cleanliness  and  Care. — In  no  other  line  of  scientific  work  are 
neatness  and  care  more  essential  than  in  quantitative  analysis. 
It  is  desirable  that  the  student  should  acquire  speed  so  that  his 
efficiency  may  be  increased  by  his  ability  to  accomplish  much 
work  in  the  time  at  his  disposal  but  speed  attained  through  care- 
less manipulation  or  through  a  sacrifice  of  a  close  study  of  the 
analytical  process  is  of  little  avail  because  the  results,  when  ob- 
tained, are  not  dependable.  Speed  should  rather  be  acquired  by 
an  intelligent  application  of  methods  that  are  thoroughly  under- 
stood. Several  experiments  may  be  performed  at  the  same  time 
without  confusion  if  the  analyst  has  cultivated  the  habit  of  delib- 
erate and  clear  thinking.  It  is  highly  important  that  as  far  as 
possible  the  operations  should  be  systematized  so  that  the  mind 
may  be  left  free  for  the  more  important  problems  that  are  not 
of  a  routine  nature.  For  example  a  complete  system  of  marking 
vessels  and  materials  makes  impossible  errors  due  to  confusion 
and  lack  of  identification.  Apparatus  should  be  kept  scrupu- 
lously clean,  note  books  and  other  records  with  perfect  system 
and  the  desk  always  cleared  of  apparatus  that  is  not  in  use.  Re- 
agents should  be  added  carefully  and,  whenever  practicable, 
should  be  measured  approximately  so  that  in  case  of  error  the 
defect  may  be  corrected.  A  disregard  of  this  precaution  often 
makes  it  necessary  to  begin  over  again  a  determination  that 
might  have  been  saved. 

1 


2  QUANTITATIVE  ANALYSIS 

All  of  these  points  will  be  amplified  as  the  work  proceeds  and 
as  occasion  offers.  They  are  here  mentioned  in  order  to  give 
some  idea  of  the  requirements  of  the  work  that  is  to  follow. 

Limit  of  Accuracy. — A  question  frequently  asked  by  beginners 
in  quantitative  analysis  concerns  the  degree  of  agreement  that 
is  regarded  as  reasonable  and  necessary  for  the  results  of  deter- 
minations made  in  duplicate.  This  question  can  never  be 
answered  without  qualification.  The  ideal  of  all  scientific 
work  is  absolute  accuracy.  How  closely  this  ideal  may  be  ap- 
proached will  depend  partly  upon  the  individual  analyst,  but  also 
upon  the  possibilities  of  the  method  with  which  he  happens  to  be 
working.  The  latter  is  a  variable  quantity.  Certain  inorganic 
methods  are  capable  of  giving  results  that  are  quite  reliable  to 
within  one  hundredth  of  one  percent  while  other  (particularly 
organic)  methods  give  only  an  approximation  of  whole  percents. 
Thus  no  definite  limit  to  the  accuracy  of  methods  in  general  can 
be  set.  Each  case  must  be  considered  by  itself  and,  while  the 
student  will  use  the  utmost  possible  care  in  all  cases,  he  must, 
for  a  time,  be  content  to  allow  his  instructor  to  be  the  judge  of 
the  accuracy  that  may  be  reasonably  required. 

A  very  common  fallacy  is  to  the  effect  that  no  part  of  the  work 
need  be  performed  more  carefully  than  that  part  which  is  necessarily 
least  accurate.  For  example,  it  is  said  that  if  a  certain  part  of  an 
analytical  process  involves  an  unavoidable  error  of  0.10  percent, 
it  is  a  waste  of  time  to  attempt  to  avoid  errors  in  other  parts  of 
the  work  amounting  to  0.05  percent  or  even  0.09  percent.  This 
is  a  most  unfortunate  attitude  for  the  analyst.  This  logic  would 
lead  one  to  the  conclusion  that  if  a  method  cannot  give  results 
nearer  than  0.10  percent  to  the  truth  it  should  be  given  an  excel- 
lent chance  to  depart  0.10  percent +0.09  percent  from  the  truth 
or  0.10  per  cent +nX  0.09  percent,  if  there  are  n  other  places 
where  errors  may  occur.  Of  course  it  is  not  necessarily  true 
that  all  errors  will  have  the  same  algebraic  sign  and  they 
may,  to  a  certain  extent,  counteract  each  other  in  effect.  It 
is  important  to  note,  however,  that  there  is  no  assurance  that 
they  do  counteract  each  other  instead  of  accumulating.  This 
misconception,  no  doubt,  arises  from  the  unquestioned  fact  that 
when  a  certain  minimum  error  is  unavoidable  it  is  not  wise  to 
expend  an  undue  amount  of  time  in  rectifying  other  possible 


GENERAL  PRINCIPLES  3 

errors  where  the  ratio  of  these  errors  to  the  larger  error  is  very 
small,  because  in  such  a  method  the  analytical  results  have  little 
significance  as  expressed  in  small  fractions.  For  example,  in 
the  determination  of  volatile  combustible  matter  in  coal  it  is 
difficult  to  obtain  results  from  analyses  performed  in  duplicate, 
agreeing  more  closely  than  0.2  to  0.5  percent,  because  of  the 
impossibility  of  exactly  duplicating  heating  conditions.  It  is 
then  not  profitable  to  weigh  the  sample  so  accurately  as  to  justify 
an  agreement  within  0.005  percent  unless  there  are  several  other 
places  where  rectification  of  similar  errors  is  possible,  simply 
because  the  results  will  have  no  significance  in  this  remote  decimal 
place  and  will  not  be  expressed  to  the  third  or  even  the  second 
place.  If,  however,  there  are  five  other  points  in  the  method 
where  errors  may  be  made  as  small  as  0.005  percent  but  by  undue 
haste  or  neglect  might  be  as  large  as  0.05  percent,  then  the  argu- 
ment already  referred  to  would  lead  one  to  commit  an  error  as 
large  as  0.5  percent +5X0.05  percent  =  0.75  percent  where  it 
could  have  been  kept  as  low  as  0.5  percent+5X  0.005  percent 
=  0.525  percent. 

There  still  remains  to  be  answered  a  legitimate  question  re- 
garding the  number  of  decimal  places  to  be  reported.  The  opera- 
tions of  multiplication  and  division  that  are  involved  in  calcula- 
tions of  analyses  often  give  five  or  more  figures  in  decimal  places 
and  yet  even  the  novice  understands  that  these  figures  have  no 
significance  since  the  probability  of  their  correctness  is  very  slight. 
A  rule  that  is  very  generally  followed  is  to  report  one  decimal 
place  farther  than  the  one  that  is  considered  to  be  certainly  correct. 
Usually  the  student  has  little  evidence  concerning  the  accuracy  of 
his  work  other  than  the  agreement  of  his  duplicate  determinations. 
If  two  analyses  of  an  iron  ore  give  52.75  percent  and  52.71  per- 
cent, respectively,  it  would  be  considered,  from  this  standpoint 
alone,  that  52.7  percent  is  certainly  correct  and  that  the  average 
of  52.73  percent  is  a  close  approximation  for  the  second  decimal 
place.  This  average  would  therefore  be  reported.  If  the  re- 
sults were  52.75  percent  and  52.67  percent  the  disagreement 
would  be  0.08  percent  and  there  is  again  agreement  in  the  first 
decimal  place  although,  at  first  sight,  this  would  not  appear  to  be 
true.  The  average  52.71  percent  would  then  be  reported.  If 
the  results  were  52.75  percent  and  52.45  percent  the  disagree- 


4  QUANTITATIVE  ANALYSIS 

ment  would  be  0.30  percent.  Evidently  the  first  decimal  place  is 
but  an  approximation  and  the  average  52.6  percent  would  be 
reported. 

The  rule  for  reports  will  then  be  as  follows:  Determine  the 
disagreement  of  the  two  determinations  by  subtracting  the  smaller 
percent  from  the  larger.  Calculate  the  average  and  report  as  far 
as  the  first  significant  figure  in  the  difference. 

Classes  of  Methods. — Quantitative  analysis  is,  for  the  sake  of 
convenience,  divided  into  two  general  classes  which  are  desig- 
nated as  Gravimetric  and  Volumetric.  The  general  nature  of  the 
work  is  indicated  by  these  terms,  since  the  first  class  consists  of 
analyses  made  by  means  of  measurements  of  weight  while  the 
second  class  consists  of  those  made  by  means  of  measurements  of 
volume.  The  discussion  of  volumetric  analysis  will  be  left  for  a 
later  section  of  the  book  because  the  complete  volumetric  proc- 
ess cannot  be  carried  on  without  a  knowledge  of  gravimetric 
methods.  Gravimetric  analysis  will  therefore  comprise  the  first 
part  of  the  course. 

The  problem,  as  it  presents  itself  to  the  quantitative  analyst, 
is  to  take  a  substance  whose  qualitative  composition  is  at  least 
partially  known  and  so  to  treat  it  that  a  part  or  all  of  the  constitu- 
ents may  be  expressed  in  terms  of  percents.  Thus  we  speak  of 
the  "analysis"  of  a  substance  and  of  the  "determination"  or 
"estimation"  of  the  constituents,  the  last  two  terms  being  used 
with  practically  the  same  meaning. 


CHAPTER  II 
GRAVIMETRIC  ANALYSIS 

The  gravimetric  determination  of  any  constituent  of  a  com- 
pound or  mixture  usually  depends  upon  the  precipitation  of 
this  constituent  in  such  a  form  that  it  may  be  separated  and 
weighed.  This  makes  it  necessary  that  the  following  conditions 
shall  be  fulfilled:  (a)  The  precipitate  must  have  an  extremely 
small  solubility  in  order  that  the  amount  lost  in  the  filtrate  may 
be  negligible,  (b)  The  precipitate  must  be  of  such  a  nature  that 
it  may  be  readily  retained  upon  the  filter  and  washed  free  from 
all  impurities,  (c)  The  precipitate  must  be  susceptible  to  drying 
or  strong  heating,  changing  its  composition  either  not  at  all, 
or  in  a  perfectly  definite  and  well-known  manner,  (d)  The  pre- 
cipitate should,  if  an  electrolyte,  be  a  strongly  ionized  one.  The 
last  condition  will  be  understood  when  the  nature  of  precipita- 
tion is  considered. 

The  determination  of  calcium  in  a  calcium  compound  may  be 
taken  as  an  illustration  of  the  gravimetric  process.  A  definite 
weight  of  sample  is  taken,  dissolved  and  the  calcium  precipitated 
as  oxalate.  This  is  separated  by  filtration,  washed  free  from 
soluble  salts  and  then  strongly  heated  and  weighed  as  pure  cal- 
cium oxide.  It  is  known  from  the  formula  for  calcium  oxide 
(expressing  as  it  does  the  results  of  many  careful  analyses)  that 
40  07 
56 7)7  °^  ^S  we^h"t  is  calcium.  That  is, 

40.07P 

5607       =  weight  of  calcium, 

where  P  =  weight  of  calcium  oxide  found. 

Since  this  experimentally  determined  weight  of  calcium  was  ob- 
tained from  a  known  weight  of  the  calcium  compound  it  follows 
that 

weight  of  calcium  X  100  , 

— g—  -  =  percent  of  calcium  in  the  sample, 

S  representing  the  weight  of  sample  used. 

5 


6  QUANTITATIVE  ANALYSIS 

Combining  these  two  expressions: 

40.07  X  100P  =         ent  of  caicium  in  the  sample.        (2) 
56.07S 

Factors. — It  is  seen  from  an  inspection  of  the  above  formula 
that  so  long  as  determinations  of  calcium  are  being  made  in  this 

manner  the  constant  fraction  -  '  56  07 will  enter  into  a11  cal~ 

culations.     The  calculation  will  be  somewhat  shortened  if  these 
factors  are  collected  into  the  decimal  fraction  71.464. 
Expression  (2)  then  becomes 

71  464P 

-  =  percent  of  calcium.  (3) 

fe 

This  is  a  typical  form  of  expression  for  the  experimentally  deter- 
mined percent  of  any  element,  radical  or  compound.  The  deci- 
mal fraction  71.464  is  known  as  a  gravimetric  factor,  which  may 
be  defined  as  the  percent  of  a  given  element  or  group  of  elements 
in  or  equivalent  to  a  pure  compound  weighed.  The  pure  substance 
weighed  usually  contains  the  element  or  radical  which  is  being 
determined  but  does  not  always  do  so. 

If  the  factor  in  general  is  represented  by  F,  equation  (3)  may 
be  written 

TTP 

-F-=  required  percent.  (4) 

Atomic  weights  have  been  calculated  from  the  results  of  the 
most  careful  experimental  work  of  modern  investigators.  For 
this  reason  the  factor  is  the  most  reliable  of  all  of  the  experimental 
data  used  by  the  analyst  and  the  calculation  may  be  carried  far- 
ther than  that  of  analytical  percents.  Five  significant  figures 
should  usually  be  recorded. 

Factor  Weights. — In  equation  (4),  F  is  a  constant  for  all  deter- 
minations of  the  particular  element  or  group  of  elements  for  which 
it  has  been  calculated.  It  is  possible  so  to  choose  the  weight  of 
sample  taken  as  to  simplify  the  calculation  represented  by  equa- 
tion (4) .  For  instance,  by  taking  a  sample  weight  equal  in  grams 

F 
to  the  value  of  the  factor,  g  =  1  and  equation  (4)  becomes 

P  =  required   percent. 


GRAVIMETRIC  ANALYSIS  7 

In  such  a  case  the  weight  of  the  precipitate,  expressed  in  terms 
of  grams  or  fractions,  would  indicate,  without  calculation,  the 
required  percent  of  the  constituent  being  determined. 

A  weight  of  sample  equal  in  grams  to  the  value  of  the  factor  is 
usually  too  large  a  quantity  to  be  handled  readily  and  a  definite 
fraction  of  this  weight  (0.5,  0.2,  0.1,  0.01,  etc.)  may  be  used  in- 
stead. Any  such  weight  is  called  a  factor  weight,  which  may  be 
denned  as  a  quantity  equal  in  weight  units  to  the  value  of  the  gravi- 
metric factor,  or  to  some  simple  fraction  of  this  value. 

The  use  of  such  weights  involves  adjusting  the  quantity  of 
the  sample  on  the  balance  to  a  certain  specified  value.  If  this 
device  is  to  result  in  the  saving  of  time  it  is  obvious  that  the 
weighing  must  not  require  any  extra  time.  Therefore  the  use  of 
factor  weights  is  generally  confined  to  the  determination  of  such 
constituents  as  occur  in  small  amounts  in  a  sample  that  is  to  be 
analyzed,  so  that  relatively  large  amounts  of  the  sample  may  be 
used  and  the  accuracy  of  the  weighing  may  be  correspondingly 
less  without  impairing  the  accuracy  of  the  determination. 

Use  of  Logarithms. — Most  of  the  results  of  quantitative  analy- 
sis involve  chiefly  multiplications  and  divisions.  On  this  account 
a  four  or  five  place  table  of  logarithms  will  be  found  extremely 
serviceable.  Many  students  hesitate  to  use  such  tables  because 
they  find  that  mistakes  are  more  readily  made  and  that  the  cal- 
culations require  more  time  than  is  the  case  when  ordinary 
arithmetical  calculations  are  employed.  If  this  is  true  it  is  be- 
cause of  a  lack  of  facility  in  handling  such  tables  and  this  comes 
through  lack  of  practice.  The  beginner  in  this  work  is  strongly 
urged  to  make  use  of  his  tables  even  though  this  should,  at  first, 
involve  more  work  and  care  than  direct  arithmetical  solutions. 
Wherever  a  constant  occurs  in  a  solution  the  logarithm  of  this 
constant  may  be  recorded  and  used  in  all  similar  calculations. 
This  is  particularly  helpful  in  the  case  of  factors. 

Chemists*  Slide  Rules. — A  slide  rule  may  be  substituted  for 
the  logarithmic  table  but  it  is  not  accurate  beyond  the  third 
significant  figure  and  should  not  be  used  for  the  calculation  of 
careful  analyses.  Chemists'  slide  rules  have  been  devised,  having 
marked  at  the  proper  points  the  constants  that  represent  the 
analytical  factors.  By  the  use  of  such  a  rule  a  percent  may  be 
read  from  one  adjustment  of  the  slide. 


QUANTITATIVE  ANALYSIS 


FIG.   1. — Chemist's  slide  rule. 


Following  is  the  international  table  of  atomic  weights  for  1918. 
INTERNATIONAL  ATOMIC  WEIGHTS,  1918 


Symbol 

Atomic 
weight 

Symbol 

Atomic 
weight 

Al 

27  1 

Molybdenum  

Mo 

06  0 

Antimony  

Sb 

120  2 

Neodymium  

Nd 

144  3 

A 

39  88 

Ne 

20  2 

Arsenic 

As 

74  96 

Nickel 

Ni 

58  68 

Barium  

Ba 

137  37 

Niton  

Nt 

222  4 

Bismuth  

Bi 

208  0 

Nitrogen  

N 

14  01 

Boron 

B 

11  0 

Os 

190  9 

Bromine  

Br 

79  92 

o 

16  00 

Cadmium  

Cd 

112  40 

Palladium 

Pd 

106  7 

Caesium. 

Cs 

132  81 

p 

31  04 

Calcium  

Ca 

40  07 

Pt 

195  2 

Carbon 

c 

12  005 

J£ 

on    in 

Cerium  

Ce 

140  25 

Pr 

140  9 

Chlorine  

-      Cl 

35  46 

Ra 

226  0 

Chromium  

Cr 

52  0 

Rh 

102  9 

Cobalt     . 

Co 

58  97 

Rb 

OK     AK 

Columbium  

Cb 

93.1 

Ruthenium 

Ru 

101  7 

Copper  

Cu 

63  57 

Sa 

150  4 

Dysprosium  

Dy 

162  5 

Sc 

44    i 

Erbium  

Er 

167  7 

0» 

7Q   9 

Europium  

Eu 

152  0 

Si 

00      <3 

Fluorine  

P 

19.0 

Silver 

ACT 

107   RS 

Gadolinium  

Gd 

157.3 

Sodium  

Na 

23  00 

Gallium 

Ga 

69  9 

q_ 

Germanium  

Ge 

72  5 

g 

oo    nc 

Glucinum  

Gl 

9  1 

Ta 

Gold  

Au 

197  2 

TP 

Helium  

He 

4  00 

Th 

Holmium  

Ho 

163  5 

T1 

Hydrogen  

H 

1.008 

Th 

ooo   4 

Indium 

In 

mo 

Iodine  

I 

126.92 

Tin 

Tm 

Sn 

168.5 

Indium  

Ir 

193   1 

Ti 

Iron  

Fe 

55  84 

Krypton  

Kr 

82  92 

Lanthanum  

La 

139.0 

Vanadium.  . 

v 

51  0 

Lead  
Lithium  

Pb 
Li 

207  .  20 
6.94 

Xenon  

Xe 

Vh 

130.2 

Lutecium  

Lu 

175.0 

Yttrium  

Yt 

88  7 

Magnesium  

Mg 

24.32 

Zinc 

7n 

Manganese  
Mercury  

Mn 
Hg 

54.93 
200.6 

Zirconium  

Zr 

90.6 

GRAVIMETRIC  ANALYSIS 


Problem 

1.  Calculate  the  factors  and  their  logarithms  indicated  in  the  follow- 
ing table,  recording  in  the  proper  spaces  for  future  use.  The  atomic 
weights  as  recorded  in  the  preceding  table  should  be  used  and  the  factors 
should  be  calculated  in  five  significant  figures. 

TABLE  OF  GRAVIMETRIC  FACTORS  AND  LOGARITHMS 


Found 

Required 

Factor 

Log  Factor 

CaO 

Ca 

AgCl 

Ag 

Cl 

AgBr 

Br 

Agl 

I 

A120, 

Al 

BaS04 

Ba 

S04 

S03 

s 

SrS04 

Sr 

K2PtCl6 

K 

KC1 

K2S04 

KC1O4 

K 

KC1 

K2S04 

Na2SO4 

Na 

NaCl 

Na 

Mg2P2O7 

Mg 

P206 

. 

P 

Mn2P2O7 

Mn 

Mg2As2O7 

As 

GENERAL  OPERATIONS 

Certain  operations  enter  into  the  majority  of  gravimetric 
analyses  and  the  general  conduct  of  these  will  be  discussed  briefly, 
principles  and  precautions  being  indicated.  It  will  be  readily 
understood  that  variations  in  standard  procedure  will  be  de- 
pendent upon  the  nature  of  the  substance  being  analyzed  and 
such  variations  will  be  discussed  as  the  necessity  arises. 

Preparation  of  Samples. — The  object  of  all  preliminary  work 
with  samples  is  to  make  it  possible  to  obtain,  for  the  actual  analy- 


10  QUANTITATIVE  ANALYSIS 

sis,  a  portion  that  shall  truly  represent  the  average  composition 
of  the  entire  material  at  hand.  This  matter  is  likely  to  be  treated 
lightly  by  the  beginner,  but  proper  sampling  is  often  one  of  the 
most  difficult  problems  of  quantitative  analysis.  It  is  generally 
necessary  to  use  a  quantity  of  1  gm  or  less  and  if  the  substance 
is  not  homogeneous  this  small  quantity  may  have  an  average  com- 
position that  is  very  different  from  the  average  composition  of 
the  entire  material  being  investigated.  No  matter  how  carefully 
an  analysis  may  be  performed  or  how  accurate  the  results  ob- 
tained, if  the  substance  used  does  not  represent  the  average  of 
the  substance  originally  at  hand  the  results  become  nearly  or  en- 
tirely valueless.  If  the  substance  is  practically  homogeneous  the 
operation  of  sampling  involves  nothing  more  difficult  than  grind- 
ing down  to  a  degree  of  fineness  required  for  the  work.  This  is 
the  case  when  the  substance  is  an  approximately  pure  chemical 
compound,  such  as  will  be  used  for  the  earlier  exercises. 

The  gross  sample,  as  the  analyst  receives  it,  may  be  in  the 
form  of  lumps,  as  is  frequently  the  case  with  minerals,  or  it  may 
be  in  the  form  of  small  pieces,  crystals,  powder,  or  solution.  In 
any  case  except  that  of  liquid  samples,  the  object  is  to  reduce  the 
size  of  pieces  to  that  required  for  the  analysis  (usually  a  rather 
fine  powder)  and  at  the  same  time  to  select  from  the  total  mass 
such  a  quantity  as  is  required  for  the  experimental  work*  The 
original  sample  is  often  quite  large.  It  may  vary  from  a  few 
grams,  or  less,  to  many  pounds.  It  is  obviously  unnecessary 
and  practically  impossible  to  grind  the  entire  amount  into  a  fine 
powder.  The  operation  then  resolves  itself  into  a  thorough  mix- 
ing and  progressive  grinding  and  dividing.  Many  forms  of 
both  hand  and  power  grinders  are  in  common  use.  For  the  first 
exercises  nothing  more  complicated  than  a  porcelain  mortar  and 
pestle  will  be  required. 

Mixing  and  Dividing. — The  mixing  and  dividing  is  best  carried 
out  by  using  a  sheet  of  oilcloth  or  paper  and  a  spatula.  In  many 
laboratories  it  is  customary  to  use  oilcloth,  particularly  for  mixing 
minerals.  This  is  convenient  but  offers  the  possibility  of  con- 
tamination ("salting")  of  one  sample  by  the  remnant  of  one  that 
has  preceded  it.  It  is  better  to  use  a  large  sheet  of  tough,  flexible 
paper,  which  can  be  discarded  after  using.  The  sample,  after 
having  been  broken  down  to  the  proper  maximum  size  of  pieces, 


GRAVIMETRIC  ANALYSIS 


11 


is  placed  on  the  paper  and  thoroughly  mixed  by  rolling  diagonally 
across  the  paper  and  alternating  the  direction  of  rolling  as  illus- 
trated in  Fig.  2.  The  proper  rapid  manipulation  of  the  paper  is 
attained  only  after  considerable  practice.  One  precaution  is 
essential:  the  corner  of  the  paper  that  is  lifted  must  be  drawn 
across,  low  down,  in  such  a  manner  that  the  pile  of  material  is  not 
caused  to  slide  along  the  paper  but  is  turned  over  upon  itself  so 
that  the  bottom  is  brought  entirely  to  the  top.  In  this  way  only 
can  a  segregation  of  larger  and  smaller  particles  be  prevented. 
Since  the  larger  and  smaller  particles  usually  have  different  com- 
position it  is  essential  that  they  should  be  thoroughly  mixed. 


FIG.  2. — Manipulation  of  paper  when  mixing  samples. 

The  number  of  times  that  the  sample  is  rolled  before  dividing  will 
depend  upon  the  degree  of  homogeneity  and  the  accuracy  re- 
quired in  the  analysis.  In  the  assaying  of  gold  and  silver  ores 
it  is  not  unusual  to  require  one  hundred  times. 

Quartering. — When  the  first  mixing  is  finished  the  pile  is 
made  approximately  circular  and  is  then  divided,  by  means  of  a 
spatula,  into  quarters.  Opposite  quarters  are  carefully  scraped 
to  another  sheet  of  paper,  ground  finer  if  necessary,  remixed  and 
quartered  as  before.  This  process  of  grinding,  rolling,  and  quar- 
tering is  continued  until  a  sample  is  finally  obtained,  small 
enough  in  quantity  and  fine  enough  in  texture  to  serve  the  pur- 
pose of  the  final  weighing  and  analysis.  The  maximum  size  of 


12  QUANTITATIVE  ANALYSIS 

particles  to  be  allowed  in  any  particular  mixing  and  quartering 
will  depend  upon  the  total  quantity  of  material  being  handled 
in  this  operation.  No  particle  should  be  so  large  that  its  inclusion 
in  any  quarter  would  cause  the  average  composition  of  this  quar- 
ter to  be  appreciably  different  from  the  average  composition  of 
the  entire  pile.  This  means  that  the  ratio  of  the  size  of  the  largest 
particle  to  the  size  of  the  quarter  should  not  be  greater  than  a 
certain  maximum  value.  What  this  maximum  value  shall  be 
must  be  arbitrarily  determined  by  the  nature  of  the  sample  and 
the  degree  of  accuracy  required  in  the  analysis.  It  is  obvious 
that  the  part  can  perfectly  represent,  in  composition,  the  whole 
only  when  the  largest  particle  is  infinitesimal.  It  is  equally 
obvious  that  this  limit  is  impossible  and  unnecessary  in  practice 
and  we  may  say  that,  in  general,  the  ratio  of  the  largest  particle 
to  the  portion  that  includes  it  should  not  be  greater  than  0.01 
percent.  If  this  condition  is  met,  then,  after  thorough  mixing  of 
the  sample,  the  chance  inclusion  or  exclusion  of  any  given  particle 
cannot  modify  the  results  of  the  analysis  to  any  appreciable 
extent. 

The  maximum  size  of  the  particles  to  be  obtained  in  the  final 
portion  that  is  to  be  weighed  and  used  in  the  analysis  must  be 
determined,  not  only  from  the  above  considerations,  but 
also  by  the  nature  of  the  operation  to  follow  the  weighing. 
This  is  usually  solution  or  fusion.  If  the  substance  is  considered 
to  be  almost  absolutely  homogeneous  and  if  it  is  easily  soluble  (as, 
for  example,  a  crystal  of  cupric  sulphate)  then  the  grinding  need 
be  carried  no  farther  than  is  necessary  to  permit  the  easy  adjust- 
ment, between  fairly  narrow  limits,  of  the  weight  taken  for  analy- 
sis. In  such  a  case,  if  a  sample  of  0.3  to  0.5  gm  is  required,  then 
no  particle  should  weigh  more  than  about  0.1  gm.  If,  however, 
the  process  of  solution  or  fusion  is  a  difficult  one  to  accomplish 
or  if  the  material  is  far  from  being  homogeneous,  the  grinding 
is  carried  much  farther,  in  order  to  provide  a  very  large  surface 
of  contact  between  the  particle  and  the  solution  or  flux,  or  in 
order  to  conform  to  the  rule  of  maximum  size  of  particles,  stated 
above.  In  many  cases,  as  with  minerals,  the  maximum  size  of 
particles  is  fixed  by  causing  the  sample  to  pass  through  a  sieve 
having  meshes  of  stated  dimensions.  A  gold  ore  may  be  ground 
to  pass  a  sieve  having  100  or  200  meshes  to  the  linear  inch.  In 


GRAVIMETRIC  ANALYSIS 


13 


such  a  case  one  should  not  make  the  mistake  of  grinding  and 
sifting  a  portion  until  a  sufficient  quantity  is  passed,  discarding 
the  remainder.  This  would  cause  an  error  because  the  particles 


FIG.  3. — Vertical  section  of  a  pile  of  heterogeneous  material, 
b 


FIG.  4. — Division  by  quartering. 


that  resist  grinding  longest  are  less  brittle  and  have  a  composition 
different  from  that  of  the  particles  which  pulverize  easily. 

The  reason  for  dividing  into  quarters  after  each  mixing  and  for 
selecting  opposite  quarters  will  be  understood  from  the  following: 


14 


QUANTITATIVE  ANALYSIS 


Close  examination  of  the  pile  of  unmixed  material  will  reveal  the 
fact  that,  even  after  the  most  thorough  and  careful  mixing,  it  is 
not  entirely  homogeneous.  Around  the  circumference  of  the 
base  the  particles  are  coarser  and  they  may  be  gathered  toward 
one  side.  Around  the  apex  of  the  conical  pile  there  is  a  collection 
of  coarser  particles.  If  we  simply  dig  in  at  random  for  the  por- 
tion to  be  removed  the  lack  of  homogeneity  will  alter  the  char- 
acter of  the  portion  removed. 


Fio.  5. — Riffle;  for  automatic  division  of  dry  samples. 

Fig.  3  shows  how  quartering  will  properly  divide  a  pile  with 
coarse  material  over  the  top.  Fig.  4  shows  how  the  opposite 
quarters,  no  matter  in  what  direction  the  cuts  be  made,  will  ob- 
tain the  average  of  a  non-homogeneous  pile,  while  a  cut  into 
halves  will  do  so  only  in  case  the  cut  is  made  in  the  direction  ab. 
In  these  diagrams  the  conditions  are  purposely  exaggerated. 

An  automatic  sampler,  called  a  riffle,  is  illustrated  in  Fig. 
5.  The  nature  of  its  action  is  easily  seen. 

In  case  the  substance  to  be  analyzed  is  a  liquid,  the  operation 


GRAVIMETRIC  ANALYSIS  15 

of  sampling  is  usually  a  simple  one,  consisting  of  thorough  mixing 
before  the  removal  of  the  proper  quantity  for  the  analysis. 

Solution.  —  After  the  sample  of  solid  substance  has  been  prop- 
erly selected  and  weighed,  the  next  operation  is  usually  one  of 
solution.  What  the  solvent  shall  be  is  determined  by  the  nature 
of  the  case.  It  may  be  water,  concentrated  or  dilute  acids,  bases 
or  salts,  organic  solvents,  or  a  solid  that  is  to  act  as  a  dissolving 
flux  when  heated  with  the  substance.  In  any  event  it  is  desirable 
to  use  a  relatively  small  amount  of  the  solvent,  not  only  because  it 
must  finally  be  entirely  removed,  but  also  because  all  precipitates 
dissolve  to  some  extent  and  it  is  only  by  keeping  the  amount  of 
solvent  down  to  the  least  quantity  that  is  workable  that  the  loss 
of  precipitate  is  reduced  to  the  practicable  minimum. 

Precipitation.  —  The  process  of  producing  precipitates  for  use  in 
quantitative  analysis  is  one  that  has  received  much  study  from 
the  quantitative  standpoint.  Every  substance  has  a  more  or  less 
definite  solubility,  under  definite  conditions,  and  it  is  necessary 
to  reduce  this  (already  small)  solubility  to  the  least  possible 
quantity.  According  to  the  ionic  hypothesis  reactions  between 
most  inorganic  and  many  organic  compounds  are  reactions  of  the 
ions  of  the  compounds  that  change. 

Mass  Law.  —  The  reaction  between  potassium  chloride  and 
silver  nitrate  is,  under  proper  conditions,  nearly  complete  — 
nearly  enough,  in  fact,  that  we  regard  it,  for  practical  analytical 
purposes,  as  complete.  Both  of  these  salts  are  highly  ionized 
in  moderately  concentrated  solutions.  When  the  solutions  are 
mixed,  silver  chloride  is  formed  and  precipitated.  The  usual 
expression  for  the  reaction  is 

KCl+AgNO3<=±AgCl+KNO3. 

This  really  expresses  merely  the  substances  as  originally  taken  for 
making  the  solution  and  the  products  as  they  are  when  removed 
from  solution.  The  equation  that  expresses  the  real  reaction  is 

Ag+Cl^AgCl,  (1) 

also,  to  some  extent, 

(2) 


Reaction  (1)  is  the  one  that  is  important  in  this  instance. 


16  QUANTITATIVE  ANALYSIS 

'  The  principle  known  as  the  Mass  Law,  as  formulated  in  1867 
by  Guldberg  and  Waage,1  states  that  the  velocity  of  any  chemical 
reaction  is  directly  proportional  to  the  active  masses  of  the  re- 
acting substances.  In  a  reversible  reaction  this  applies  to  the 
reverse  as  well  as  to  the  direct  reaction,  and  in  the  case  already 
cited  this  means  that  the  velocity  is  proportional  to  the  concen- 
trations of  the  reacting  bodies,  whether  these  are  ions  or  molecules. 
Stated  symbolically  for  reaction  (1),  we  have 

vel  =  kCA+gCc-i          and 
vd=k'CAgcl 

—  *  4— 

where  vel  and  vel  are  the  velocities  of  the  reactions4aking  place 
in  the  direction  of  the  arrows,  k  and  k'  are  constants  and  C^g,  C^ 
and  CAgci  the  concentrations,  stated  in  weight  equivalents  per 
unit  volume,  of  silver  ions,  chlorine  ions  and  silver  chloride  mole- 
cules at  any  given  instant.  At  the  moment  of  adding  the  silver 
nitrate  to  the  potassium  chloride  CAg  and  C^have  their  maximum 
values  while  CAgci=0.  As  the  reaction  proceeds  CAgcl  increases 
while  CA+a  and  Cji  decrease,  owing  to  the  formation  of  silver 

chloride  molecules.  For  this  reason  vel  decreases  while  vel  in- 
creases. Equilibrium  occurs  when  the  direct  and  reverse  re- 
actions proceed  with  equal  velocity.  Therefore  at  equilibrium 

vel  =  vel  and 


Therefore     C;gCci  =  ^CAgcl  =  KCAgcl,  where  K-p       (3) 

Solubility  Product.  —  Equation  (3)  is  an  expression  of  the  fact 
that  at  equilibrium  there  is  a  definite  and  constant  relation 
between  the  concentrations  of  the  reacting  bodies  and  of  the 
bodies  formed.  The  laws  of  solubility  tell  us  that  there  is  simi- 
larly a  constant  relation  between  the  concentration  of  dissolved 
substance  and  that  of  the  same  substance  undissolved  after  pre- 

1  Ostwald's  Klassiker,  No.  104. 


GRAVIMETRIC  ANALYSIS  17 

cipitation  has  begun  and  the  stable  condition  known  as  saturation 
is  reached.     In  the  case  of  silver  chloride, 


If  concentration  is  taken  to  mean  the  ratio  of  the  weight  of 
solute  (expressed  in  gram  equivalents)  to  the  weight  of  solution, 
then  CAgcl_solid  must  equal.  1,  no  matter  how  much  or  how  little 
solid  may  be  present.  Therefore  K'CAgcl_aolid  has  a  constant 
value  : 

K'          and 

K''  (4) 

Equation  (4)  is  an  expression  of  the  fact  that  in  a  solution  satu- 
rated with  a  precipitated  substance,  the  product  of  the  concen- 
tration of  the  ions  of  this  substance  is  a  constant  quantity.  This 
product  is  known  as  the  solubility  product  and  it  is  of  great  impor- 
tance in  quantitative  analysis.  This  product  is  a  maximum 
value  for  the  condition  known  as  saturation.  It  is  exceeded  when 
the  solution  is  supersaturated  as  every  solution  is  before  precipi- 
tation begins.  Such  precipitates  as  calcium  oxalate  and  silver 
chloride  attain  the  solubility  product  comparatively  quickly, 
magnesium  ammonium  phosphate  slowly. 

According  to  condition  (a)  on  page  5  it  becomes  necessary 
that  the  solubility  product  as  well  as  the  concentration  of  dis- 
solved molecules  shall  be  extremely  small,  if  the  substance  is  to 
be  useful  for  quantitative  purposes,  as  otherwise  the  results  of 
the  analysis  will  be  vitiated  by  an  appreciable  loss  of  the  substance 
being  precipitated.  It  should  be  noticed  also  that  it  is  the  solu- 
bility product  that  is  constant  and  not  the  concentration  of  the 
individual  ions.  It  is  usually  true  that  a  single  gravimetric 
determination  concerns  a  single  radical  and  not  the  entire  com- 
pound; thus,  in  the  case  already  considered,  if  the  chlorine  radical 
were  being  quantitatively  determined  the  chloride  would  be 
weighed  and  silver  nitrate  used  as  the  reagent.  It  is  possible 
so  to  increase  the  factor  representing  the  concentration  of  the  ion 
which  is  not  being  determined  that  the  other  factor  must  assume  a 
very  low  value.  As  an  illustration,  suppose  that  a  sample  of  a 
soluble  chloride  is  weighed,  dissolved  in  water  and  a  solution  of 
silver  nitrate  added  until  all  of  the  chlorine  is  precipitated  except 


18  QUANTITATIVE  ANALYSIS 

that  represented  by  an  ordinary  saturated  solution  of^silver 
chloride.  At  18°  100  gm  of  water  will  dissolve  1.7  XlO"4  gm 
or  1.19X10"6  gram  equivalents  of  silver  chloride.1  In  such  a 
highly  dilute  solution  ionization  is  nearly  complete  (i.e.,  K  is 
very  large)  and  since  there  is  present  practically  the  same  number 

of  gram  equivalents  of  Ag  and  Cl  as  represents  the  number  of 
gram  equivalents  of  dissolved  silver  chloride,  CA+gCci  =  (practi- 
cally) (1.19 X10~6)2= 1.42 X10-12.  This  is  the  solubility  pro- 
duct of  silver  chloride.  If  to  this  solution  0.001  gram  equivalent 
of  silver  ions  is  added  (a  condition  approximately  attained,  with- 
out appreciable  dilution,  by  the  addition  of  3.5  cc  of  5  per- 
cent silver  nitrate  solution) 

^AgCci  =(1.19X10"6)2  becomes  upon  substitution  of 

c;g=ixio-3, 

10-3Cci=1.42X10"12,  whence 
Cci=  1.42X10-' 

Thus  by  adding  the  precipitating  reagent  but  3.5  cc  in  excess  of 
the  amount  required  to  form  silver  chloride  completely,  the 
amount  of  chlorine  remaining  in  solution  has  been  reduced  from 
1.19  X  10~6  gram  equivalent  to  1.42  X  10~9  gram  equivalent; 
stated  in  terms  of  the  actual  weight  of  chlorine,  0.00004  gm 
remaining  in  solution  has  been  reduced  to  0.00000005  gm, 
a  quantity  so  small  that  it  has  absolutely  no  significance  in  any 
but  the  most  refined  methods  of  analysis,  such,  for  instance,  as 
are  involved  in  atomic  weight  determinations.  From  this  reason- 
ing follows  the  general  rule  that  the  precipitating  reagent  should 
always  be  added  in  a  certain  slight  excess  over  the  amount  equiva- 
lent to  the  substance  being  precipitated.  Of  course  this  is  done 
also  as  a  matter  of  convenience,  since  it  is  obviously  impossible 
to  add  exactly  the  equivalent  amount  of  the  reagent  and  a  defi- 
ciency would  be  fatal  to  the  success  of  the  experiment. 

Attention  has  already  been  called  (page  5)  to  the  desirability 
of  having  highly  ionized  electrolytes  for  precipitates.  This  can 
now  be  understood  since  the  molecular  form  of  the  dissolved  sub- 
stance constitutes  the  irrecoverable  portion.  The  effect  of  an 
excess  of  reagent  upon  the  solubility  of  the  precipitate  becomes 

lKohlrausch  and  Rose,  Z.  physik.  Chem.,  12,  234  (1893). 


GRAVIMETRIC  ANALYSIS  19 

less  as  we  pass  to  precipitates  whose  ionization  is  small  and 
becomes  negligible  or  zero  with  non-electrolytes. 

Colloids. — There  is  a  certain  class  of  precipitates  with  which  we 
cannot  deal  in  so  definite  a  manner.  This  is  the  class  known  as 
colloids.  Certain  important  distinctions  are  to  be  made  between 
the  colloids  and  the  crystalloids.  While  the  latter  have  certain 
very  definite  effects  upon  the  freezing-point,  vapor  pressure  and 
osmotic  pressure  of  solvents,  the  former  have  little  that  is  appre- 
ciable. The  solubility  of  the  crystalloids  in  various  solvents  is  a 
tolerably  definite  and  constant  quantity,  temperature  being  speci- 
fied, while  there  is  no  definite  solubility  for  the  colloids.  The 
solubility  of  those  crystalloids  which  ionize  is  affected  in  a  definite 
and  calculable  manner  by  other  electrolytes.  The  solubility  of 
the  colloids  is  affected  by  the  presence  of  electrolytes  but  not  in 
the  same  definite  way.  Examples  of  well  known  colloids  are 
ferric,  chromium  and  aluminium  hydroxides,  arsenic  sulphide, 
and  many  organic  compounds  such  as  the  albumens.  It  is  now 
tolerably  well  established  that  the  colloidal  "solutions"  contain 
molecular  aggregates  that  are  large  enough  to  exhibit  more  of  the 
properties  of  invisible  mechanical  suspensions  than  of  true 
solutions,  although  there  is  little  doubt  that  the  difference 
between  the  pseudo  solution  of  a  colloid  and  the  true  solution  of 
a  crystalloid  is  one  of  degree  of  molecular  association  rather  than 
of  kind,  since  most  true  solutions  contain  associated  molecules. 
In  order  to  distinguish  the  true  solution  from  the  pseudo  solution 
of  a  colloid  the  latter  is  called  a  "sol"  and  if  water  is  the  solvent 
the  sol  is  a  "hydrosol." 

The  importance  of  colloidal  sols  to  the  analyst  lies  in  the  fact 
that  they  do  not  respond  to  the  effect  of  excess  of  reagent  when 
attempting  a  precipitation  and  that  the  colloid  will  remain  in 
invisible  suspension  to  an  extent  that  would  normally  represent 
a  greatly  supersaturated  solution.  The  absence  of  effect  of 
excess  of  reagent  is,  no  doubt,  due  to  the  absence  of  any  but  a 
very  slight  concentration  of  ions  in  the  solution  of  the  precipi- 
tating colloid.  Colloidal  sols  are  broken  down  with  precipitation 
(flocculation)  of  the  colloid  by  the  addition  of  certain  electrolytes. 
The  flocculated  colloid  may,  under  certain  conditions,  return  to 
the  sol.  Certain  colloids  will  not  so  return  after  drying.  Colloids 
of  the  former  class  are  known  as  "reversible"  and  those  of  the 


20  QUANTITATIVE  ANALYSIS 

latter  class  are  called  " irreversible."  Some  examples  of  reversi- 
ble colloids  are  dextrine,  gums,  albumens  and  many  organic 
colloids,  also  silver  chloride  and  molybdic  acid.  Examples  of 
irreversible  colloids  are  metal  hydroxides,  stannic  acid,  arsenic 
sulphide  and  colloidal  metals.  Reversible  colloids  may  often,  by 
certain  treatment,  be  changed  into  irreversible  colloids.  Thus 
strong  heating  causes  the  change  of  reversible  silica  into  irre- 
versible silica. 

Enlargement  of  Particles  of  the  Precipitate. — Complete  separa- 
tion of  a  precipitate  by  filtration  requires  that  the  smallest  indi- 
vidual particles  of  the  precipitate  shall  be  larger  than  the  largest 
pores  of  the  filter,  although  any  further  considerable  increase  in 
size  is  unnecessary  and  undesirable,  because  thorough  washing 
is  then  more  difficult.  Amorphous  precipitates  offer*  some  diffi- 
culty, on  account  of  the  fact  that  sols  are  likely  to  form  and  these 
cannot  be  separated  by  the  finest  filters.  Many  crystalline  pre- 
cipitates, notably  barium  sulphate  and  calcium  oxalate,  have  a 
tendency  to  precipitate  in  very  fine  crystals  so  that,  even  when 
the  finest  grades  of  filter  paper  are  used,  appreciable  quantities 
of  the  precipitate  are  lost.  This  tendency  can  be  partly  over- 
come by  observing  certain  precautions  during  the  process  of 
precipitation,  such  as  adding  the  reagent  slowly,  stirring  the 
solution  vigorously,  heating  the  solution  while  adding  the  pre- 
cipitant, etc. 

It  has  already  been  noticed  that  a  state  of  supersaturation 
always  precedes  precipitation.  The  extent  to  which  super- 
saturation  occurs  depends,  among  other  things;  upon  the  rate  at 
which  the  precipitating  substance  is  formed  in  the  solution  and 
this,  in  turn,  depends  upon  the  rate  of  addition  of  the  precipitant. 
The  formation  of  crystal  nuclei  requires  an  appreciable  amount  of 
time.  If  the  reagent  is  added  very  rapidly,  a  relatively  large 
degree  of  supersaturation  is  produced  before  the  substance  which 
is  being  formed  begins  to  crystallize.  If,  on  the  other  hand,  the 
reagent  is  added  slowly  and  thoroughly  mixed  by  stirring,  crystals 
begin  to  form  before  any  considerable  supersaturation  occurs. 
This  condition  is  very  important  to  the  formation  of  large 
crystals.  If  there  is  but  slight  supersaturation  at  the  beginning 
of  precipitation  and  the  solution  is  vigorously  stirred  as  more 
reagent  is  added,  a  relatively  small  number  of  crystal  nuclei 


GRAVIMETRIC  ANALYSIS  21 

may  serve  to  maintain  a  condition  of  approximate  equilibrium 
with  the  solution.  If  large  supersaturation  occurs  before  pre- 
cipitation begins,  and  if  the  rapid  addition  of  reagent  is  continued 
many  crystal  nuclei  will  form  at  all  parts  of  the  solution.  If 
we  regard  the  quantity  of  precipitate  ultimately  formed  as  the 
same  in  either  case  it  is  evident  that  the  solutions  from  which 
large  numbers  of  crystals  form  will  produce  the  finer  precipitates. 
From  this  follows  the  rule  that  the  precipitating  reagent  should 
always  be  added  slowly  and  the  solution  should  be  stirred  during  the 
addition  of  the  reagent. 

Even  after  the  most  careful  attention  to  the  method  of  precipi- 
tating it  occasionally  happens  that  a  precipitate  is  produced  in 
such  a  fine  state  of  division  that  a  portion  passes  through  the 
paper.  When  this  occurs  it  is  usually  possible  to  enlarge  the 
crystals  to  a  size  that  will  admit  a  ready  separation  by  filtration, 
by  allowing  to  stand  for  some  time  in  contact  with  the  mother 
liquor,  with  or  without  the  application  of  heat.  This  enlargement 
is  not  merely  a  coherence  of  small  particles  to  form  larger  ones, 
but  is  a  real  enlargement  of  individual  crystals,  the  number  of 
particles  decreasing  as  the  average  mass  increases.  Ostwald1 
demonstrated  that  this  change  in  the  form  of  the  crystalline  aggre- 
gate is  due  to  the  fact  that  small  crystals  have  a  greater  solution 
tension  than  the  large  ones.  The  result  of  digestion  in  contact 
with  the  solvent  is  that  the  solution  containing  a  definite  con- 
centration of  the  dissolved  precipitate  cannot  remain  in  equilib- 
rium with  both  large  and  small  crystals.  If  in  equilibrium  with 
the  larger  ones,  and  thus  saturated  with  respect  to  these,  it  is 
under-saturated  with  respect  to  the  smaller  ones  so  that  these 
dissolve  to  some  extent.  The  solution  then  becomes  supersatu- 
rated with  respect  to  the  larger  crystals,  and  some  of  the  sub- 
stance precipitates  (crystallizes)  on  these.  The  end  of  such  a 
process  is  the  total  disappearance  of  the  small  particles  and  the 
appearance  of  a  smaller  number  of  particles  having  a  larger 
average  size.  Hulett2  showed  that  the  solubility  of  barium 
sulphate  could  be  increased  from  0.00229  gm  per  liter  (the 
solubility  of  the  precipitated  salt)  to  0.00618  gm  per  liter  by 
pulverizing  the  crystals. 

1  Z.  physik.  Chem.,  34,  495  (1900). 

2  Ibid.,  34,  69  (1900). 


22  QUANTITATIVE  ANALYSIS 

Filtration. — Materials  generally  used  for  filtering  are  combus- 
tible (organic)  or  non-combustible  (inorganic).  The  precipitate 
is  to  be  weighed  later  and  it  is  necessary  that  some  means  be  found 
for  either  entirely  destroying  the  filtering  medium  or  discounting 
its  weight.  Paper  filters  are,  at  present,  used  for  the  majority 
of  gravimetric  .analyses  although  several  inorganic  substitutes 
are  being  used  to  a  greater  extent  than  formerly  because  they  pos- 
sess the  advantage  of  having  no  action  on  most  precipitates  at 
high  temperatures. 

Filter  Paper. — Paper  for  qualitative  work  is  necessarily  made 
with  great  care  since  it  must  combine  the  quality  of  considerable 
strength  with  that  of  a  uniform  porosity.  For  quantitative 
purposes  the  paper  must  have  even  greater  uniformity  of  texture; 
it  is  nearly  always  removed  by  burning,  after  the  precipitate 
has  been  thoroughly  washed,  and  it  is  essential  that  the  inorganic 
matter  which  is  always  found  in  organic  fibers  and  left  as  ash 
shall  be  either  in  sufficiently  small  quantity  to  be  negligible  or 
that  its  quantity  shall  be  uniform  so  that  a  definite  weight  may  be 
subtracted  from  the  weight  as  found  for  each  precipitate.  The 
better  grades  of  quantitative  papers  have  been  washed  with 
acids  so  that  much  of  the  inorganic  matter  has  been  removed. 
This  results  also  in  softening  the  fiber,  making  it  a  better  filtering 
medium  although  more  frail  and  subject  to  rupture  if  suction  is 
applied.  For  the  highest  grade  of  paper  hydrochloric  acid  and 
hydrofluoric  acid  are  used  and  the  weight  of  ash  for  a  paper 
9  cm  in  diameter  is  reduced  to  as  low  as  0.00011  gm.  On  account 
of  the  fact  that  this  weight  is  negligible  in  ordinary  determina- 
tions such  paper  is  often  erroneously  called  "ashless"  paper. 

Much  trouble  will  be  obviated  in  the  use  of  paper  filters  if  some 
care  is  exercised  in  the  selection  of  funnels.  A  circular  paper  is 
usually  folded  into  quadrants,  then  opened  out  to  form  a  cone,  the 
sides  of  which  include  an  angle  of  60°.  Funnels  as  purchased  for 
chemical  work  are  supposed  to  be  made  with  a  60°  angle  but  com- 
paratively few  are  exactly  of  this  form.  Others  have  the  correct 
angle  between  the  sides  but  near  the  apex  of  the  cone  the  shape  is' 
irregular  so  that  the  corresponding  part  of  the  paper  is  not  prop- 
erly supported. 

Reduction  by  Burning  Paper. — Certain  precipitates  are 
changed  by  ignition  in  contact  with  organic  matter,  in  such  a 


manne 


GRAVIMETRIC  ANALYSIS  23 


manner  that  the  composition  of  the  resultant  substance  is  un- 
certain. Examples  are  compounds  of  easily  reducible  metals, 
such  as  silver,  platinum,  lead,  etc.  If  silver  chloride  is  heated 
in  contact  with  burning  paper  it  is  partially  reduced  to  metallic 
silver  but  the  amount  of  reduction  is  not  known  in  any  given 
case,  so  that  no  factor  can  be  used  to  obtain  the  weight  of  either 
silver  or  chlorine.  In  such  a  case  the  precipitate  may  be  merely 
dried  without  strong  heating,  or  it  may  be  treated  by  a  process 
such  that  the  altered  portion  will  be  reconverted  into  the  original 
form,  or,  finally,  it  may  be  filtered  on  a  filter  of  some  inorganic 
material  that  is  unaltered  by  high  temperatures.  The  first 
method  of  treatment  is  objectionable  because  it  involves  drying 
and  weighing  the  paper  both  before  and  after  filtration  and  wash- 
ing. The  cellulose  fiber  is  somewhat  soluble  in  almost  any  liquid, 
also  it  is  practically  impossible  to  dry  it  to  the  same  degree  of 
hydration.  This  leaves  the  second  method  of  treatment  (re- 
conversion of  the  changed  precipitate)  as  the  only  desirable 
one,  if  paper  is  to  be  used  as  the  filter.  In  the  case  of  silver  chloride 
the  paper  and  precipitate  are  dried  and  most  of  the  precipitate 
removed  and  preserved.  The  paper  is  then  burned  in  air,  this 
reducing  a  compartively  small  part  of  the  precipitate.  This 
small  amount  of  reduced  silver  is  treated  in  the  crucible 
with  nitric  acid,  which  redissolves  it,  and  with  hydrochloric 
acid,  which  reconverts  the  silver  nitrate  to  silver  chloride. 
The  acids  are  evaporated,  the  remainder  of  the  (unchanged) 
silver  chloride  is  added  and  the  whole  is  heated  and  weighed. 

Reduction  of  Pressure  for  Filtration. — In  most  cases  the  only 
pressure  needed  or  desired  for  causing  the  liquid  to  filter  is  that 
due  to  gravity.  If  the  funnel  is  properly  made  and  the  paper 
fits  well  the  stem  will  fill  with  liquid  and  this  increases  the  speed 
of  filtration  on  account  of  the  length  of  the  column.  Some 
precipitates,  particularly  those  of  a  colloidal  nature,  clog  the  pores 
of  the  filter  and  render  filtration  a  slow  and  tedious  process. 
In  such  cases  it  is  necessary  to  use  some  means  for  diminishing 
the  pressure  beneath  the  liquid.  The  funnel  is  inserted  in  a  rubber 
stopper,  placed  in  the  top  of  a  pressure  flask  or  bell  jar,  to  which 
is  attached  a  suction  pump.  Since  there  is  no  support  for  the 
apex  of  the  filter  paper,  a  supporting  cone  of  platinum  or  strong 
paper  is  placed  in  the  funnel  under  the  paper.  The  cone  need 


24 


QUANTITATIVE  ANALYSIS 


not  be  used  if  the  paper  is  one  that  has  been  partially  parch- 
mentized  or  " hardened"  by  treatment  with  sulphuric  acid. 

Inorganic  Filters.— Many  chemists  prefer  to  use  inorganic 
materials  for  such  precipitates  as  are  reduced  by  contact  with 
burning  paper,  and  for  this  purpose  the  crucible  devised  by 
Gooch1  is  widely  used.  This  piece  of  apparatus  takes  the  place 
of  both  filter  paper  and  crucible,  the  precipitate  being  either 
simply  dried  or  strongly  heated  directly  in  the  filter,  which  has 


FIG.  6. — Filtering  crucible  and  bell  jar. 

previously  been  weighed.  It  consists  of  a  tall  crucible  of 
platinum,  with  a  bottom  having  fine  perforations.  This  is 
placed  in  a  holder  which  can  be  fitted  to  a  flask  or  bell  jar  for 
use  with  diminished  pressure.  Into  the  crucible  is  poured  a 
small  amount  of  finely  shredded  asbestos  fiber  suspended  in 
distilled  water,  the  water  is  drawn  out  and  the  asbestos  sucked 
down  over  the  perforated  bottom  making  a  thin  felt  which  is  an 
admirable  substitute  for  the  usual  paper.  The  crucible  may 

1  Proc.  Am.  Acad.,  Feb.  13,  1878 


GRAVIMETRIC  ANALYSIS  25 

then  be  rinsed  with  alcohol  to  promote  rapid  drying;  it  is  dried 
and  weighed  and  is  then  ready  for  use  as  a  filter.  In  using  the 
Gooch  crucible  it  is  essential  that  the  suction  pump  be  running 
when  any  liquid  is  poured  into  the  crucible  as  otherwise  the 
felt  will  be  stirred  up  and  disintegrated  and  some  of  the  fiber 
will  pass  through  the  perforations.  The  loss  of  asbestos  is  the 
most  frequent  source  of  error  and  even  with  the  greatest  care  it 
occasionally  happens  that  small  amounts  are  lost  by  washing 
through.  The  asbestos  for  this  purpose  should  be  as  nearly  pure 
silicate  as  possible  and  free  from  oxides  of  iron  or  other  metals. 
It  is  first  thoroughly  shredded,  then  is  digested  with  concentrated 
hydrochloric  acid  to  remove  all  soluble  material  and  is  finally 
washed  free  from  hydrochloric  acid  and  soluble  chlorides.  The 
purified  material  is  kept  in  bottles  covered  with  distilled  water, 
ready  for  use.  Asbestos  cannot  be  used  in  any  case  where  the 
solution  to  be  filtered  is  basic  because  it  is  somewhat  soluble  in 
bases.  In  case  strong  ignition  is  required  the  crucible  of  platinum 
is  fitted  with  a  cap  which  covers  the  bottom  portion,  thus  pre- 
venting any  loss  of  asbestos  during  heating. 

The  porcelain  modification  of  the  Gooch  crucible  was  devised 
by  Caldwell.1  It  is  not  well  adapted  to  strong  heating  because 
of  its  liability  to  crack  and  also  because  the  asbestos  felt  curls  up 
and  partly  disintegrates  and  will  inevitably  cause  a  loss  of  the 
fiber. 

A  method  has  been  devised2  for  making  a  platinum  sponge 
of  such  texture  that  it  is  suitable  for  filtering.  This  has,  as  yet, 
not  found  a  very  extended  use.  Experiments  have  also  been 
conducted  with  a  view  to  adapting  porous  material,  such  as 
unglazed  porcelain,  "alundum"  (vitreous  aluminium  oxide), 
etc.,  to  the  purpose  of  quantitative  filters.  Such  materials 
may  be  used  extensively  in  the  future.  They  possess  the  very 
decided  advantage  that  there  is  no  possible  loss  of  loose  material 
such  as  is  liable  to  occur  during  the  use  of  the  ordinary  Gooch 
filter. 

Any  filtrate  obtained  in  the  processes  of  quantitative  analysis 
should  be  received  in  a  beaker  or  other  vessel  which  has*  been 

1  J.  Am.  Chem.  Soc.  13,  105  (1891). 

2  Munroe:  Chem.  News,  68,  101  (1888)  and  Snelling:  J.  Am.  Chem.  Soc., 
31,  456  (1909). 


26  QUANTITATIVE  ANALYSIS 

thoroughly  cleaned.  It  is  often  thought  that  the  nature  of  the 
receiver  is  unimportant  because  the  filtrate  is  to  be  finally  dis- 
carded. It  frequently  happens,  however,  that  a  filter  paper 
breaks,  allowing  the  precipitate  to  escape,  or  the  precipitate  is  so 
fine  as  to  run  through  the  pores,  or  it  is  discovered  that  precipita- 
tion from  the  filtrate  is  not  complete.  In  any  of  these  cases  the 
filtrate  must  be  returned  to  the  original  precipitating  vessel 
and  if  the  receiving  vessel  were  not  clean  the  determination  is 
invalidated. 

Washing. — The  soluble  products  of  reactions  of  precipitation, 
as  well  as  soluble  impurities  originally  present,  must  be  washed 
away  from  the  precipitate  on  the  filter.  If  the  precipitate  itself 
exerted  no  action  upon  the  dissolved  substances  washing  would  be 
comparatively  easy  as  will  be  evident  if  the  process  is  considered 
in  detail. 

If  we  assume  that  the  filtrate  is  allowed  to  drain  away  from 
the  precipitate  until  a  definite  small  quantity  a  remains,  that  the 
wash  water  is  then  added  to  make  a  volume  6,  stirred  up  with  the 
precipitate  and  allowed  to  drain  until  volume  a  again  remains — 
and  that  this  process  of  dilution  and  draining  is  repeated  with 
each  additional  washing,  then  each  addition  of  wash  water  reduces 

the  concentration  of  dissolved  matter  by  the  fraction  r  of  the 

previous  concentration.  If  the  concentration  in  the  original 
mother  liquor  is  represented  by  c  the  first  addition  of  water 

reduces  this  to-^Xc,  the   second    to  T X^Xc  =  (~\  c,  the  nth 

(a\  n 
kj  c.     After  draining  the  last  wash  water  the  quantity  of 

soluble  impurity  remaining  with  the  precipitate  is(^)   ca.    If, 

for  example,  c  =  5  percent  (which  is  greater  than  the  usual  con- 
centration of  dissolved  impurities)  a  =  l  cc  and  6  =  10  cc, 
then  after  one  washing  the  amount  of  impurity  remaining  is 

•j^X 0.05 XI  =0.005    gm;  after    two   washings  the    amount    is 
/  1\  2 
\TO/    X0-05><1  =0.0005  gm;  after  three  washings  there  remains 

0.00005  gm.  The  last  is  a  quantity  that  would  not  be  appreci- 
able to  the  ordinary  analytical  balance. 


GRAVIMETRIC  ANALYSIS 


27 


Interference  by  Adsorption.  —  The  above  method  of  reasoning 
is  not  strictly  valid  because  the  precipitate  itself  exercises  an 
influence  upon  the  solution,  resulting  in  diminished  efficiency 
of  the  washing.  DeVille1  first  demonstrated  the  fact  that 
wherever  a  solution  is  in  contact  with  a  solid,  the  former  is 
slightly  more  concentrated  in  the  region  adjoining  the  surface 
of  contact.  This  difference  in  concentration  is  due  to  a  mutual 
attraction  between  the  molecules  of  solid  and  those  of  solute. 
Where  this  surface  is  merely  the  wall  of  the  containing  vessel 
the  difference  in  concentration  is  not  made  evident  by  any 
ordinary  means  of  measurement  because  the  thickness  of  the  more 
concentrated  layer  is  very  slight  and  the  interior  surface  of  the 
containing  vessel  is  not  large.  A  precipitate,  on  the  other  hand, 
has  a  very  much  larger  surface,  owing  to  the  usual  fine  state  of 
division,  the  same  statement  being  true  with  regard  to  the  filter 
paper.  The  portion  a  of  the  solution  is  therefore  always  one  of 


greater  concentration  than  c  or  vj  nc,  and  the  amount  of  impurity 

remaining  is  greater  than  (r)  ca.     This  action,  known  as  "ad- 

sorption," does  not  greatly  obstruct  the  washing  of  precipitates 
that  are  decidedly  crystalline  in  character  but  causes  much 
trouble  in  the  case  of  amorphous  and  colloidal  precipitates,  pos- 
sibly because  of  the  very  great  surface  possessed  by  these  bodies. 
The  great  surface  exposed  by  hydrosols  is  illustrated  by  the 
figures  of  the  following  table  adapted  from  the  work  of  Wo. 
Ostwald.2  A  cube  having  a  length  of  side  equal  to  1  cm  is 
subdivided  into  smaller  cubes,  with  this  result: 


Length  of  side 

Number    of 
cubes 

Total  surface 

1  0  cm  .  . 

1 

6  sq  cm 

0.1  cm  .  . 

10* 

60  sq  cm 

0.01  cm  

10« 

600  sq  cm 

0.001     cm     (Diam.     of      blood     corpuscles  =  about 
0.007  cm). 
1  .  0  0(0  .  0001  cm;   Diam.  of  small  coccus)  

0.1  ft  ... 

10* 

101' 
101* 

6,000  sq  cm 

6  sq  meters 
60  sq  meters 

0  .01  ft  (limit  of  ultramicroscopic  -visibility)  
1.0  ttft  (  =  0.000001  mm;  Diam.  of    smallest  colloid 
particles) 
0  .  1  nfi  (Diam.  of  elementary  molecules)  .  . 

10" 

10" 

10*« 

600  sq  meters 
6,000  sq  meters 

60,000    sq  meters 

1  Ann.  chim.  phys.,  38,  5  (1853). 
*  Grundriss  der  Kolloidchemie,  85. 


28 


QUANTITATIVE  ANALYSIS 


While  colloidal  particles  are  not  to  be  regarded  as  cubes,  their 
surface  would  vary  with  continued  subdivision  in  the  same  way. 
At  the  moment  of  precipitation  a  substance  having  a  surface, 
relatively  so  enormous,  may  show  the  effect  of  adsorption  to  a 
marked  degree,  much  of  soluble  salts  being  carried  out  of  the 
solution.  Flocculation  no  doubt  diminishes  the  surface  consid- 
erably but  the  flocculated  colloid  still  possesses  a  much  greater 
surface  than  the  same  weight  of  a  crystalline  solid  may  have. 
The  hydroxides  of  iron,  aluminium  and  chromium  retain  dis- 
solved salts  or  bases  with  great  tenacity  and  are  extremely  diffi- 
cult of  purification  by  washing. 
The  number  of  washings  neces- 
sary for  the  satisfactory  purifica- 
tion of  a  given  precipitate  will 
depend  upon  the  nature  of  both 
precipitate  and  dissolved  sub- 
stance and  must  be  learned  by 
.  experience.  A  safe  plan  to  fol- 
low is  that  of  testing  the  wash- 
ings until  they  are  found  to  be 
practically  free  from  the  sub- 
stance in  question.  In  deciding 
for  what  substance  the  test  shall 
be  made  in  the  washings  one 
must  be  guided  by  the  reactions 
that  are  known  to  take  place 

no.  7,-Wash  bottle  for  distilled  water,  during  precipitation   and   by  a 

knowledge  of  what  other  sub- 
stances may  have  been  present  with  the  element  or  radical 
being  precipitated.  This  will  be  dealt  with  in  each  specific  case 
in  the  exercises  that  follow. 

Wash  Bottles. — The  simplest  apparatus  for  use  in  washing  pre- 
cipitates is  shown  in  Fig.  7.  It  consists  of  an  ordinary  flask 
of  convenient  size  fitted  with  tubes  and  rubber  stopper  as  shown. 
The  tube  (a)  may  be  continuous,  but  the  flexibility  produced  by  a 
rubber  connection  is  advantageous  in  directing  the  stream  of 
water.  For  use  with  hot  water  the  neck  of  the  flask  should  be 
wrapped  with  cork,  paper,  or  heavy  twine  for  the  protection  of 
the  hands.  The  usual  equipment  includes  at  least  two  wash 


GRAVIMETRIC  ANALYSIS 


29 


bottles,  one  being  for  hot  water  and  one  for  cold  water.  A  mis- 
take that  is  often  made  is  that  of  allowing  the  hot  water  bottle 
to  remain  over  a  flame  or  hot  plate  when  not  in  use,  keeping  the 
water  boiling  meanwhile.  Boiling  promotes  solution  of  the  glass 
of  the  flask  so  that  the  water  may  become  unfit  for  use.  The 
nozzle  (b)  of  the  wash  bottle  may  be  made  in  either  of  two  ways. 
A  glass  tube  may  be  drawn  out  until  a  capillary  tube  is  produced 
and  then  cut  off  where  the  bore  is  such  as  to  give  a  fine  stream 
of  proper  dimensions.  The  edges  are  then  rounded.  A  better 
method  is  to  cut  off  a  piece  of  tubing  and  fuse  one  end  until 
it  has  contracted  to  the  proper  diameter.  This 
tube  possesses  the  advantage  that  it  is  not  easily 
broken  by  contact  with  the  funnel. 

For  use  with  organic  solvents  that  dissolve 
rubber  a  wash  bottle  is  used,  having  a  ground 
glass  stopper  instead  of  a  rubber  one  and  the 
delivery  tube  is  of  one  piece,  omitting  the 
rubber  connection. 

A  wash  bottle  of  any  design  should  be  so  con- 
structed as  to  furnish  a  very  fine  stream  of  the 
wash  liquid.  The  stream  is  directed  against  the 
upper  part  of  the  paper  in  such  a  way  as  to 
wash  thoroughly  both  paper  and  precipitate. 
It  is  never  directed  against  the  funnel  above  the 
paper  as  the  precipitate  will  almost  invariably  creep  up  the  glass. 

Drying  of  Precipitates. — Unless  the  precipitate  is  to  be  removed 
from  the  paper  it  is  generally  unnecessary  to  dry  it  completely 
before  placing  in  the  crucible  for  ignition.  It  should  be  allowed 
to  drain  thoroughly  before  removing  from  the  funnel,  after  which 
the  paper  may  be  folded  and  placed  directly  in  the  crucible. 
It  sometimes  happens  that  the  precipitate  is  reduced  or  otherwise 
affected  by  contact  with  carbon  or  reducing  gases  from  the  burn- 
ing paper;  such  precipitates  must  be  largely  removed  before 
burning  the  paper  and  this  involves  previous  drying  in  order  to 
prevent  the  sticking  of  the  precipitate  to  other  materials  with 
which  it  may  come  in  contact. 

Ovens. — A  glance  at  the  pages  of  the  catalogues  of  dealers 
in  chemical  apparatus  will  impress  one  with  the  fact  that  there 
are  available  many  types  of  ovens  for  such  purposes.  These 


FIG.  8. — Incor- 
rect (a)  and  cor- 
rect (b)  forms  for 
nozzles. 


30 


QUANTITATIVE  ANALYSIS 


types  need  not  be  described  here.  It  is  sufficient  to  notice  that 
the  oven  must  possess  at  least  two  features:  circulation  of  air 
through  the  drying  chamber  in  order  to  remove  the  water  vapor 
as  it  is  formed,  and  a  fairly  accurate  means  of  controlling  the 
temperature.  Electrically  heated  ovens  are  more  convenient 
than  those  heated  by  gas  and,  considering  the  length  of  life, 
are  probably  not  more  expensive.  There  are  cases  where  the 
precipitate  is  affected  by  oxygen  or  carbon  dioxide.  Such 
a  precipitate  must  be  dried  in  an  atmosphere  of  some  gas,  such 
as  hydrogen  or  nitrogen,  toward  which  it  is  chemically  inert,  and 
the  oven  must  be  provided  with  means  for  passing  a  current  of  gas 
through  it. 


FIG.  9. — Wash  bottle  for  organic  solvents. 

Desiccators. — In  addition  to  devices  for  drying  at  elevated 
temperatures  we  have  also  those  for  drying  at  the  ordinary 
temperature  of  the  room.  Substances  that  are  not  definitely 
and  decidedly  hygroscopic  will  lose  most  of  their  moisture  by 
simple  exposure  to  the  air  but  this  is  obviously  an  inconvenient 
procedure  and  involves  much  loss  of  time.  Evaporation  of 
moisture  can  be  hastened  in  one  or  both  of  two  ways  without 


GRAVIMETRIC  ANALYSIS 


31 


raising  the  temperature:  (a)  by  enclosing  the  moist  substance 
in  a  vessel  which  also  contains  a  strongly  hygroscopic  material 
and  (b)  by  keeping  the  atmosphere  which  surrounds  the  material 
practically  free  from  water  vapor  by  mechanical  means,  such 
as  exhausting  by  means  of  an  air  pump  or  by  passing  a  dry  gas 
through  the  vessel.  The  vessel  known  as  a  " desiccator"  is 
of  such  form  that  the  drying  agent,  such  a  sulphuric  acid  or 
calcium  chloride,  may  be  contained  in  the  lower  portion,  while 
the  substance  to  be  dried  is  placed  above.  A  desiccator  for 
drying  under  reduced  pressure  is  shown  in  Fig.  10. 


FIG.   10. — Desiccator  for  drying  under  reduced  pressure. 

In  order  to  understand  the  action  of  the  various  forms  of 
desiccators  it  is  necessary  to  recall  the  physical  law  that  any 
moist  substance,  if  confined  in  a  vessel  at  a  given  temperature 
will  continue  to  lose  moisture  until  a  definite  pressure  of  water 
vapor  is  established,  when  equilibrium  between  liquid  and  vapor 
phases  is  accomplished.  If  the  pressure  of  the  vapor  is  reduced 
by  extraneous  means  evaporation  begins  in  an  attempt  to  re- 
establish equilibrium  and  so  long  as  the  vapor  pressure  is  kept 


32 


QUANTITATIVE  ANALYSIS 


reduced  evaporation  continues.  It  is  important  to  note,  however, 
that  the  vapor  pressure  to  be  considered  is  not  the  total  pressure 
(such  as  that  of  the  atmosphere)  but  is  the  partial  pressure  of  the 
vapor  of  water.  The  same  result  is  therefore  finally  accomplished 
by  pumping  out  the  mixture  of  air  and  water  vapor,  by  simply  dis- 
placing this  mixture  by  means  of  any  gas  that  has  been  freed  from 
water  vapor  or  by  having  present  and  in  contact  with  the  confined 
atmosphere  any  substance  that  readily  absorbs  moisture.  The 
simplest  desiccators  involve  no  principle  other  than  that  of  con- 
finement with  a  drying  agent.  A  small  desiccator  of  this  descrip- 
tion like  Fig.  11  is  used  by  the  analyst,  not  for  drying  precipitates 


FIG.  11. — Ordinary  desiccator. 

but  for  keeping  crucibles,  precipitates  and  small  amounts  of  mate- 
rials in  a  dry  atmosphere,  previous  to  weighing.  In  such  cases 
the  materials  have  already  been  dried  and  the  only  function  of 
the  desiccator  is  to  prevent  the  absorption  of  moisture. 

The  desiccator  is  prepared  for  use  by  partly  filling  the  lower 
chamber  with  the  proper  drying  agent,  a  triangle  or  perforated 
porcelain  plate  for  supporting  small  objects  being  then  placed 
upon  the  shoulder  above.  The  ground-glass  joint  of  the  cover 
is  lightly  smeared  with  vaseline  to  make  it  impervious  to  air. 
If  calcium  chloride  is  used  as  the  drying  agent  a  small  piece  of 


GRAVIMETRIC  ANALYSIS  33 

sodium  hydroxide  may  be  added  to  keep  the  atmosphere  free 
from  carbon  dioxide. 

Drying  Agents. — The  drying  agents  commonly  used  in  desic- 
cators and  for  drying  gases  are  sulphuric  acid,  calcium  chloride 
and  phosphorus  pentoxide.  The  efficiency  of  any  drying  agent 
will  depend  upon  the  pressure  of  water  vapor  that  is  maintained 
when  equilibrium  between  the  agent  and  the  surrounding  atmos- 
phere is  established.  Every  definite  chemical  substance  which 
combines  with  water  maintains  a  definite  tension  of  water  vapor 
at  a  definite  temperature.  If  this  tension  is  large  the  compound 
is  a  poor  drying  agent.  If  as  large  or  larger  than  that  of  the 
substance  to  be  dried  it  does  not  act  at  all  or  even  adds  moisture 
to  the  latter.  If  the  aqueous  tension  is  exceedingly  small  the 
substance  is  a  good  drying  agent.  The  rapidity  of  action  depends 
also  upon  the  relative  surface  exposed  to  the  atmosphere.  Thus 
a  granular  or  porous  solid  will  absorb  moisture  more  rapidly 
than  a  liquid,  its  aqueous  tension  being  the  same. 

Phosphorus  pentoxide  is  the  most  hygroscopic  of  the  three 
substances  mentioned  above  and  is  therefore  the  most  efficient 
drying  agent.  In  addition  to  its  greater  cost  its  use  is  also  limited 
by  the  fact  that  it  becomes  viscous  when  moist  and  that  a  large 
amount  of  heat  is  evolved  when  it  combines  with  moisture. 
Sulphuric  acid  ranks  next  to  phosphorus  pentoxide  in  efficiency 
but  it  is  not  much  used  in  desiccators  that  are  to  be  carried  about 
the  laboratory  because  of  its  tendency  to  splash  against  crucibles 
or  other  articles  carried  in  the  desiccator.  Calcium  chloride, 
although  the  least  efficient  of  all,  is  the  most  convenient  for  many 
purposes  and  is  generally  used  for  desiccator's  and  for  the  drying  of 
gases  in  many  analytical  processes. 

Ignition  of  Precipitates. — The  term  " ignition"  is  used  in  this 
connection  in  a  sense  somewhat  beyond  its  ordinarily  accepted 
meaning,  since  it  is  applied  to  the  heating  to  high  temperatures  of 
substances  that  are  entirely  incombustible.  The  purposes  of 
ignition  are  to  destroy  the  filter,  if  paper  has  been  used,  to  expel 
the  last  traces  of  moisture  and  volatile  impurities  that  have  not 
been  removed  by  washing  and  to  cause  the  precipitate  to  change 
in  a  definite  manner,  if  a  change  is  to  be  made.  If  a  paper  filter 
has  been  used  it  is  carefully  removed  from  the  funnel  by  slipping 
up  the  side.  It  is  then  folded  as  indicated  in  Fig.  12,  the  object 


34 


QUANTITATIVE  ANALYSIS 


being  so  to  enclose  the  precipitate  that  loss  is  impossible.  If 
it  is  to  be  dried  and  removed  it  is  then  placed  in  the  oven  on  a 
cover  glass. 

Reducible  Precipitates.— The  method  of  treating  a  precipi- 
tate that  is  affected  by  burning  paper  is  as  follows:  After  drying 
completely  the  paper  and  cover  glass  are  placed  on  a  sheet  of 


FIG.  12. — Folding  a  filter  paper  for  ignition.          » 

glazed  paper.  This  paper  is  to  prevent  possible  loss  of  traces 
of  the  precipitate  during  removal  from  the  filter  and  should  be 
black  if  the  precipitate  is  white,  or  white  if  the  precipitate  is 
dark  in  color.  The  paper  is  carefully  opened  and,  by  use  of  a 
spatula  of  horn,  steel  or  platinum,  the  precipitate  is  carefully 
removed  and  placed  in  the  cover  glass.  In  doing  this  the  pre- 


FIG.   13. — Paper  held  by  platinum  wire  for  ignition. 

cipitate  must  be  removed  as  completely  as  possible  but  the  paper 
must  not  be  scraped,  as  otherwise  fiber  will  be  removed  with  the 
precipitate.  The  paper  is  now  re-folded  and  placed  in  a  crucible 
which  has  been  ignited  and  weighed,  where  it  is  carefully  burned. 
The  objectionable  action  of  the  paper,  whereby  some  of  the  pre- 
cipitate is  changed,  is  usually  that  of  reduction.  It  is  obvious 


GRAVIMETRIC  ANALYSIS  35 

that,  if  the  small  amount  of  precipitate  remaining  on  the  paper  is 
to  escape  this  action  to  some  extent,  the  burning  of  the  paper 
must  be  performed  under  conditions  favorable  to  vigorous  oxida- 
tion. Burning  in  an  atmosphere  of  pure  oxygen  would  seem  to  be 
the  best  remedy  but  this  is  usually  impracticable.  Slow  combus- 
tion, at  as  low  a  temperature  as  will  support  combustion  and  with 
an  excess  of  air,  is  easily  carried  out.  One  method  is  that  of 
burning  on  a  platinum  wire.  The  paper  is  tightly  rolled  and  held 
by  a  heavy  platinum  wire  over  the  crucible  which  is  placed  on  the 


FIG.  14. — Crucible  inclined  for  accelerating  combustion. 

glazed  paper.  The  paper  is  fired  by  touching  with  the  outer  edge 
of  the  burner  flame  and  is  allowed  to  burn  slowly,  the  ash  drop- 
ping into  the  crucible.  After  treating  the  partially  changed  pre- 
cipitate to  convert  it  into  the  original  condition  the  main  portion 
is  brushed  in,  using  a  small  pencil  brush  of  camel's  hair,  and  the 
whole  is  heated  to  the  desired  temperature,  then  cooled  in  the 
desiccator  and  weighed.  If  burning  on  the  wire  is  considered 
unnecessary  the  paper  is  placed  directly  in  the  crucible  and  ig- 
nited by  placing  the  burner  under  it.  The  proper  method  of 


36  QUANTITATIVE  ANALYSIS 

heating  crucibles  in  order  to  oxidize  the  contents  is  described 
below.  The  brush  already  mentioned  should  be  free  from  loose 
hairs  and  should  be  rounded  on  the  end.  In  using  it  is  drawn 
sidewise  in  such  a  manner  that  very  little  of  the  precipitate 
enters  the  brush  itself. 

Oxidation  in  the  Crucible.— The  crucible  is  almost  invariably 
heated  by  means  of  a  naked  flame,  being  supported  on  a  tri- 
angle by  means  of  some  kind  of  stand.  When  the  object  is  to 
oxidize  the  paper  or  precipitate  the  crucible  is  placed  on  its  side 
and  the  cover  leaned  against  it  as  shown  in  Fig.  14.  The  burner 


FIG.  15. — Correct  position  FIG.  16. — Incorrect  position 

of  inclined  crucible.  of  inclined  crucible. 

is  placed  under  the  bottom  of  the  crucible  in  such  a  position  that 
the  gaseous  products  of  the  burner  cannot  enter  the  crucible. 
The  uprising  current  of  warm  air  strikes  the  cover  and  is  deflected 
into  the  crucible,  thus  providing  an  oxidizing  atmosphere  about 
the  paper.  If  the  flame  from  the  burner  is  applied  only  enough 
to  keep  the  paper  burning  the  desired  condition  is  attained. 
No  harm  results  if  the  volatilized  combustible  material  from  the 
paper  burns  with  a  flame  above  the  crucible.  After  the  paper 
is  thoroughly  charred  the  temperature  is  gradually  raised  to 
complete  the  combustion. 

The  proper  position  of  the  crucible  on  the  triangle  is  shown 
in  Fig.  15.  If  placed  as  in  Fig.  16  the  crucible  is  liable  to  fall 
back  and  it  may  even  sometimes  fall  through  and  cause  a  loss 
of  the  determination. 

Even  in  cases  where  the  burning  paper  has  no  reducing  action 
upon  the  precipitate  it  is  still  desirable  to  complete  the  com- 
bustion of  the  paper  at  a  comparatively  low  temperature.  Crys- 
talline precipitates  that  are  ordinarily  regarded  as  infusible  will 
often  undergo  softening  at  the  sharp  corners  of  the  crystals. 


GRAVIMETRIC  ANALYSIS  37 

This  causes  a  certain  sticking  together  which  results  in  the  enclo- 
sure of  a  small  amount  of  carbon  in  such  a  way  as  to  make  its 
oxidation  extremely  difficult.  If  the  paper  containing  the 
precipitate  is  heated  to  a  high  temperature  at  the  very  beginning 
it  is  often  almost  impossible  to  make  it  white.  One  of  the  best 
examples  of  this  action  is  in  the  ignition  of  magnesium  ammo- 
nium phosphate  to  convert  it  into  magnesium  pyrophosphate. 
Premature  heating  of  this  substance  to  very  high  temperatures 
will  frequently  result  in  a  black  or  gray  material  that  cannot  be 
whitened  by  long  ignition. 

Decomposition  in  the  Crucible. — After  oxidation  of  the  paper 
is  completed  the  temperature  is  raised  in  order  to  volatilize 
completely  any  volatile  impurities  that  may  remain  and  to  cause 
whatever  decomposition  is  desired.  Since  oxidation  is  no  longer 
an  object  the  crucible  is  placed  in  an  upright  position  and  the 
cover  is  placed  over  the  top.  This  gives  an  opportunity  for  the 
flame  to  bear  directly  on  the  bottom  of  the  crucible  where  the 
precipitate  lies.  The  cover  also  largely  prevents  loss  of  heat 
due  to  convection  currents  of  air  within  the  crucible. 

Porcelain  Crucibles. — The  most  commonly  used  crucibles  are 
made  of  a  high  grade  of  porcelain.  Such  a  crucible  will  withstand 
temperatures  as  high  as  can  be  attained  by  use  of  the  ordinary  air 
and  gas  blast  lamp  without  more  than  a  trifling  loss  of  weight  and 
they  possess  the  decided  advantage  of  low  cost.  They  cannot 
be  used  for  fusions  because  most  fluxes,  particularly  those  of  a  ba- 
sic nature,  attack  the  glaze  as  well  as  the  porcelain  itself.  It  is 
because  of  this  susceptibility  to  the  action  of  fusible  salts  which 
act  as  fluxes  that  the  life  of  a  porcelain  crucible  is  limited.  In 
spite  of  the  most  careful  washing  there  will  always  remain  with 
the  precipitate  traces  of  fusible  materials  and  these  will,  in  time, 
cause  destruction  of  the  glaze  lining  of  the  crucible.  After  the 
crucible  is  thus  roughened  it  is  difficult  to  clean  it  and  it  becomes 
unsuitable  for  further  use. 

Porcelain  for  Chemical  Uses. — The  cutting  off  of  imports 
by  the  war  left  America  in  a  situation  with  regard  to  chemical 
porcelain,  similar  to  that  which  existed  in  the  case  of  glassware. 
This  country  had  depended  very  largely  upon  foreign  porcelain 
for  chemical  uses  and  German  and  Austrian  ware  had  practically 
monopolized  the  field.  American  industry  has  since  developed 


38  QUANTITATIVE  ANALYSIS 

an  excellent  porcelain  and  the  Bureau  of  Standards  has  made 
comparative  tests1  of  two  American  makes  (Coors  and  Guernsey) 
one  Japanese  ware,  marked  "S.  C.  P."  and  two  German  wares 
(Royal  Berlin  and  Bavarian).  The  tests  included  resistance 
to  sudden  heating  and  cooling,  also  treatment  with  a  number  of 
acids  and  bases,  in  solution  and  fused.  It  was  concluded  that 
the  Japanese  porcelain  was  fully  equal  to  the  Royal  Berlin  in 
every  respect.  American  ware  suffered  in  the  heat  tests, 
although  it  was  stated  that  later  samples  had  withstood  the 
test  satisfactorily. 

Platinum  Crucibles. — Crucibles  of  platinum  are  very  desirable 
for  ignition  and  are  almost  essential  for  fusions.  Platinum  fuses 
at  1770°  and  does  not  soften  enough  to  preclude  its  use  much 
below  this  temperature.  It  resists  the  action  of  all  single  acids 
if  these  are  pure.  It  is  readily  dissolved  by  solutions  of  chlorine 
and,  on  this  account,  by  aqua  regia.  Even  "chemically  pure" 
hydrochloric  acid  often  contains  traces  of  chlorine  and  will 
slightly  attack  platinum.  Platinum  easily  alloys  with  most 
metals  and  so  should  not  be  heated  in  contact  with  compounds  of 
easily  reducible  metals,  particularly  if  carbon  be  present.  When 
heated  for  a  long  time  in  contact  with  carbon  it  slowly  dissolves 
this  and  becomes  brittle  because  of  the  presence  of  the  carbide 
of  platinum.  This  is  noticed  when  the  crucible  is  heated  in  a 
reducing  flame  and  on  this  account  it  is  necessary  carefully  to 
adjust  the  flame  so  that  the  tip  of  the  inner  cone  is  below  (not 
against)  the  bottom  of  the  crucible.  A. flame  showing  yellow 
must  never  be  used.  Platinum  crucibles  cannot  be  heated  in 
contact  with  alkali  hydroxides  although  they  are  not  attacked 
by  alkali  carbonates.  Compounds  of  phosphorus  are  reduced 
to  some  extent  by  heating  with  carbon  and  the  phosphorus  readily 
combines  with  platinum,  causing  destruction  of  the  crucible. 
Ignition  of  phosphates  thus  requires  especial  care  if  this  action 
is  to  be  prevented. 

Formerly  platinum  crucibles  and  dishes  contained  a  small 
percentage  of  iridium,  added  for  the  purpose  of  hardening  the 
metal  and  giving  greater  resistance  to  mechanical  wear.  Such 
an  alloy,  however,  is  more  readily  attacked  by  reagents  and  it 
is  more  volatile  and  practically  pure  platinum,  is  now  used  almost 
1  Bur.  Stand.  Tech.  Paper,  105. 


GRAVIMETRIC  ANALYSIS  39 

exclusively.  It  was  also  formerly  the  custom  to  form  crucibles 
and  dishes  by  spinning  the  metal.  This  gives  an  article  with  a 
fine  surface  as  apparent  to  the  naked  eye  but  it  results  in  the 
formation  of  minute  surface  cracks  and  scales  and  disintegration 
is  aided  by  this  process.  The  crucible  of  the  present  day  is 
given  its  surface  by  hammering.  This  results  in  a  more  uneven 
surface  but  the  particles  of  metal  are  firmly  welded  together 
and  the  hammered  crucible  has  a  life  appreciably  longer  than 
that  of  the  spun  crucible. 

Very  great  differences  are  observed  in  the  platinum  ware  as 
furnished  by  different  refiners  and  manufacturers  and  many 
troubles  are  experienced  in  the  use  of  such  ware  for  accurate 
work.  The  continued  advance  in  the  cost  of  platinum  has  made 
it  correspondingly  difficult  to  obtain  satisfactory  ware  and  a  com- 
mittee was  appointed  by  the  American  Chemical  Society  to 
investigate  the  subject  and  to  make  recommendations  to  the 
Society.  Two  reports  of  this  committee  have  been  made.1 
An  investigation  of  the  quality  of  platinum  ware  has  been  made 
also  by  the  Bureau  of  Standards.2 

In  these  reports  the  objections  to  inferior  platinum  ware  are 
summarized  as  follows: 

1.  Undue  loss  of  weight  on  ignition. 

2.  Undue  loss  of  weight  on  acid  treatment,  especially  after  strong  ignition. 

3.  Unsightly  appearance  of  the  surface  of  the  ware  after  strong  ignition, 
especially  in  the  early  stages  of  heating. 

4.  Adherence  of  dishes  and  crucibles  to  triangles. 

5.  Basicity  of  the  surface  of  the  ware  after  strong  ignition. 

6.  Blistering. 

7.  Development  of  cracks  after  continued  heating. 

So  far  as  these  matters  have  been  studied  the  causes  are  stated 
as  follows: 

Loss  on  Ignition  is  chiefly  due  to  the  presence  of  iridium  which 
is  added  to  stiffen  and  harden  the  ware.  The  loss  at  1200° 
is  more  than  twice  as  great  as  at  1100°  for  nearly  all  ware  used 
in  the  experiments. 

Appearance  of  the  Surface  after  Ignition. — Even  with  good  ware 
the  surface  is  frosted  after  strong  ignition.  This  is  due  to  sur- 

1  J.  Ind.  Eng.  Chem.,  3,  686  (1911),  Ibid.,  6,  512  (1914). 

2  Bur.  Stand.  Tech.  Paper,  254. 


40  QUANTITATIVE  ANALYSIS 

face  crystallization,  but  the  crystallization  should  be  fine  and 
evenly  distributed.  Poor  ware  becomes,  when  ignited,  unevenly 
coated  with  a  whitish  layer  and  with  brown  stains,  the  latter 
being  due  to  small  amounts  of  iron.  Sometimes  the  entire  inner 
surface  becomes  stained  brown  from  the  latter  cause. 

Adherence  of  Crucibles  and  Dishes  to  Triangles  is  caused  by 
welding  of  the  platinum  of  the  ware  with  the  triangle.  This 
cannot  well  be  overcome  but  is  not  serious  at  ordinary  blast-lamp 
temperatures  unless  platinum  triangles  are  used. 

Basicity  of  the  Surface  after  Ignition  is  due  to  the  presence  of 
traces  of  calcium  alloyed  with  the  platinum.  Calcium  oxide  is 
produced  when  the  crucible  or  dish  is  heated  and  this  is  made 
evident  by  testing  with  moist  red  litmus  paper. 

Blistering  is  found  to  occur  appreciably  only  witji  ware  of 
earlier  manufacture  and  is  not  now  an  important  objection. 

Cracking. — No  cause  has  been  determined  for  this  form  of 
deterioration. 

Specifications. — It  is  recommended  that  purchasers  specify 
that  platinum  ware  must  show  no  marked  uneven  discoloration 
on  heating,  must  give  no  test  for  iron  after  heating  for  two 
hours  and  that  the  rate  of  loss  per  hour  at  1100°  over  a  period 
of  four  hours  shall  not  exceed  0.2  mg  and  that  5  percent  of 
rhodium  be  substituted  for  iridium  as  a  hardening  agent. 

While  platinum  possesses  properties  that  make  it  an  extremely 
valuable  metal  for  the  chemist,  its  use  is  greatly  limited  by  its 
present  high  cost.  Because  of  this  fact,  available  substitutes 
are  always  in  demand. 

Care  of  Platinum. — Platinum  ware  will  deteriorate  rapidly 
unless  precautions  are  taken  in  its  use  and  care. 

1.  Handle  carefully  to  avoid  bending.     Use   clean   crucible 
tongs  and  do  not  allow  the  tongs  to  come  into  contact  with  fused 
materials  within  the  crucibles  or  dishes. 

2.  For  cleaning  apply  the  appropriate  solvent,  according  to 
the  nature  of  the  material  to  be  removed.     Chromic  acid  is 
suitable  for  removing  organic  matter,   hydrochloric  or  nitric 
acids  for  insoluble  carbonates  or  metallic  oxides  and  fusing  with 
sodium  carbonate  for  silica  or  silicates,  or  with  sodium  pyro- 
sulphate  for  such  metals  or  metallic  oxides  as  resist  the  action 
of  acids. 


GRAVIMETRIC  ANALYSIS  41 

3.  Do  not  heat  platinum  in  contact  with  the  inner  cone  of  the 
laboratory  burner,  as  brittleness  results  from  such  exposure. 

4.  Do  not  heat   compounds  of  lead,   tin,   bismuth,   arsenic, 
antimony  or  zinc  in  contact  with  platinum.     Reduction  may 
occur,  the  reduced  metal  alloying  with  the  platinum. 

5.  Do  not  attempt  to  remove  fusions  from  platinum  crucibles 
or  dishes  by  means  of  files,  glass  rods  or  other  hard  tools.     Use  a 
rubber-tipped  rod  or  solvents. 

6.  Dull  surfaces  should  be  polished  lightly  with  wet  emery 
slime  or  fine  carborundum. 

Platinum  Substitutes. — The  increasing  scarcity  of  platinum 
has  made  the  introduction  of  substitutes  a  practical  necessity. 
While  it  is  true  that  pure  platinum  possesses  certain  properties 
that  cannot  be  duplicated  by  any  other  metal  or  alloy,  yet  certain 
alloys  have  been  found  to  be  suitable  for  making  into  crucibles 
and  dishes  that  will  serve  for  many  of  the  operations  of 
the  analytical  laboratory,  in  place  of  the  platinum  that  has  been 
in  use.  Two  of  these  will  be  mentioned. 

Palau. — This  is  a  trade  name  for  an  alloy  containing  about 
80  percent  gold  and  20  percent  palladium.  The  alloy  is  some- 
what darker  in  color  than  platinum  but  resembles  it  otherwise. 
The  melting  point  is  1370°.  This  is  higher  than  the  melting 
point  of  gold  but  it  is  400°  lower  than  that  of  platinum. 

The  Bureau  of  Standards  found  that  a  crucible  of  this  material 
was  comparatively  free  from  iron  and  that  its  loss  on  heating 
to  1200°  was  less  than  that  of  a  platinum  crucible  containing 
2.4  percent  of  iridium.  The  resistance  to  acids,  sodium  hydrox- 
ide and  ferric  chloride  solutions  and  to  fused  sodium  carbonate 
is  comparable  with  that  of  platinum.  The  ware  is  therefore 
suitable  for  fusions  with  sodium  carbonate  but  it  is  decidedly 
attacked  by  fused  pyrosulphates.  Crucibles  of  palau  could 
not  be  used  for  work  at  high  temperatures  on  account  of  its 
relatively  low  melting  point. 

Rhotanium. — Fahrenwald1  described  a  series  of  six  gold- 
palladium  alloys,  some  of  which  compare  very  favorably  with 
platinum  in  many  of  the  essential  properties.  Unfortunately 
the  percentage  composition  of  these  alloys  is  not  definitely 
stated  but  it  is  to  be  inferred  that  gold  ranges  from  60  to  90 

1  J.  Ind.  Eng.  Chem .,  9,  590  (1917). 


42  QUANTITATIVE  ANALYSIS 

percent  and  that  rhodium  is  contained  in  some  members  of  the 
series.  The  melting  points  range  from  1150°  to  1450°  for  the 
series.  Presumably  the  alloy  having  the  highest  melting  point 
contains  the  highest  proportion  of  palladium  but  increasing 
palladium  also  increases  the  rate  of  attack  by  some  reagents. 
It  would  appear  that  rhotanium  of  properly  chosen  composition 
might  well  replace  platinum  for  most  analytical  purposes, 
excluding  processes  where  hot  concentrated  nitric  acid  is  used. 
The  alloys  cannot  be  used  as  anodes  in  electroanalysis. 

Gold  shows  about  the  same  resistance  to  the  action  of  reagents 
as  does  platinum,  but  its  relatively  low  melting-point  (1035°) 
makes  it  unsuitable  for  crucibles  or  other  articles  that  must  be 
strongly  heated. 

Silica. — Crucibles  have  recently  been  made  of  fused  silica. 
This  resists  the  action  of  chemicals  better  than  does  glass  and 
the  melting-point  is  such  that  no  ordinary  air-gas  flame  will  fuse 
the  crucible.  Pure  quartz  fuses  at  1600°  and  amorphous  silica 
at  1750°-!  780°.  Moreover  the  coefficient  of  expansion  is  very 
small  (5.4X10~7  at  temperatures  between  0°  and  1000°)  so  that 
sudden  heating  or  cooling  does  not  cause  cracking.  Silica  cru- 
cibles have  not  yet  taken  the  place  of  those  of  porcelain,  largely 
because  of  their  higher  cost. 

Alundum. — Recently  a  highly  refractory  form  of  aluminium 
oxide  has  been  utilized  for  crucibles.  This  is  commercially 
known  as  "Alundum."  Alundum  does  not  fuse  below  2050°, 
does  not  much  soften  at  1775°  and  its  coefficient  of  expansion  is 
7.8  X10-6.1 

Triangles. — The  discussion  concerning  the  relative  merits  of 
various  materials  used  for  crucibles  will  apply  equally  well  to 
those  used  for  supporting  triangles.  Porcelain  is  the  cheapest 
serviceable  material  and  will  do  for  supporting  crucibles  of  any 
other  material  as  well  as  those  of  porcelain  itself.  The  familiar 
"pipe  stem"  triangle  is  constructed  of  three  tubes  of  refractory, 
unglazed  porcelain,  held  together  by  a  frame  of  iron  wire  (Fig. 
17).  The  chief  objection  to  this  form  lies  in  the  fact  that  the 
relatively  large  tubes,  lying  on  three  sides  of  the  crucible,  obstruct 
the  flame  and  cause  a  very  noticeable  decrease  in  efficiency.  An 
improved  form  is  shown  in  Fig.  18.  The  projections  serve  to  sup- 

1  Saunders:  Trans.  Am.  Electrochem.  Soc.,  19,  333  (1911). 


GRAVIMETRIC  ANALYSIS  43 

port  the  crucible  and  allow  much  better  contact  with  the  flame. 
Any  porcelain  triangle  becomes  practically  useless  if  cracked 
because  the  supporting  iron  frame  is  thus  exposed  to  the  flame 
and  soon  oxidizes  and  breaks. 


FIG.  17. — Common  "pipe  stem"  triangle. 

Platinum  triangles  offer  the  great  advantage  of  long  life  and 
they  also  obstruct  the  flame  very  little  on  account  of  the  small 
size  of  the  framework.  Here  again  the  high  cost  of  platinum  pre- 
cludes its  extensive  use.  A  common  form  of  platinum  triangle 


FIG.  18. — Triangle  with  projections. 

is  shown  in' Fig.  19.  A  less  expensive  form  can  be  constructed 
by  stretching  heavy  platinum  wire  on  an  iron  framework,  like 
Fig.  20  or  Fig.  21.  This  is  not  very  satisfactory  because  the 
weight  of  the  crucible  causes  lengthening  and  sagging  of  the  wire 


44  QUANTITATIVE  ANALYSIS 

at  high  temperatures.  The  screws  shown  in  Fig.  21  can  be  used  in 
taking  up  this  slack  but  the  wire  eventually  weakens  and  breaks. 
Alloys  of  nickel  and  chromium  having  high  fusing  points  and 
considerable  resistance  to  oxidation  at  high  temperatures  have 
been  adapted  to  the  construction  of  chemists'  triangles.  The 


Fio.  19. — Heavy  platinum  triangle. 

proportion  of  the  two  elements  may  be  varied  somewhat  but 
the  presence  of  iron,  even  in  small  amounts,  makes  the  alloy 
oxidizable  in  the  flame  and  the  resulting  oxide  combines  with  the 
silicious  glaze  of  porcelain  crucibles,  thus  changing  the  weights 
of  the  latter.  As  it  is  becoming  increasingly  difficult  to  obtain 


Fio.  20. — Triangle  of  platinum  wire. 

nickel-chromium  triangles  that  are  free  from  iron  it  is  likely 
that  their  use  must  soon  be  restricted  to  qualitative  work  or  other 
work  at  not  very  high  temperatures. 

Probably  the  best  triangle  that  is  now  obtainable  at  a  moderate 
cost  is  one  of  the  pipe-stem  form,  made  from  silica  tubes  on  a 


GRAVIMETRIC  ANALYSIS  45 

frame  of  nickel  wire.  These  may  be  used  with  either  porcelain 
or  platinum  crucibles  and  they  are  not  easily  cracked  by  sudden 
heating  or  cooling. 


FIG.  21. — Another  form  of  platinum  wire  triangle. 

Burners. — The  burner  that  is  to  be  used  by  the  analyst  may  be 
anything  from  the  cheapest  and  simplest  burner  of  the  Bunsen 
type  to  the  most  expensive  and  complicated  burner  obtainable. 
The  purchaser  has  his  choice  and  probably  certain  advantages 


FIG.  22. — Alloy  triangle. 

are  possessed  by  each  burner.  The  only  feature  that  is  really 
essential  is  independent  regulation  of  air  and  gas  supply.  The 
requirements  are  quite  different  in  different  cases  and  the  analyst 
must  have  at  his  disposal  all  kinds  of  flame,  from  the  yellow  illumi- 


46 


QUANTITATIVE  ANALYSIS 


nating  flame  to  the  most  intensely  hot  and  oxidizing  flame,  and 
he  requires  very  small  and  very  large  flames  of  each  class.  In  order 
to  obtain  this  variety  of  flame  there  must  be  some  method  of 
regulating  the  gas  supply  without  changing  the  pressure  at  the 
gas  valve,  since  this  also  changes  the  amount  of  air  drawn  in  at 
the  mixer.  The  simplest  form  of  Bunsen  burner  does  not  permit 
this  gas  regulation  without  unscrewing  the  upper  tube  and  chang- 
ing the  gas  jet  by  the  use  of  pliers.  Such  regulation  is  not  pos- 
sible in  practice. 


FIG.  23. — Section  of  E.  and  A.  burner.        FIG.  24.— Section  of  Teclu  burner. 

Two  forms  of  adjustable  burners  are  shown  in  Figs.  23  and  24. 
In  the  E.  and  A.  form  the  gas  flow  is  regulated  by  the  large 
milled  disc  near  the  bottom  of  the  burner  and  the  air  supply 
by  the  ring  which  screws  up  and  down  at  the  base  of  the  burner 
tube.  In  the  Teclu  burner  (Fig.  24)  the  gas  is  controlled  by  the 
screw  on  the  side  of  the  base  while  the  disc  at  the  bottom  of  the 
cone  controls  the  air  supply. 

It  is  important  to  note  that  in  both  of  these  burners  the  regu- 
lation of  gas  flow  is  not  accomplished  by  altering  the  pressure 


GRAVIMETRIC  ANALYSIS  47 

under  which  it  is  delivered  but  by  changing  the  size  of  the  orifice 
in  the  burner.  The  maximum  pressure  is  thus  used  at  all  times 
and  the  result  is  a  better  mixture  of  gas  with  air  than  is  obtainable 
by  regulating  the  gas  cock  of  the  supply  line. 

A  very  common  error  on  the  part  of  students  lies  in  careless- 
ness with  regard  to  the  regulation  of  flames.  If  a  relatively 
cool  flame  is  required  and  if  a  deposit  of  carbon  is  not  objection- 
able the  air  should  be  excluded  from  the  mixer.  If,  on  the  other 
hand,  the  highest  efficiency  of  the  burner  is  desired,  careful 
regulation  of  the  air  and  gas  is  necessary.  The  inner  blue  cone 
should  be  well  defined  and  it  should  not  show  a  yellow  tip.  If 
air  is  admitted  more  than  that  required  to  completely  burn  the 
gas  with  production  of  a  blue  flame,  the  result  is  a  roaring  and 
fluttering  flame.  This  means  that  more  air  is  being  admitted 
than  can  be  used  and  this  air,  in  being  heated  by  the  flame, 
lowers  the  temperature  of  the  latter. 

Blast  Lamps. — The  blast  lamp  is  used  for  obtaining  tempera- 
tures higher  than  are  attainable  by  use  of  the  ordinary  burner.  In 
this  burner  the  gas  is  used  in  large  quantities  and  air  is  delivered 
under  pressure  so  that  a  flame  is  produced  more  intensely  hot 
than  the  slower  burning  flame  of  the  common  burner.  Where 
extremely  high  temperatures  are  necessary  the  oxyhydrogen  blow- 
pipe is  used  but  this  is  rarely  the  case  for  analytical  operations. 

Meker  Burner. — A  somewhat  radical  departure  from  the  older 
types  is  found  in  the  Meker  burner.  This  is  shown  in  section 
in  Fig.  25.  The  air  is  drawn  in  through  several  holes  in 
the  base  of  the  tube.  The  delivery  of  the  gas  under  pressure 
into  the  inverted  cone  which  forms  the  burner  tube  causes  a 
greater  reduction  of  pressure  within  the  tube  than  is  the  case 
with  burners  having  cylindrical  tubes.  The  result  is  a  greater 
inflow  of  air,  making  possible  the  combustion  of  a  greater  amount 
of  gas  in  a  given  space,  and  also  more  complete  mixing  of  gas 
and  air. 

The  nickel  grid  through  which  the  mixture  flows  at  the  top  of 
the  burner  causes  the  gas  to  burn  exactly  as  though  each  mesh 
were  a  small  individual  burner.  The  tip  of  the  inner  reducing 
cone  of  each  small  flame  is  usually  about  one  millimeter  above 
the  top  of  the  burner  and,  as  all  of  the  small  flames  unite  to  form 
one  large  one,  the  result  is  a  highly  concentrated  flame,  every 


48 


QUANTITATIVE  ANALYSIS 


part  of  which  is  oxidizing  in  character  except  a  zone  of  about  one 
millimeter  in  depth,  immediately  above  the  top  of  the  burner. 
This  is  a  distinct  advantage,  especially  in  heating  platinum 
articles,  since  platinum  is  easily  damaged  by  heating  in  a  re- 
ducing flame. 


FIQ.  25. — Section  of  M6ker  burner. 

The  flame  of  the  Me*ker  burner  is  nearly  as  hot  as  that  of  the 
ordinary  blast  lamp  using  the  same  gas  and  it  may  be  substi- 
tuted for  the  blast  lamp  in  many  cases.  There  is  also  a  Me"ker 
blast  lamp,  similar  in  construction  to  the  one  already  described 
but  using  air  under  pressure. 

Fusion. — For  the  purposes  of  quantitative  analysis  the  fusion 
of  materials  is  almost  always  accomplished  with  the  end  in  view  of 
producing  more  soluble  substances  through  the  interaction  of 


GRAVIMETRIC  ANALYSIS  49 

an  added  agent,  called  a  flux,  and  the  refractory  material. 
For  instance,  most  of  the  natural  silicates  are  practically  insoluble 
in  water  and  all  ordinary  reagents  and  therefore  they  cannot 
be  analyzed  by  ordinary  methods.  By  a  preliminary  heating 
to  a  high  temperature  in  contact  with  a  basic  substance  like  sodium 
carbonate,  a  fusible  mixture  of  new  compounds  is  formed  and 
these  will,  for  the  most  part,  be  soluble  in  water  and  hydro- 
chloric acid  so  that  the  solution  may  be  submitted  to  precipita- 
tion and  filtration  processes  for  the  separation  and  determination 
of  the  elements.  Similarly,  refractory  and  insoluble  metallic 
oxides  may  be  heated  with  sodium  pyrosulphate  with  the  for- 
mation of  a  fused  mass  consisting  of  soluble  sulphates  of 
the  metals. 

The  necessary  qualities  of  any  useful  flux  are  (1)  that  it  must 
be  of  such  a  nature  as  to  be  capable  of  reacting  with  the  re- 
fractory body  when  heated  with  it  and  (2)  that  the  resulting 
compounds  shall  fuse  at  the  prevailing  temperature.  To  these 
the  analyst  adds  a  third  requisite:  (3)  that  the  resulting  com- 
pounds shall  be  soluble  in  water  or  in  the  laboratory  reagents. 
The  first  condition  is  met  by  choosing  as  the  flux  a  substance 
of  opposite  nature  to  that  of  the  refractory  sample.  That 
is,  if  the  latter  is  of  an  acid  nature  (as  silica  and  polysilicates) 
the  flux  should  be  basic,  and  conversely. 

No  general  statement  can  be  made  with  regard  to  the  relative 
fusibility  of  various  compounds,  as  based  upon  the  chemical 
composition  of  these  compounds.  It  may  be  noted  that  re- 
fractory silicates  are  usually  made  more  readily  fusible  by  reduc- 
ing the  ratio  of  silica  to  metal  oxide  by  introducing  more  metals, 
and  particularly  by  the  introduction  of  the  alkali  metals.  Both 
of  these  points  are  made  by  using  alkali  metal  carbonates  as 
fluxes,  since  the  net  result  of  the  reaction  at  high  temperatures 
is  to  expel  carbon  dioxide  and  to  combine  the  alkali  metal 
oxide  with  the  refractory  silicate.  This  will  explain  why  these 
carbonates  are  almost  always  chosen  as  fluxes  for  silicates. 
A  reaction  such  as  the  following  may  occur  when  orthoclase 
is  fused  with  sodium  carbonate: 

2KAlSi3O8+5Na2CO3-+K2SiO3+5Na2SiO3 
+2NaAlO2+6CO2, 


50  QUANTITATIVE  ANALYSIS 

a  more  or  less  complicated  mixture  of  aluminates  and  silicates 
of  the  alkali  metals  being  formed. 

Basic  Fluxes.  —  Sodium  carbonate,  potassium  carbonate  and 
the  double  sodium-potassium  carbonate  are  the  most  important 
of  the  basic  fluxes  that  are  used  for  analytical  purposes.  These 
are  used  chiefly  for  fusion  with  silica  and  the  refractory  silicates. 
Such  fluxes  as  calcium  oxide,  used  for  fluxing  silicates  in  the 
blast  furnace  for  iron,  are  of  little  use  for  analytical  purposes, 
partly  because  the  resulting  compounds  are  not  soluble  and 
partly  because  metals  that  are  to  be  determined  in  the  sample 
are  introduced  by  the  use  of  such  materials. 

Acid  Fluxes.  —  Fluxes  of  an  acid  nature  are  valuable  chiefly 
for  forming  fusible,  soluble  compounds  when  heated  with  metallic 
oxides  or  salts  that  are  over-saturated  with  metallic  oxides. 
The  most  useful  of  such  fluxes  are  the  pyrosulpHates  and  the 
biborates  of  sodium  and  potassium. 

Acid  sulphates  are  often  used  instead  of  pyrosulphates.  When 
the  former  are  heated  they  give  off  water  and  they  are  completely 
converted  into  pyrosulphates  by  heating  to  higher  temperatures  : 

2NaHS04->Na2S207+H2O. 

Because  of  the  excess  of  sulphur  trioxide  in  the  pyrosulphate, 
this  readily  reacts  with  metallic  oxides  when  heated  with  them: 


The  biborates  likewise  combine  with  metallic  oxides  because 
of  their  excess  of  boric  anhydride. 

Fe2O3+3Na2B4O7->2Fe(BO2)3+6NaBO2. 
Weighing.  —  The  balance  is  the  only  kind  of  weighing  apparatus 
that  is  independent  of  the  numerical  value  of  the  force  of  gravity 
so  that  it  actually  measures  mass  and  not  weight.  In  its  simplest 
form  it  consists  of  a  beam,  supported  at  its  middle  point  on  a 
fulcrum,  having  suspended  at  its  ends,  at  equal  distances  from 
the  middle  fulcrum,  two  platforms  for  holding,  respectively, 
the  objects  to  be  weighed  and  the  weights.  It  is  thus  seen  to 
be  an  apparatus  for  comparing  masses  and  if  it  is  mechanically 
perfect  and  if  the  standard  weights  are  correct,  then  the  mass 
of  the  object  is  given  by  the  mass  of  the  weights  that  counter- 
balance it  when  the  balance  is  at  equilibrium. 


GRAVIMETRIC  ANALYSIS 


51 


Description  of  Balance. — The  balance  as  used  by  the  analyst  is 
a  much  more  sensitive  and  carefully  built  piece  of  apparatus  than 
the  ordinary  balance  and  must  be  capable  of  giving  weights  that 
are  correct  to  within  0.0001  gm.  In  order  to  reduce  the  friction 
of  the  bearings  to  the  lowest  possible  value  these  are  made  in  the 
form  of  a  knife  edge  supported  by  a  polished  block-,  the  material 
of  the  bearings  being  some  substance  that  is  quite  hard,  this 
usually  being  agate.  The  method  of  weighing  is  to  place  the 


FIG.  26. — Analytical  balance. 

object  to  be  weighed  in  one  pan  and  to  add  weights  to  the  other 
until  equilibrium  is  attained,  judging  this  condition  by  observing 
when  a  pointer,  attached  to  the  beam,  swings  equal  distances  to 
the  right  and  left  of  an  experimentally  determined  "zero  point" 
on  a  scale.  The  whole  balance  is  enclosed  in  a  glass  case  in  order 
to  exclude  dust  and  to  prevent  the  interference  of  air  currents. 
The  case  is  provided  with  levelling  screws  and  a  spirit  level  or 
plumb  bob. 

To  prevent  needless  wear  on  the  knife  edges  the  beam  and  pans 
should  be  provided  with  suitable  rests  so  that  the  knife  edges  may 


52 


QUANTITATIVE  ANALYSIS 


not  be  in  contact  with  their  bearings  when  the  balance  is  not  being 
used.  These  rests  are  necessary  also  for  the  proper  control  of  the 
action  of  the  balance,  as  will  be  noticed  when  exercises  with  the 
balance  are  described.  The  rests  for  the  beam  and  for  the  pan 
supports  are  usually  operated  by  one  piece  of  mechanism.  The 
exact  construction  of  this  varies  in  different  balances  but  the 
action  of  the  beam  rests  belongs  to  one  of  two  classes,  the  vertical 
and  the  circular.  In  the  first  (Fig.  27)  the  rests  move  vertically 
upward.  The  chief  defect  of  this  action  is  the  fact  that  if  the 
beam  is  caught  at  any  position  except  a  horizontal  one  the  knife 


FIG.  27. — Beam  rests  with  vertical  action. 

edges  are  caused  to  slide  upon  their  bearings,  causing  unnecessary 
wear.  In  the  circular  action  (Fig.  28)  the  arms  of  the  arrests  have 
the  same  length  as  the  arms  of  the  beam  and  they  move  about  the 
same  axis.  No  matter  in  what  position  the  beam  is  caught  or 
how  sudden  the  motion  of  arrest,  no  damage  results  to  the  knife 
edges.  All  designs  of  mechanism  for  this  purpose  include  devices 
for  automatically  placing  beam  and  pan  bearings  in  their  proper 
position  in  case  they  have  been  accidentaUy  twisted  out  of  posi- 
tion. These  are  shown  at  (a),  Fig.  28.  The  rests  under  the  pans 
are  not  for  the  purpose  of  lifting  knife  edges  from  their  bearings, 
but  merely  for  steadying  the  pans  and  for  controlling  the  move- 
ments of  the  balance  when  it  is  in  use.  In  some  balances  these 


GRAVIMETRIC  ANALYSIS 


53 


are  operated  by  the  same  mechanism  that  operates  the  other 
rests,  while  in  others  a  separate  knob  is  provided. 

Sensibility. — The  sensibility  of  the  balance  is  stated  in  terms  of 
the  displacement  of  the  beam  by  a  given  excess  of  load  in  one  pan. 
More  specifically,  it  is  the  number  of  scale  divisions  of  displace- 
ment of  zero  point  by  an  excess  of  1  mg  in  one  pan. 

The  sensibility  is  affected  by  a  number  of  factors.  The  bal- 
ance, in  order  to  possess  stability,  must  have  the  center  of  gravity 
of  the  moving  parts  slightly  below  the  point  of  the  middle  knife 
edge.  This  distance  is  one  of  the  determining  factors  of  the  sen- 


FIG.  28. — Beam  rests  with  circular  action. 


sibility.  If  large,  the  sensibility  must  be  relatively  small,  since  a 
given  displacement  of  the  zero  point  will  involve  a  relatively 
large  displacement  of  the  center  of  gravity  and  will,  in  conse- 
quence, require  a  greater  difference  in  load.  Every  balance  has 
some  provision  for  arbitrarily  altering  the  sensibility  by  altering 
the  location  of  the  center  of  gravity.  The  most  common  device 
is  a  weight  that  can  be  moved  up  and  down  the  pointer. 

The  three  knife-edge  bearings  must  lie  in  the  same  plane,  as 
well  as  parallel  to  each  other.  Since  the  pans  swing  freely  upon 
their  own  bearings,  the  whole  load  of  the  pans  is  applied  at  these 
points.  If  the  plane  of  these  bearings  were  below  that  of  the 
middle  bearing  it  could  easily  be  that  the  center  of  gravity 
would  lie  between  these  planes  and  then  an  increase  in  load 


54 


QUANTITATIVE  ANALYSIS 


would  lower  the  center  of  gravity  with  reference  to  the  central 
bearing  and  thus  decrease  the  sensibility.  If  their  plane  were 
above  that  of  the  middle  bearing  an  increase  in  load  would 
raise  the  center  of  gravity  to  a  point  above  the  middle  point 
of  support  and  give  instability  to  the  balance  so  that  if  dis- 
placed from  its  normal  horizontal  position  the  beam  would  not 
return.  When  the  three  bearings  lie  in  the  same  plane  an  in- 
crease in  load  will  raise  the  center  of  gravity  but  can  never  raise 
it  to  the  level  of  the  middle  knife  edge.  The  above  method 
of  reasoning  supposes  that  the  balance  beam  is  perfectly  rigid, 
a  property  that  is  never  attained  in  practice.  Increase  in  load, 


FIG.  29. — Illustrating  the  principle  of  moments. 

therefore,  does  actually  cause  decrease  in  sensibility  because 
the  beam  is  somewhat  distorted,  causing  the  center  of  gravity 
to  be  lowered.  In  order  to  combine  great  strength  with  lightness 
of  weight  and  so  minimize  the  distortion  of  the  beam,  makers 
have  tried  many  designs  and  many  alloys  in  the  manufacture 
of  balance  beams. 

Another  property  that  has  a  large  influence  upon  the  sensibility 
is  the  length  of  the  arms  of  the  beam.  It  is  a  well  known  principle 
of  physics  that  when  a  balance  is  in  equilibrium  the  product 
of  the  weight  of  one  side  into  the  length  of  the  corresponding 
arm  must  equal  the  product  of  the  other  weight  into  the  length 
of  its  arm.  This  is  the  "principle  of  moments."  In  Fig.  29, 
aw  =  a'w'.  The  greater  the  inequality  of  the  statical  moments 
(aw;  and  a'w')  when  an  excess  of  weight  lies  on  one  side  the  greater 


GRAVIMETRIC  ANALYSIS  55 

will  be  the  displacement  of  zero  point  and  this  inequality  will 
be  greater  when  a  and  a'  (the  lengths  of  the  arms)  are  large. 
Lengthening  the  arms  also  causes  slower  swinging  so  that  sensi- 
bility gained  by  this  means  results  in  loss  of  time  in  weighing. 
This  fact  sets  a  practical  limit  to  the  length  of  the  beam. 

The  following  properties  have  been  discussed  as  having  an 
influence  upon  the  sensibility  of  the  balance:  1.  Distance  between 
the  center  of  gravity  and  point  of  support.  2.  Coincidence 
of  the  planes  of  the  three  bearings.  3.  Length  of  the  arms 
of  the  beam.  4.  Reduction  of  friction  to  a  minimum  by  finely 
ground  knife  edges.  In  addition  to  these  should  be  mentioned 
the  weight  of  the  beam.  A  heavy  beam  makes  a  balance  that 
is  less  sensitive  than  a  balance  having  a  light  one. 

In  addition  to  those  features  which  affect  the  sensibility, 
certain  others  are  essential  if  the  balance  is  to  weigh  accurately. 
It  is  extremely  difficult  to  construct  a  beam  having  absolutely 
the  same  distance  between  the  central  knife  edge  and  the  two  end 
ones.  Obviously  any  difference  involves  a  slight  difference  in 
the  two  weights  required  to  bring  the  balance  to  equilibrium. 
From  the  equation  aw  =  a'w',  if  a^a'  then  w^w'  and  the  weight 
of  the  substance  which  is  being  weighed  is  not  the  same  as  that 
of  the  weights  which  counterbalance  it.  The  relative  lengths  of 
the  arms  must  be  determined  for  each  balance  and  the  observed 
weights  corrected  if  these  lengths  are  appreciably  different. 
Even  if  the  discrepancy  in  lengths  is  sufficiently  small  to  be 
negligible  it  may  be  magnified  by  a  change  in  temperature  or  by 
a  change  in  load,  if  the  beam  is  not  absolutely  uniform  in  material 
and  structure.  This  last  condition  is  impossible  of  attainment 
in  practice.  It  will  be  made  clear  in  the  course  of  the  work  in 
gravimetric  analysis  that  it  is  not  the  absolute  weight  that  is 
important  in  most  cases  but  only  relative  weights  because  the 
object  of  quantitative  analysis  is  to  determine  the  proportionate 
parts  of  the  constituents  of  a  compound  or  mixture.  From  the 
equation  aw  =  a'w',  if  a  and  a'  are  not  equal  the  error  in  w' 


bears  in  all  cases  a  definite  ratio  to  w1 '. 

While  the  balance  gives  the  true  mass  of  the  object  and  is  in- 
dependent of  the  magnitude  of  the  force  of  gravity,  this  expres- 
sion is  true  only  if  the  buoyancy  of  the  air  acts  upon  weights  and 
objects  alike.  This  can  be  the  case  only  when  the  density  of 


56 


QUANTITATIVE  ANALYSIS 


weights  is  the  same  as  the  density  of  objects,  a  condition  that  is 
not  fulfilled  in  the  majority  of  cases.  A  correction  must  there- 
fore be  introduced  in  such  cases  in  order  to  find  the  true  weight 
of  the  object.  The  amount  of  correction  is  negligible  in  gravi- 
metric analysis  and  becomes  serious  only  when  the  total  weight 
is  considerable.  The  method  for  applying  this  correction  will 
be  explained  in  the  section  dealing  with  volumetric  analysis. 
(See  p.  180.) 

Weights. — Sets  of  analytical  weights  as  purchased  frequently 
include  weights  as  small  as  1  mg.  These  are  rarely  used 
because  the  balance  provides  a  more  convenient  method  for 
making  the  final  adjustment,  in  the  form  of  a  " rider"  or  small 
weight  of  fine  platinum  or  aluminium  wire  which  may  be  shifted 
to  various  positions  on  the  beam.  The  manner  in  which  the 


FIG.  30. — Balance  beam. 

beam  is  divided  varies  with  the  balances  of  different  manufac- 
turers. The  lowest  subdivision  should  be  at  most  0.1  mg.  The 
weight  of  the  rider  will  depend  upon  the  manner  of  numbering  the 
milligram  divisions  and  the  weight  will  be  represented  by  the 
number  which  is  directly  over  the  terminal  knife  edge.  This  is 
because  when  the  rider  is  placed  on  this  division  it  is  essentially  the 
same  as  though  it  were  in  the  pan  below.  To  correctly  indicate 
weights  it  must  then  weigh  the  number  of  milligrams  indicated 
by  this  division,  which  may  be  5,  6,  10,  12,  or  any  other  number. 
A  mechanism  is  provided  for  lifting  and  ad  justing  the  rider  with- 
eniout  opng  the  balance  case.  This  may  be  a  simple  sliding  hook, 
or  an  elaborate  carrier,  such  as  are  found  in  more  expensive 


GRAVIMETRIC  ANALYSIS 


57 


balances.  It  is  essential  that  the  carrier  be  capable  of  quickly 
and  easily  shifting  the  rider  without  danger  of  throwing  it  from 
the  beam. 

The  Chain  Rider.— The  "Chainomatic"  balance  entirely  dis- 
penses with  a  separate  rider.  One  end  of  a  small  gold  chain  is 
permanently  attached  to  the  balance  beam.  The  other  end  of 
this  chain  is  fastened  to  a  hook  which  may  be  moved  up  and 
down  a  scale,  this  action  being  controlled  by  a  knob  outside  the 
balance  case.  Movement  of  the  hook  on  the  scale  varies  in  a 
definite  manner  the  length  of  side  of  the  loop  which  is  supported 


Fia.  31.  —  Usual  form  of  a  set  of  analytical  weights. 


by  the  beam  and  this  may  be  adjusted  while  the  beam  is  in 
motion.  This  is  a  distinct  advance  in  balance  design,  although 
this  improvement  adds  considerably  to  the  cost  of  the  balance. 
Every  balance  is  rated  for  a  certain  maximum  load,  it  being 
understood  that  this  is  the  load  for  each  pan  and  not  the  total 
load.  The  normal  load  is  fixed  by  the  strength  of  the  knife  edges 
and  by  the  capacity  of  the  beam  to  resits  deformation  under  stress. 
If  the  knife  edges  are  short  and  ground  to  exceeding  fineness  they 
are  injured  more  readily  by  a  load  than  if  they  are  slightly  more 
blunt.  If  the  beam  is  overloaded  it  is  temporarily  deformed  to 


58  QUANTITATIVE  ANALYSIS 

such  an  extent  that  there  is  an  unusual  loss  of  sensibility,  due  to 
the  excessive  lowering  of  the  center  of  gravity.  It  is  thus  evi- 
dent that  the  weighing  of  heavy  objects  requires  correspondingly 
more  sturdy  balances  and  these  will,  of  course,  be  less  sensitive. 
The  usual  form  of  a  set  of  metric  weights  is  shown  in  Fig.  31. 
The  largest  weight  should  not  be  heavier  than  the  maximum 
load  for  which  the  balance  is  rated  and  the  least  weight  should 
be  such  that,  used  in  conjunction  with  the  rider,  10  mg  may 
be  made  up.  The  larger  weights  are  constructed  of  brass  or 
bronze,  plated  with  platinum  or  gold  to  prevent  corrosion.  The 
fractional  pieces  are  of  platinum  in  the  better  sets  or  of  aluminium 
in  the  cheaper  ones.  Even  with  weights  plated  with  platinum 
or  gold  it  is  comparatively  easy  to  damage  the  surface  by  careless 
handling  or  by  allowing  chemicals  to  touch  the  weights. 

Printed  directions  for  setting  up  always  accompany  a  new 
balance.  The  following  rules  deal  only  with  the  balance  set  up 
and  ready  for  use. 

Cleanliness. — The  pans,  beam,  bearings  and  all  other  parts 
inside  the  glass  case  must  at  all  times  be  kept  free  from  dust  and 
chemicals.  A  camel's  hair  brush  1  inch  wide  should  be  provided 
for  this  purpose.  It  is  not  permissible  to  weigh  any  soluble  mate- 
rial in  direct  contact  with  the  pans  because  some  of  this  will  in- 
variably stick  to  the  pan  and  eventually  it  may  cause  corrosion. 
Volatile  acids  must  never  be  brought  inside  the  balance  case  un- 
less securely  stoppered  in  an  air-tight  container. 

Adjustment. — The  balance  is  levelled  by  means  of  the  screws 
provided  for  that  purpose.  Examination  is  made  to  determine 
whether  the  knife  edges  are  in  the  proper  position  with  respect  to 
their  bearings.  The  pan  rests  are  released  to  determine  whether 
the  pans  hang  vertically  from  the  stirrups,  or  whether  they  swing 
horizontally  when  released.  If  so  this  swinging  is  stopped  by 
momentarily  touching  the  pans  by  the  pan  rests,  repeating  the 
operation  until  the  pans  hang  quietly  upon  release. 

To  Set  the  Beam  in  Motion. — Various  methods  are  used  for 
starting  the  oscillation  of  the  balance  about  the  central  bearing. 
One  pan  may  be  lightly  touched  with  a  small  camel's  hair  brush. 
This  is  not  an  easy  process  to  carry  out  properly  because  it  is 
difficult  to  control  the  impulse  given  to  the  pan.  Another 
method  is  to  raise  the  door  and  fan  one  of  the  pans  slightly  with 


GRAVIMETRIC  ANALYSIS  59 

the  hand.  This  is  open  to  the  same  objection  as  is  the  first 
method  and,  in  addition,  it  defeats  the  primary  aim  of  the  glass 
case  which  is  to  prevent  the  interference  of  air  currents.  Even 
the  slight  current  started  by  the  hand  does  not  at  once  die  and  it 
must  be  a  disturbing  influence  for  some  time  after  the  balance  case 
is  closed.  A  better  method  than  either  of  the  above  mentioned  is 
to  lower  the  rider  to  the  beam  just  before  releasing  the  latter,  then 
to  catch  up  the  rider  with  the  carrier  after  releasing  and  allowing 
the  proper  start.  A  short  practice  will  enable  the  operator  to 
give  just  the  desired  impulse  to  the  beam  to  make  the  pointer 
swing  over  from  five  to  ten  divisions  on  either  side  of  the  zero 
of  the  scale. 

The  rests  should  be  so  adjusted  that  the  three  knife  edges  are 
lifted  from  their  bearings  when  the  rests  are  raised,  but  the  distance 
between  the  edge  and  bearing  should  be  barely  perceptible.  If 
this  distance  is  unduly  large  the  shock  to  the  delicate  knife  edge  is 
so  great  that  this  edge  is  soon  dulled  or  chipped  with  a  consequent 
loss  of  sensibility. 

To  Determine  the  Zero  Point. — The  zero  point  may  be  de- 
fined as  that  point  on  the  scale  at  which  the  pointer  would  eventually 
come  to  rest  from  swinging  over  the  scale.  It  is  never  observed  by 
allowing  the  pointer  to  actually  come  to  rest  because  such  in- 
fluences as  minute  air  currents  would  either  prevent  this  con- 
summation entirely  or  would  cause  the  observation  of  a  fictitious 
zero  point.  The  effect  of  these  influences  is  counteracted  by 
allowing  the  pointer  to  swing  a  number  of  times  to  the  right  and  to 
the  left,  taking  the  average  of  the  indications.  Proceed  as 
follows : 

If  the  balance  operates  all  of  its  rests  by  one  mechanism  care- 
fully lower  these  rests  and  set  the  beam  in  motion  as  directed 
above.  If  the  pan  supports  are  controlled  by  a  separate  button 
lower  the  beam  and  stirrup  rests  first,  then  the  pan  rests.  Allow 
the  pointer  to  swing  three  or  four  times  in  each  direction  and  re- 
cord the  number  of  scale  divisions  over  which  it  swings,  taking 
the  last  reading  on  the  same  side  as  the  first.  Record  in  two 
columns  and  take  the  average  of  each  column.  Subtract  the  less 
average  from  the  greater  and  divide  the  remainder  by  2.  This 
gives  the  zero  point  if  the  proper  direction  is  assigned  to  it. 


60 


QUANTITATIVE  ANALYSIS 


Example: 


Left 

Right 

8.25 
7.75 
7.25 
7.00  . 
6.75 
6  50 

6.00 
5.50 
5.00 
4.75 
4.50 

Average    7  .  25 

Average  5.15 

rj  rtr r   I  r 

-  =  1.05.     Therefore   the   zero   point   is  1  division  to 
2 

the  left  of  the  zero  of  the  scale.  Two  methods  of  procedure 
are  now  open  to  the  operator.  He  may  either  make  his  weighings 
with  reference  to  this  observed  zero  point  or  he  may  adjust 
the  balance  so  that  the  observed  zero  point  is  the  actual  zero  of 
the  scale,  using  for  this  purpose  the  small  screws  provided  on  the 
ends  of  the  beam.  The  first  method  is  preferable  for  the  beginner 
because  any  attempt  to  change  the  adjustment  will  probably 
result  in  more  serious  derangement.  After  skill  has  been  gained 
by  practice  time  will  be  gained  and  much  calculation  will  be 
saved  if  the  observed  zero  point  is  adjusted  to  coincide  with  the 
ideal  zero  of  the  scale.  A  method  for  making  a  close  approxi- 
mation of  the  zero  point  without  resorting  to  calculations  on 
paper  will  be  explained  in  a  later  paragraph.  The  zero  point 
changes  and  it  must  be  determined  each  day,  or  more  often  if 
necessary. 

To  Determine  the  Sensibility. — The  sensibility  in  the  case 
of  the  analytical  balance  has  already  been  denned  as  the  number 
of  scale  divisions  that  the  zero  point  is  displaced  by  an  excess  in 
weight  of  1  mg  on  one  side.  That  the  sensibility  varies  with 
the  total  load  has  already  been  explained.  To  determine  the 
sensibility  with  zero  load,  first  determine  the  zero  point  of 
the  balance.  Place  the  rider  on  the  beam  at  the  division  marked 
1  and  redetermine  the  zero  point.  The  difference  is  the  sensi- 
bility. Determine  the  sensibility  when  both  pans  are  loaded 
with  5,  10,  25  and  50  gm,  respectively.  Record  the  results  on 
a  card  and  place  this  in  the  balance  case  for  future  reference. 


GRAVIMETRIC  ANALYSIS  61 

To  Determine  the  Relative  Lengths  of  the  Arms.  —  Weigh 
a  small  object,  such  as  a  crucible,  placing  it  on  the  left  pan  and 
the  weights  on  the  right,  then  place  the  object  on  the  right  pan 
and  the  weights  on  the  left.  If  the  arms  of  the  balance  are  of 
unequal  length  these  weights  will  not  be  the  same.  Let  W  = 
the  true  weight  of  the  object,  TF'  =  the  sum  of  the  weights  when 
the  object  is  on  the  left  pan,  a  =  the  weight  added  to  W  when  the 
object  is  placed  on  the  right  pan,  r  =  the  length  of  the  right  arm 
and  Z  =  that  of  the  left  arm.  From  the  principle  of  moments 


W'r  = 
Wr=( 

WW'r2=W(W'+a)l2 


This  is  the  proper  method  for  testing  the  equality  of  arms. 
If  the  arms  are  equal  then  a  =  0  and  7  =  1.     If  a  is  negative, 


<  1  and  the  right  arm  is  therefore  shorter  than  the  left,  while 


if  a  is  positive  -7  >  1  and  the  right  arm  is  the  longer.     As  has 

already  been  explained,  the  question  of  equality  has  little  or  no 
importance  for  purely  analytical  work.  Inasmuch  as  the  chemist 
has  uses  for  his  balance  in  other  lines  of  work  it  should  be  tested. 
To  Weigh.  —  With  all  of  the  rests  raised  the  object  to  be 
weighed  is  placed  on  the  left  pan  by  means  of  the  crucible  tongs 
or  by  some  other  method  that  avoids  contact  with  the  fingers. 
The  pan  rests  are  lowered  and  raised  momentarily  until  the 
pans  stop  swinging  on  their  bearings,  then  these  rests  are  lowered 
and  fastened  down  unless  all  of  the  rests  are  governed  by  one 
mechanism.  By  means  of  the  weight  forceps  place  one  of  the 
weights,  judged  to  be  somewhat  heavier  than  the  object,  on  the 
right  pan.  Lower  the  beam  rest  slowly  until  the  pointer  just 
starts  to  move  to  the  right  or  left.  If  to  the  right  the  weight  is 
too  light,  if  to  the  left  it  is  too  heavy.  If  it  is  too  light  raise  the 


62  QUANTITATIVE  ANALYSIS 

beam  rest  and  exchange  the  weight  for  the  next  heavier  one  and 
repeat  the  trial  until  the  first  weight  is  found  that  is  too  heavy. 
Remove  this  and  replace  by  the  next  lighter  one.  Continue  the 
addition  of  weights,  trying  after  each  addition  and  adding  always 
the  next  consecutive  weight  lighter  than  the  one  used  in  the 
preceding  trial.  When  the  weights  below  1  gm  (milligram 
pieces)  are  reached  the  difference  between  the  loads  on  the  two 
pans  is  so  small  that  the  pan  rests  will  now  readily  control  the 
movements  of  the  balance.  If  these  rests  are  operated  by  a 
separate  knob  they  are  then  raised  and  the  beam  and  stirrup 
rests  lowered,  and  the  process  of  trial  is  continued  until  the  range 
covered  by  the  rider  is  reached.  The  balance  case  is  now  closed, 
and  the  pans  again  steadied  if  they  have  shown  a  tendency 
toward  lateral  swinging.  Make  the  preliminary  ^ rider  trials 
by  placing  the  rider  on  the  division  estimated  to  be  nearest  the 
proper  one,  slightly  releasing  the  pan  rests  until  the  pointer  starts 
to  move.  Arrest  the  motion  and  move  the  rider  by  whole  milli- 
gram divisions  to  the  right  or  left,  as  may  be  required,  until  the 
milligram  nearest  to  the  correct  weight  is  reached.  Estimate 
the  proper  fraction  of  a  milligram,  set  the  beam  in  motion  as 
already  directed  and  drop  the  rider  on  the  estimated  division. 
By  observing  the  swinging  determine  the  zero  point  and  calcu- 
late from  this  and  the  sensibility  with  this  load  what  change 
should  be  made  in  the  position  of  the  rider  to  bring  the  balance 
to  equilibrium  on  the  zero  as  determined  with  no  load.  Repeat 
the  trial  with  the  rider  on  this  calculated  position  and  shift  if 
necessary  to  obtain  the  exact  weight.  Raise  all  of  the  rests  and 
read  the  weights  as  follows:  Observe  the  empty  places  in  the 
box.  If  the  weights  have  been  systematically  placed  in  the  box, 
none  being  removed  except  those  on  the  pan,  the  empty  places 
will  give  the  correct  weight.  Record  this  weight  in  the  data 
book.  Confirm  by  counting  the  weights  as  they  lie  on  the  pan. 
Reconfirm  by  counting  as  they  are  removed  and  replaced  in  the 
box.  This  gives  three  readings  and  if  these  are  carefully  made 
a  mistake  is  practically  impossible.  In  recording  the  weights 
mental  addition  may  be  made  if  they  are  taken  in  order,  pro- 
ceeding from  the  larger  ones  to  the  smaller  ones.  This  is  because, 
as  the  sets  of  weights  are  made,  no  one  order  of  digits  can  total 
more  than  9.  Each  order  can  be  mentally  added  and  recorded 


GRAVIMETRIC  ANALYSIS  63 

with  the  certainty  that  no  other  order  will  change  the  one  read. 
Thus,  if  there  are  on  the  pan  the  following  pieces :  10  gm,  5  gm, 
1  gm,  200  mg,  100  mg,  50  mg,  20  mg,  10  mg,  and  5  mg,  and  on 
the  rider  3.2  mg,  we  should  read  and  record  thus:  Of  whole 
grams  16,  of  tenths  (100  mg)  3,  of  hundredths  8,  of  thousandths 
8,  of  ten-thousandths  2.  Writing  in  the  same  order  we  should 
have  16.3882  gm. 

Use  of  Rests. — The  beam  and  stirrup  rests  must  be  used  when 
changing  weights  heavier  than  1  gm  for  two  reasons:  First,  the 
shock  of  the  weight  against  the  pan  must  not  be  allowed  to 
communicate  itself  to  the  knife  edges  when  on  their  bearings 
and  second,  the  pan  rests  are  held  up  by  a  spring  that  will  not 
support  an  excess  load  of  more  than  1  gm  in  one  pan.  In  other 
words,  these  rests  would  not  control  the  balance  at  this  point 
in  the  experiment.  When  the  milligram  pieces  are  being  ex- 
changed the  shock  of  impact  with  the  pan  is  so  small  that  the 
knife  edges  are  not  damaged  and  the  pan  rests  offer  an  easier 
method  of  control. 

Trial  of  Weights. — In  making  a  trial  of  a  weight  the  pointer 
should  be  allowed  to  move  only  far  enough  to  indicate  the  direc- 
tion of  motion.  This  indicates  the  proper  change  to  be  made  in 
weights  as  well  as  if  it  were  allowed  to  move  half  way  across 
the  scale  and  it  does  not  derange  the  balance. 

Estimation  of  Zero  Point. — It  has  been  stated  that  in  later 
work  extended  calculations  of  zero  point  would  not  be  made. 
On  account  of  the  resistance  due  to  friction  with  the  air  and  in 
the  bearings,  any  balance  decreases  the  amplitude  of  vibration 
with  each  successive  j  ourney .  The  amount  of  such  decrease  varies 
with  different  balances  but  a  close  approximation  can  be  made 
by  simple  observation.  If  the  zero  point  of  the  unloaded  balance 
has  been  adjusted  to  coincide  with  that  of  the  scale,  in  the  final 
adjustment  of  weights  the  loaded  balance  can  also  be  brought  to 
this  ideal  zero  point  without  the  necessity  of  extended  cal- 
culations, by  simply  noting  that  the  distance  to  which  the  pointer 
swings  in  one  direction  is  a  certain  (approximate)  fraction  of  a 
division  less  than  the  distance  in  the  opposite  direction  on  the  next 
preceding  journey.  In  many  cases  of  quantitative  analysis, 
if  the  longer  (even  though  slightly  more  exact)  method  involving 
calculations  of  zero  point  were  followed,  the  time  consumed 


64  QUANTITATIVE  ANALYSIS 

in  weighing  would  be  so  great  that  the  weight  of  the  object  would 
change  appreciably  while  on  the  balance,  owing  to  absorption 
or  evolution  of  water,  carbon  dioxide,  etc. 

To  Obtain  a  Specified  Weight  of  Sample. — It  is  often  desirable, 
in  order  to  simplify  calculations,  to  use  a  certain  specified  weight 
of  sample,  as  1  gm,  0.5  gm,  5  gm,  etc.  This  is  a  difficult  operation 
if  the  ordinary  method  of  weighing  by  difference  is  used,  because 
the  sample  that  is  to  be  used  is  poured  from  a  weighing  bottle 
and  if  too  much  is  inadvertently  poured  out  it  is  not  easy  to 
return  the  excess  without  loss.  If  the  sample  is  dry  and  is 
unaffected  by  free  contact  with  air  one  can  dispense  with  the 
weighing  bottle  and  weigh  directly  on  a  watch  glass  or  scoop. 
For  this  purpose  one  may  obtain  "counterpoised  watch  glasses," 
which  are  pairs  of  glasses  the  members  of  which  possess  so  nearly 
the  same  weight  that  they  can  easily  be  balanced  by  means  of  the 
rider  so  that  their  weight  does  not  enter  into  the  calculation. 
The  method  of  obtaining  a  predetermined  weight  of  a  sample 
is  as  follows:  The  glasses  are  placed  on  the  pans  and  exactly 
balanced,  if  necessary,  by  using  the  rider.  Weights  totaling 
the  desired  quantity  are  placed  on  the  right  glass  and  the  pan 
rests  lowered,  steadying  the  pans  at  the  time  if  necessary. 
The  beam  rests  are  now  lowered  just  enough  to  allow  the  central 
knife  edge  to  come  to  its  bearing  and  the  pointer  to  perceptibly 
move  to  the  left.  The  balance  door  is  lowered  about  half  way 
and  then  the  sample,  in  a  fine  state  of  division,  is  carefully  poured 
on  the  left  glass  until  but  a  slight  excess  is  obtained,  as  evidenced 
by  the  swing  of  the  pointer  to  the  right.  (The  total  length  of 
wing  as  allowed  by  the  beam  rests  should  nob  exceed  two  scale 
divisions  as  otherwise  the  adjustment  of  the  balance  may  be 
disturbed.)  Using  the  spatula,  sufficient  sample  is  now  removed 
from  the  glass  to  make  this  side  slightly  too  light.  This  is 
held  over  the  glass  and,  by  gently  tapping  the  spatula,  the  sample 
thus  held  is  gradually  dropped  to  the  pan  until  equilibrium 
of  the  balance  is  nearly  or  quite  attained.  The  excess  is  dis- 
carded. By  repeating  this  process  once  or  twice  apparent 
equilibrium  is  quickly  attained  and  this  is  confirmed  by  closing 
the  case  and  determining  the  zero  point  by  the  usual  method. 
In  this  way  any  desired  weight  of  sample  can  be  obtained  in  a 
comparatively  short  time.  The  degree  of  accuracy  with  which 


it  is  fina 


GRAVIMETRIC  ANALYSIS  65 


it  is  finally  weighed  will  depend,  as  in  all  other  cases,  upon 
the  nature  of  the  sample,  the  total  weight  being  taken  and 
the  degree  of  accuracy  possible  at  other  points  in  the  analysis. 

To  Correct  the  Observed  Weight  for  Inequality  of  Arms. — 
Certain  investigations  in  physics  and  physical  chemistry  require 
that  the  weight  found  shall  be  the  absolute  weight,  correcting 
for  the  inequality  of  arms  and  for  buoyancy  of  the  air.  The 
former  correction  need  not  be  made  for  analytical  work  but  where 
necessary  either  of  the  following  methods  may  be  used. 

Method  of  Gauss. — Weigh  the  object  first  on  the  left  pan  and 
then  on  the  right.  Let  W  be  the  true  weight,  a  the  weights 
required  to  counterbalance  when  the  object  is  on  the  left  pan 
and  b  the  weights  when  the  object  is  on  the  right.  By  the 
principle  of  moments: 

Wl  =  ar 
U  =  Wr 
W2lr  =  ablr 


Therefore  the  true  weight  is  the  square  root  of  the  product  of 
the  two  observed  weights.  Where  the  inequality  of  arms  is  very 
slight  (the  usual  case)  the  arithmetical  mean  of  the  two  weights 
is  a  sufficiently  close  approximation  to  the  square  roo't  of  the 
product. 

Method  of  Borda. — This  is  also  known  as  the  method  of 
substitution.  Place  the  object  to  be  weighed  on  one  pan  and 
counterbalance  with  any  other  material,  such  as  similar  weights 
or  dry  sand.  Remove  the  object  and  substitute  accurate 
weights  until  the  balance  is  again  in  equilibrium.  These 
weights  are  necessarily  the  same  in  value  as  the  object  for 
which  they  substitute,  irrespective  of  the  relative  length  of 
the  arms. 

Use  of  Arm  Ratio. — Weigh  the  object,  as  usual,  on  the  left 
pan  with  the  weights  on  the  right.  Multiply  the  observed 

weight  by  the  ratio  v-     This  gives  the  true  weight.     If  the  arms 

of  the  balance  are  equal  in  length,  v  =  1  and  the  observed 
weight  is  the  true  weight. 

5 


66  QUANTITATIVE  ANALYSIS 

Calibration  of  Weights. — For  analytical  purposes  it  is  not 
necessary  that  the  various  pieces  in  a  set  of  weights  shall  have 
the  exact  values  indicated  by  the  stamp.  This  is  because  an 
analysis  is  always  reported  as  a  percent  or  as  some  similar 
ratio.  The  only  requirement  is,  therefore,  that  the  pieces  shall 
have  the  correct  relation  to  each  other.  That  is,  the  piece 
marked  "1  gm"  need  not  weigh  exactly  one  gram.  Indeed  it 
might  conceivably  have  any  other  value  that  is  reasonably 
near  to  one  gram;  but  it  is  necessary  that  its  weight  shall  be 
one-tenth  of  that  of  the  piece  marked  "  10  gm, "  ten  times  that  of 
the  piece  marked  "100  mg,"  etc.,  or  that  the  deviation  from 
these  ratios  be  known  and  corrected  in  calculations  of  the  weights 
of  objects. 

Commercial  weights  are  seldom  accurately  adjusted  unless 
the  cost  is  high.  Therefore  a  calibration  should  always  be  made 
and  correction  applied  in  such  cases  as  are  made  necessary  by 
errors  in  manufacture.  Also,  if  standard  pieces  are  available 
it  is  desirable  to  correct  each  set  to  true  gram  values,  rather 
than  to  merely  relative  values,  since  the  chemist  frequently 
uses  his  weights  for  other  than  analytical  purposes. 

Before  beginning  the  calibration  see  that  in  all  cases  where  there 
is  more  than  one  piece  of  a  given  denomination  the  different 
pieces  bear  some  distinctive  mark.  A  good  plan  is  to  make 
small  dots  by  means  of  a  prick  punch.  This  marks  without 
damaging  the  plate  or  changing  the  weight  of  the  pieces.  One 
of  the  10-gram  pieces  may  be  marked  (•)  and  the  other  (••);  the 
three  1-gram  pieces  (•),  ("),  (•••)>  and  similarly  with  other  dupli- 
cate or  triplicate  pieces. 

Calibration  of  weights  is  essentially  a  comparison  of  the  dif- 
ferent pieces  of  a  set  with  each  other  and  with  a  standard, 
followed  by  a  calculation  of  either  relative  or  absolute  values. 
The  method  to  be  followed  in  making  this  comparison  will  depend 
upon  whether  the  arms  of  the  balance  have  been  found  to  be  of 
equal  length,  within  reasonable  experimental  limits.  The 
analytical  problem  sometimes  requires  an  accuracy  represented 
by  a  maximum  error  of  0.001  percent,  although  the  maximum 
permissible  error  is  usually  larger  than  this.  In  order  to  meet 
these  requirements  the  weights  should  be  calibrated  with  the 

same  degree  of  accuracy  and  if  the  ratio  of  arm  lengths,  -,-»  is 


GRAVIMETRIC  ANALYSIS  67 

not  greater  than  1.00001  nor  less  than  0.99999,  calibration 
by  direct  comparison  of  weights  on  the  two  sides  of  the  balance 
may  be  employed.  If  the  ratio  of  arms  is  not  within  these 
limits  the  method  of  direct  comparison  may  still  be  used,  pro- 
vided that  a  correction  is  applied.  The  apparent  weight  of  the 
piece  on  the  right  side,  in  terms  of  the  piece  on  the  left,  is  mul- 
tiplied by  the  ratio  j>  this  giving  the  true  comparative  values. 

The  principle  of  Gauss  may  also  be  used  in  making  the  com- 
parison on  a  balance  having  unequal  arms  but  this  involves  so 
many  calculations  that  it  is  generally  better  to  use  the  method 
already  described,  or  else  the  method  of  substitution,  the  latter 
being  based  upon  the  principle  of  Borda.  Since  these  methods 
are  independent  of  the  arm  ratio  the  latter  need  not  be  determined. 

Finally  it  may  be  stated  that  the  most  accurate  values  are  ob- 
tained by  comparing  each  piece  with  a  standard  piece  of  the  same 
denomination.  But  as  this  involves  the  use  of  an  accurately 
standardized  complete  set  and  as  such  a  set  is  necessarily  quite 
expensive  this  method  is  generally  impracticable.  Instead,  one 
standard  piece  (usually  a  1-gram  piece)  may  be  used  and  all  other 
values  calculated  from  this.  Of  course  the  unavoidable  experi- 
mental error  is  thus  cumulative  in  the .  larger  pieces  and  the 
work  must  be  done  with  extreme  care  if  the  corrections  are  to  have 
any  real  significance. 

Calibration.  Method  of  Direct  Comparison. — Determine  the  zero 
point  of  the  unloaded  balance.  Place  the  1-gram  piece  marked  (•)  on 
the  left  pan  and  the  one  marked  (••)  on  the  right.  Adjust  the  rider,  if 
necessary,  to  maintain  the  zero  point  at  its  first  position.  If  the  rider 
is  not  required  to  restore  equilibrium  and  if  the  balance  arms  have  been 
found  to  be  of  equal  length,  then  the  two  pieces  are  of  equal  value.  If 
the  rider  is  necessary  on  the  right  arm  the  weight  on  that  side  is  less 
in  value  than  the  one  on  the  left.  If  the  rider  was  used  on  the  left  arm 
the  reverse  is  true.  Record  the  result  as 

1-  =  i-  -f-  or  —  n  milligram, 

n  being  the  difference  between  the  values  of  the  two  pieces.  If  the 
balance  arms  are  unequal  this  relative  value  for  1"  is  to  be  multiplied 

bvr' 
Compare  similarly  the  other  pieces  of  the  set,  as  follows: 


68  QUANTITATIVE  ANALYSIS 

Gram  Pieces 

1-  with  standard  piece,  if  one  is  available 

!•••  with  1-  or  1- 

2  with  1-  +  1- 

5  with  2  +  1-  +  1-  +  1- 

10-  with  5  +  2  +  1-  +  1"  +  I"' 

10-  with  10" 

20  with  10-  +  10" 

50  with  20  +  10-  +  10"  +  5  +  2  +  1-  +  !••  +  1- 

Milligram  Pieces 

500  with  200  +  100-  +  100"  +  50  +  20  +  10-  +  10"  +  5  + 

rider  at  5 

200  with  100-  +  100" 

100-  with  50  +  20  +  10-  +  10"  +  5  +  rider  at  5 

50  with  20  -f-  10-  +  10-  +  5  +rider  at  5 

20  with  10-  +10" 

10-  with  10" 

10-  with  5  +  rider  at  5 

5  with  rider  at  5 

Also  compare  all  milligram  pieces  plus  the  rider  at  5  with  1-  or  with  the 
standard  piece  if  the  latter  has  been  used. 

If  a  standardized  piece  has  been  used  calculate  all  of  the  values  for  the 
other  gram  pieces  from  this;  otherwise  select  the  one  of  the  gram  pieces 
of  the  set  which  appears  to  be  most  nearly  normal  with  respect  to  the 
other  pieces  and  call  this  1.0000  gram,  calculating  values  for  the  rest  of 
the  gram  pieces  upon  this  arbitrary  assumption. 

In  the  case  of  the  milligram  pieces  calculate  provisional  values  for 
each  piece,  starting  with  the  arbitrary  assumption  that  the  smallest 
piece  of  the  set  is  correct.  (The  smallest  piece  is  usually  a  5-  or  10- 
milligram  piece.  Smaller  pieces  are  not  needed  because  the  adjustment 
of  the  rider  provides  smaller  subdivisions.)  Add  these  provisional 
values.  If  the  total  is  not  that  indicated  by  the  comparison  of  the 
collective  milligram  pieces  with  the  chosen  standard  then  the  provi- 

sional value  for  each  piece  is  to  be  multiplied  by  the  factor: 


gum 

This  will  give  the  new  value  for  the  piece,  based  upon  the  standard 
finally  adopted.  The  new  sum  will  equal  the  true  sum  unless  the 
dropping  of  decimals  beyond  the  fourth  place  has  impaired  the  accuracy 
of  the  calculations. 


GRAVIMETRIC  ANALYSIS  69 

The  ratio, »  should  be  calculated  as  far  as  six  decimal  places. 

This  means  that  considerable  work  will  be  involved  in  multiplying  all  of 
the  provisional  values  by  this  ratio.  It  is  much  simpler  (and  sufficiently 
accurate  in  most  cases)  to  use  a  somewhat  different  method  for  changing 
the  provisional  values  to  the  true  values,  thus : 

Subtract  the  provisional  sum  from  the  true  sum,  applying  the  correct 
sign  to  the  difference.  This  gives  the  total  correction  to  be  made. 
Apply  0.5  of  this  correction  to  the  500-milligram  piece,  0.2  to  the  200- 
milligram  piece,  0.1  to  each  of  the  100-milligram  pieces,  and  so  on. 

Method  of  Substitution. — In  calibrating  by  this  method  a  second  set 
of  weights  should  be  provided  to  serve  as  counterpoise  pieces.  This  may 
be  a  low  priced  set  of  ordinary  weights  or  a  worn  out  and  discarded  set 
of  analytical  weights,  since  the  accuracy  of  the  calibration  does  not 
depend  in  any  manner  upon  the  accuracy  of  adjustment  of  the  counter- 
poise. Also  it  is  unnecessary  to  determine  the  zero  point  of  the  unloaded 
balance,  the  only  requirement  being  that  the  adjustment  of  the  rider 
shall  be  made  so  as  to  bring  the  balance  into  equilibrium  about  any 
given  scale  division  as  zero  point  and  to  maintain  this  zero  point  through- 
out the  experiment.  This  point  may  conveniently  be  the  zero  of  the 
scale. 

Place  the  1-gram  piece  marked  (•)  on  the  right  pan  and  a  1-gram 
piece  of  the  counterpoise  set  on  the  left.  Adjust  the  rider  so  as  to  make 
the  pointer  swing  about  the  true  zero  of  the  scale  and  note  the  rider 
position.  Remove  the  piece  from  the  right  pan  and  put  in  its  place  the 
I-gram  piece  marked  (••).  Readjust  the  rider,  if  necessary,  to  restore 
the  equilibrium  of  the  balance.  If  the  rider  position  is  the  same  as 
before  the  pieces  (•)  and  (••)  have  the  same  value.  If  the  rider  was 
moved  to  the  right  in  the  second  experiment  the  piece  (••)  is  lighter 
than  the  piece  (•) ;  if  the  rider  was  moved  to  the  left  the  reverse  is  true. 

In  either  of  the  latter  cases  the  amount  of  rider  shift  is  a  measure  of 
the  numerical  difference  between  the  values  of  the  two  pieces. 

Continue  this  process  of  comparison  of  pieces,  using  the  combinations 
listed  in  the  directions  for  calibration  by  direct  comparison.  In  each 
case  the  pieces  to  be  compared  are  placed  successively  on  the  right  pan,  a 
counterpoise  piece  of  the  same  denomination  being  placed  on  the  left 
pan. 

Calculate  the  true  or  relative  values  for  all  of  the  pieces  of  the  set 
according  to  the  method  already  described. 

Reagents. — One  of  the  most  vexatious  problems  with  which 
the  analyst  has  to  deal  is  that  of  obtaining  reagents  that  are 
sufficiently  pure  to  suit  his  purpose.  Methods  of  manufacture 


70  QUANTITATIVE  ANALYSIS 

are  constantly  being  improved  and  better  chemicals  are  now 
available  than  in  the  past,  but  even  at  this  time  the  reagent 
that  is  assumed  to  be  pure  often  contains  small  quantities  of 
impurities  which  interfere  with  the  accuracy  of  analytical  proc- 
esses. Attempts  have  been  made  by  manufacturers  to  indicate 
on  the  label  the  degree  of  purity.  Thus  "c.  p."  for  " chemically 
pure,"  signifies  a  reagent  containing  no  impurity  in  a  quantity 
that  could  be  detected  by  chemical  tests.  "Com."  for  " com- 
mercial" means  a  crude  unpurified  chemical,  "medicinal" 
sufficiently  pure  for  medicinal  purposes,  "U.  S.  P."  purity  as 
specified  by  the  United  States  Pharmacopoeia,  etc.  Zinc  might 
be  labeled  "arsenic  free"  to  indicate  that  it  could  be  used  without 
a  blank  test  for  a  determination  of  arsenic  by  Marsh's  method, 
or  "iron  free"  so  that  it  could  be  used  for  reducing  solutions 
in  iron  analysis  without  a  blank  test.  If  these  labels  ever  did 
have  any  real  value  they  early  lost  it.  "c.  p."  has  been  made  a 
cover  for  a  multitude  of  shortcomings  in  packages  of  grossly 
impure  reagents.  "Medicinal"  has  meant  little  more  than  that 
the  manufacturer  hoped  that  the  substance  so  marked  might 
be  sold  to  the  unsuspecting  for  medicinal  purposes.  "Silver 
free"  lead  (for  assaying  silver  ores)  is  often  lead  from  which  the 
manufacturer  has  removed  a  certain  fraction  (or  none  at  all) 
of  the  silver  originally  contained  in  it. 

Analyzed  Chemicals. — On  account  of  the  carelessness  evident 
in  preparing  and  labeling  reagents  chemists  have  come  to  practi- 
cally disregard  all  such  indications  of  purported  purity  and  to 
rely  upon  one  or  both  of  two  sources  of  information  regarding 
the  purity  of  reagents.  These  sources  are  the  reputation  that 
the  manufacturer  bears  for  producing  reliable  chemicals  and  the 
chemist's  own  personal  test  of  the  chemicals  themselves.  Many 
manufacturers  have  now  entirely  discarded  the  abbreviation 
"c.  p."  and  publish  on  the  label  a  supposed  analysis  of  the  sub- 
stance contained  in  the  package.  "Analyzed  chemicals"  have 
thus  become  popular,  but  the  inexperienced  chemist  will  make  a 
great  mistake  if  he  forms  the  too  hasty  conclusion  that  the 
analysis  is  always  correct.  It  is  often  very  far  from  being 
correct.  The  passage  of  pure  food  and  drugs  acts  in  this  and 
many  foreign  countries  has  resulted  in  great  improvement  in 
the  matter  of  labeling  chemicals  that  are  to  be  used  for  medicinal 


GRAVIMETRIC  ANALYSIS  71 

purposes,  since  the  label  constitutes  a  legal  guarantee  as  to  the 
contents  of  the  package.  When  these  acts  are  extended  to 
include  the  reagents  used  for  scientific  purposes  the  chemist  will 
have  a  better  opportunity  for  purchasing  chemicals  of  the  degree 
of  purity  of  which  he  can  feel  reasonably  assured.  At  present 
the  only  safe  plan  is  to  make  blank  tests  for  such  impurities  as 
will  interfere  in  the  analysis  to  be  performed. 

Action  upon  Glass. — While  it  will  be  readily  conceded  that  no 
substance  can  be  made  absolutely  pure  yet  certain  reagents 
can  only  with  difficulty  be  made  even  approximately  pure. 
Examples  are  the  strong  bases,  such  as  sodium  hydroxide,  potas- 
sium hydroxide,  ammonium  hydroxide,  barium  hydroxide,  etc., 
which  readily  attack  and  dissolve  glass  so  that  they  are  always 
contaminated  with  silica.  On  this  account  their  solutions  are 
seldom  kept  as  stock  reagents  in  the  laboratory  but  are  made  from 
the  solids  as  required  excepting,  of  course,  ammonium  hydrox- 
ide which  is  a  solution  of  a  gas.  In  such  cases  the  chemist 
will  simply  require  that  interfering  substances  shall  be  absent. 
Basic  solutions  often  contain  precipitated  matter.  The  glass 
bottle  is  first  attacked,  the  solution  accumulating  alkali  silicates. 
These  are  later  hydrolyzed,  causing  the  precipitation  of  hydrated 
silica.  The  rule  must  never  be  forgotten  that  solutions  are  to 
be  filtered  just  before  using  for  analytical  purposes  unless  they 
are  already  quite  clear  and  free  from  sediment. 

Chemical  Glassware. — Glassware  that  is  to  be  used  for 
analytical  work  must  possess  certain  properties  that  are  not 
found  in  ordinary  glass,  (a)  It  must  have  a  very  slight  solubility 
in  water  and  in  solutions  of  acids,  bases  and  salts.  (6)  It  must 
have  a  low  coefficient  of  expansion  and  be  well  annealed,  in 
order  to  withstand  sudden  changes  in  temperature  as  well  as 
mechanical  shock,  (c)  Its  chemical  composition  must  be  adapted 
to  the  work  at  hand  so  that  traces  dissolving  shall  not  affect 
analytical  results.  For  example,  a  lead  glass  should  not  be  used 
for  solutions  in  which  small  amounts  of  lead  are  to  be  determined. 

Prior  to  the  beginning  of  the  world  war  manufacturing  of 
chemical  " resistance"  glassware  had  not  been  developed  to  any 
great  extent  in  America  and  most  of  such  material  was  imported. 
Jena  glass,  which  is  probably  the  best  known  of  imported  ware, 
is  essentially  a  borosilicate  of  sodium,  zinc  and  aluminium. 


72  QUANTITATIVE  ANALYSIS 

The  United  States  Bureau  of  Standards  has  made  an  extensive 
investigation1  of  the  qualities  of  two  kinds  of  imported  resistance 
glassware  (Jena  and  Kavalier)  and  of  five  American  glasses, 
most  of  the  latter  having  been  developed  within  the  past  four 
years,  although  one  or  two  American  glasses  have  had  an  excellent 
reputation  for  a  long  time.  The  qualities  tested  were  (a) 
chemical  composition,  (6)  coefficient  of  expansion,  (c)  internal 
stresses,  (d)  resistance  to  sudden  changes  of  temperature,  (e) 
resistance  to  mechanical  shock  and  (/)  solubility  in  water, 
ammonium  hydroxide,  mixed  solutions  of  ammonium  sulphide 
and  ammonium  chloride,  solutions  of  sodium  phosphate,  sodium 
and  potassium  carbonates,  and  sodium  and  potassium  hydroxides. 

As  a  result  of  this  investigation  it  may  be  stated  that  " — • — 
all  of  the  American  made  wares  tested  are  superior  to  Kavalier 
and  equal  or  superior  to  Jena  ware  for  general  chemical  laboratory 
use." 

In  the  following  pages  Pyrex  glass  will  often  be  specified  but 
it  should  be  understood  that  any  available  resistance  glass  may 
usually  be  substituted. 

Distilled  Water. — Natural  waters  always  contain  dissolved 
matter  which  unfits  them  for  use  in  analytical  work.  Besides 
such  natural  mineral  matter  and  dissolved  gases,  water  will 
always  dissolve  certain  quantities  of  the  container  when  allowed 
to  stand.  In  order  to  remove  dissolved  solids  the  water  is 
distilled  and  recondensed.  Various  forms  of  stills  are  in  use. 
In  any  form  of  such  apparatus  the  vessel  in  which  the  water  is 
boiled  should  be  so  far  separated  from  the  condensing  worm  that 
it  is  impossible  for  any  spray  to  enter  the  latter.  The  boiler 
itself  may  be  of  any  material,  but  the  condensing  worm  should 
be  of  pure  tin,  silver,  or  platinum  because  hot  water  dissolves 
most  other  metals  and  also  glass.  The  cost  of  platinum  of 
course  precludes  its  use  in  any  but  small  stills  that  are  to  be  used 
for  preparing  water  for  exact  investigations,  such  as  are  carried 
out  in  physical  chemical  work.  Pure  tin  is  the  metal  generally 
used  for  the  purpose. 

Distillation  does  not  free  water  from  dissolved  gases  and  for 
work  in  which  carbon  dioxide,  oxygen,  nitrogen,  or  ammonia  will 
interfere  it  is  necessary  to  boil  the  water  immediately  before 

'Bur.  Stand.  Tech.  Paper  107  (1908). 


in 

: 


GRAVIMETRIC  ANALYSIS  73 

using.  Boiling  should  not  be  unduly  prolonged,  since  the  water 
thus  becomes  recontaminated  with  the  material  of  the  contain- 
ing vessel. 

Transfer  of  Liquids. — The  operations  involving  pouring  rea- 
gents from  bottles,  pouring  liquids  into  a  filter  or  pouring  from 
one  vessel  to  another  are  often  so  clumsily  performed  as  to  cause 
a  loss  of  part  of  the  liquid  through  splashing  or  running  down 
the  outside  of  the  pouring  vessel,  thus  vitiating  the  results  of 
the  analysis  or  at  least  producing  a  very  disagreeable  sort  of 
uncleanness  of  the  apparatus.  When  pouring  from  a  bottle 
the  stopper  should  never  be  laid  on  the  desk  but  is  held  between 
the  fingers  of  the  right  hand.  The  bottle  is  then  grasped  in 
ch  a  way  as  to  bring  the  label  under  the  hand  and  then  a  glass 
d  is  held  in  a  vertical  position  against  the  mouth  of  the  bottle. 
The  latter  is  tilted  to  pour  out  the  required  amount  of  liquid,  the 


FIG.  32. — Form  of  label  for  laboratory  reagent  bottle. 

glass  rod  being  kept  in  position  until  the  bottle  is  returned  to  an 
upright  position.  No  liquid  should  run  down  the  outside  of  the 
bottle  in  this  case.  If  a  drop  should  escape  the  label  will  not  be 
marred  because  the  method  of  holding  the  bottle  brings  the  label 
on  the  upper  side  while  pouring.  When  pouring  from  a  beaker 
the  stirring  rod  is  used  in  the  same  way  for  preventing  splashing. 
Records. — In  no  other  part  of  the  analysis  is  system  more 
important  than  in  that  of  records.  Results  are  many  times 
rendered  worthless  by  uncertainty  regarding  the  meaning  of  the 
experimental  figures  or  regarding  the  pieces  to  which  recorded 
weights  belong.  -All  analyses  are  run  at  least  in  duplicate,  some 
in  triplicate.  If  at  any  stage  in  the  work  the  beakers,  crucibles, 
burettes,  or  other  pieces  become  interchanged  or  if  the  recorded 
weights,  volumes,  temperatures,  or  other  data  are  not  properly 


74 


QUANTITATIVE  ANALYSIS 


labeled  and  are  applied  to  the  wrong  pieces,  then  the  calculated 
results  of  the  analysis  are,  of  course,  incorrect.  At  the  very 
outset  the  duplicate  pieces  should  be  numbered  (I  and  II  unless 
other  marks  are  preferred)  in  every  case  where  the  mark  is  not 
objectionable.  It  is  not  advisable  to  mark  by  labels  or  pencil 


SAMPLE  OF_ 


-MARKED 


GRAVIMETRIC  ANALYSIS 


AMOUNT  OF  SAMPLE 
TAKEN 

WEIGHT  OF 

PERCENT 

or 

AMOUNT  OF  SAMPLE 
TAKEN 

WEIGHT  OP 

PERCENT 
OF 

FOUND 

FOUND 

I 

I 

II 

II 

AMOUNT  OF  SAMPLE 

WEIGHT  OF 

PERCENT 

AMOUNT  OF  SAMPLE 

WEIGHT  OF 

PERCENT 

FOUND 

AKEN 

FOUND 

1 

I 

• 

II 

ir 

AVERAGES 


SUBSTANCE  DETERMINED 

PERCENT 

8CB8TANOE  DETERMINED 

PERCENT 

FIG.  33. — A  convenient  blank  for  reporting  the  results  of  a  gravimetric  analysis. 

any  article  that  is  to  be  weighed  because  the  mark  itself  often 
causes  changes  in  weight  through  rubbing  off  or  absorption  of 
moisture.  For  articles  that  are  not  to  be  exactly  weighed  a 
small  label  or  pencil  such  as  is  used  for  marking  glass  and  porce- 
lain may  be  used.  The  latter  has  a  soft  core  composed  of  pig- 


GRAVIMETRIC  ANALYSIS  75 

ment  and  grease  or  paraffin  so  that  it  will  stick  to  glass.  The 
better  grades  of  glassware  are  now  furnished  with  a  spot  rough- 
ened by  sand  blasting,  this  making  it  possible  to  use  a  common 
graphite  pencil  or  a  pen  for  marking  without  a  label.  Even  in 
the  case  of  articles  that  are  not  to  be  marked  they  can  be  easily 
identified  if  the  analyst  follows  the  plan  of  always  keeping  No.  I 
on  the  left  and  No.  II  on  the  right  when  precipitating,  filtering, 
igniting  or  allowing  crucibles  to  stand  in  desiccators 

Reagent  bottles  should  be  marked  by  a  label  bearing  the  name 
of  the  student,  that  of  the  reagent,  the  concentration  of  the  solu- 
tion and  the  desk  number,  as  shown  in  figure  32. 

Many  systems  of  note-book  records  have  been  used  with  greater 
or  less  success.  If  an  ordinary  blank  note  book  is  used  it  is 
necessary  to  so  indicate  each  measured  weight  or  volume  that 
there  is  no  possibility  of  uncertainty  regarding  the  meaning  of 
the  figures.  Loose  sheets  of  paper  must  not,  under  any  circum- 
stances, be  used  for  records  of  a  permanent  character.  Loss  of  such  a 
sheet  has  frequently  caused  the  loss  of  days  or  even  weeks  of 
laboratory  work.  A  better  device  than  that  of  the  blank  paged 
note  book  is  found  in  a  systematic  record  book,  with  spaces  so 
provided  and  lettered  that  mere  recording  figures  is  all  that  is 
necessary.  Such  a  page  for  gravimetric  analysis  is  shown  in  Fig. 
33.  No  indication  of  the  identity  of  weights  or  percents  is 
necessary  beyond  the  filling  of  the  blanks  as  shown. 


CHAPTER  III 
EXPERIMENTAL  GRAVIMETRIC  ANALYSIS 

One  of  the  most  apparent  differences  between  the  methods  of 
study  as  applied  to  qualitative  and  to  quantitative  analysis  lies 
in  the  fact  that  while  in  qualitative  analysis  the  metals  and  acids 
are  grouped  according  to  their  susceptibility  to  the  action  of  cer- 
tain "group  reagents,"  in  quantitative  analysis  widely  different 
reagents  and  methods  are  often  used  for  elements  gr  acids  that 
are  closely  allied  in  most  respects.  A  general  review  of  the 
conditions  that  must  be  fulfilled  for  accurate  gravimetric  analysis 
will  serve  to  show  that  no  such  systematic  classification  as 
is  found  in  qualitative  work  is  desirable  in  quantitative  pro- 
cedure, since  the  reagent  and  method  must  always  be  selected 
which  will  give  the  precipitate  which  is  the  most  insoluble  and 
the  most  easily  separated  and  purified  and  which  assumes  the 
most  definite  form  upon  the  application  of  heat.  For  example, 
calcium,  barium  and  strontium  fall  in  the  same  periodic  as  well 
as  the  same  qualitative  group  and  yet  there  is  no  logical  reason 
for  studying  these  metals  together  in  quantitative  analysis 
because  barium  is  most  conveniently  precipitated  as  sulphate 
and  calcium  as  oxalate  from  water  solutions,  while  strontium  is 
precipitated  as  sulphate  from  solutions  containing  alcohol. 
Barium  sulphate  is  stable  when  ignited,  and  is  weighed  as  such. 
Calcium  oxalate  decomposes  and  is  weighed  as  carbonate  or  oxide 
while  strontium  sulphate  is  stable  and  is  weighed  in  this  form. 
Any  one  of  these  metals  could  be  precipitated  by  ammonium  car- 
bonate and  the  carbonate  changed  to  the  oxide  by  ignition  but 
this  would  not,  in  any  case,  prove  to  be  the  most  convenient 
or  the  most  accurate  method. 

In  view  of  these  facts  it  becomes  desirable  to  select,  for  each 
element  or  radical,  that  method  which  is,  under  the  circum- 
stances, the  most  easily  executed  and  the  most  accurate  and  to 
study  these,  not  in  order  of  qualitative  or  periodic  groups,  but 

7(3 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  77 

in  that  order  which  will  best  develop  manipulative  skill  and 
.  accuracy  in  experimental  work.  Many  methods  that  were 
formerly  used  have  become  almost  obsolete  because  of  the 
development  of  better  apparatus  or  more  rapid  methods.  In 
such  cases  the  older  methods  will  generally  receive  mention  at 
the  proper  place  and  the  more  modern  method  will  be  described 
more  fully. 

Apparatus. — Most  of  the  ordinary  apparatus  with  which  the 
quantitative  laboratory  is  usually  stocked  is  already  more  or 
less  familiar  to  the  student.  Special  forms  of  apparatus  will  be 
described  in  connection  with  the  determinations  for  which 
they  are  to  be  used.  At  the  beginning  of  the  course  all  glassware 
should  be  thoroughly  cleaned  and  other  apparatus  should  be  put 
in  first  class  order.  A  principle  that  should  never  be  forgotten 
is  that  both  accuracy  in  results  and  speed  in  working  are  pro- 
moted by  following  the  practice  of  cleaning  apparatus  as  soon 
as  its  use  in  a  given  experiment  is  finished,  so  that  it  will  be  ready 
(with  perhaps  a  single  rinsing  with  distilled  water)  for  the  next 
operation  that  may  demand  it.  The  student  who  works  with 
a  desk  full  of  soiled,  broken  or  disorderly  apparatus  or  with 
spilled  chemicals  scattered  over  the  tables  may  as  well  make  up 
his  mind  at  the  outset  that  he  will  never  be  an  analyst  of  any 
useful  sort. 

After  desk  apparatus  has  been  invoiced  and  cleaned,  two  desiccators 
like  that  shown  in  Fig.  11  are  prepared  for  use.  They  are  first  made 
clean  and  dry  and  then  a  layer,  one-half  inch  thick,  of  fused,  granular 
calcium  chloride  is  placed  in  the  bottom  and  a  short  piece  of  sodium 
hydroxide  is  placed  upon  this.  The  former  keeps  the  atmosphere 
within  the  desiccator  free  from  moisture  while  the  sodium  hydroxide 
absorbs  carbon  dioxide  and  traces  of  acid  vapors  that  sometimes  come 
from  the  calcium  chloride.  The  upper  part  of  the  desiccator  is  again 
wiped  with  a  dry  towel  to  remove  calcium  chloride  dust  and  a  clay  or 
alloy  triangle  is  bent  so  as  to  lie  freely  on  the  shoulder  above  the 
calcium  chloride.  (Perforated  porcelain  plates  are  sometimes  used 
instead  of  triangles  for  this  purpose.)  A  thin  film  of  vaseline  is  rubbed 
on  the  ground  joint  of  the  cover  and  the  latter  is  then  worked  down 
until  it  fits  well,  the  surplus  vaseline  being  removed  from  the  edge. 
The  desiccator  is  now  ready  for  use. 

Prepare  also  two  wash  bottles  (one  each  for  hot  and  cold  water) 
like  the  one  illustrated  in  Fig.  7,  using  liter  flasks  for  the  purpose.  The 


78  QUANTITATIVE  ANALYSIS 

neck  of  the  one  that  is  to  be  used  for  hot  water  should  be  wrapped  with 
cotton  cord  or  sheet  cork. 

A  number  of  stirring  rods,  4  to  6  inches  long,  should  be  made  by 
fusing  and  rounding  the  ends  of  glass  rod  or  tubing.  One  or  two  of 
these,  tipped  with  pieces  of  rubber  tubing  having  an  end  cemented 
together,  are  to  be  used  for  loosening  precipitates  from  beakers  and 
dishes.  These  are  known  as  the  chemist's  "policemen." 

CALCIUM 

Calcium  may  be  precipitated  from  ammoniacal  solution  as 
carbonate  by  alkali  carbonates  or  as  oxalate  by  alkali  oxalates. 
Since  ammonium  carbonate  or  oxalate  yields  volatile  ammonium 
salts  as  byproducts  in  the  reaction  and  since  traces  of  these  will 
be  expelled  upon  ignition  if  not  completely  washed  out  of  the 
precipitate,  the  ammonium  salts  are  always  used  in  preference 
to  those  of  sodium  or  potassium. 

Solubility. — The  solubility  of  calcium  carbonate  in  water  was 
determined  by  Kohlrausch  and  Rose1  by  conductivity  experi- 
ments and  was  found  to  be  0.013  gm  (corresponding  to  0.0052 
gm  of  calcium)  per  liter  at  18°,  but  the  solubility  is  considerably 
increased  by  ammonium  salts,  such  as  must  be  present  when 
ammonium  carbonate  reacts  with  a  calcium  salt  in  solution.  The 
solubility  of  calcium  oxalate  in  water  was  found  by  Kohlrausch 
and  Rose  to  be  0.0056  gm  of  the  crystallized  salt,  CaC2O4  H2O 
corresponding  to  0.0015  gm  of  calcium)  per  liter.  The  solubility 
is  considerably  reduced  by  an  excess  of  ammonium  oxalate.  On 
account  of  this  difference  in  solubi  ity  the  oxalate  method  is 
preferable.  Ammonium  chloride  should  be  present  in  either 
case  to  prevent  precipitation  of  traces  of  magnesium.  The 
reaction  involved  in  the  precipitation  may  be  expressed  as  follows: 

CaCl2+  (NH4)2C204-»CaC204+2NH4Cl. 

Ammonium  oxalate  does  not  readily  dissolve  in  water,  the 
saturated  solution  at  0°  containing  2.2  percent  of  the  salt.  As 
the  temperature  is  raised  the  solubility  is  increased  so  that  a 
saturated  solution  at  20°  is  easily  made  by  heating  in  contact 
with  an  excess  of  the  salt  and  allowing  to  cool.  It  is  not  best 
to  keep  a  stock  solution  of  ammonium  oxalate  because  it  readily 
*Z.  physik.  Chem.,  12,  234  (1893);  44,  197  (1903). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  79 

undergoes  hydrolysis,  yielding  ammonium  hydroxide  which 
attacks  the  glass,  and  because  there  occurs  a  decomposition  in 
solution  as  follows: 


It  is  preferable  to  make  a  small  amount  of  the  solution  as  needed 
for  the  determination. 

A  difficulty  that  is  often  encountered  by  the  inexperienced 
analyst  is  the  formation  of  a  precipitate  of  calcium  oxalate  which 
is  so  finely  crystalline  that  it  passes  through  the  pores  of  the 
filter,  making  its  complete  separation  impossible.  Refiltration 
of  the  portion  that  has  passed  through  will  partially  remedy  this 
trouble  but  when  a  precipitate  is  found  to  be  too  finely  divided 
for  filtration  the  only  satisfactory  cure  is  found  in  digestion. 
Certain  grades  of  filter  paper  are  made  for  filtering  fine  precipi- 
tates, their  structure  being  very  dense.  This  renders  filtration 
less  rapid  than  would  be  the  case  with  papers  of  ordinary  density. 
The  difficulty  nearly  or  quite  disappears  if  the  proper  conditions 
are  observed  during  the  precipitation.  It  has  been  explained 
(page  20)  that  too  rapid  precipitation  causes  the  formation  of  a 
large  number  of  small  particles  rather  than  a  small  number  of 
large  particles.  Two  conditions  are  found  to  be  suitable  for  the 
formation  of  large  crystals  of  calcium  oxalate:  (1)  boiling  tem- 
perature for  solutions  of  calcium  salt  and  ammonium  oxalate 
and  (2)  slow  addition  of  reagent.  These  conditions  will  be 
elaborated  in  the  directions  for  the  determination. 

Calcium  may  also  be  determined  by  precipitating  as  sulphate 
from  a  solution  containing  alcohol1  or  volumetrically  by  pre- 
cipitating as  oxalate  and  titrating  with  a  standard  solution  of 
potassium  permanganate.  The  latter  method  will  be  described 
in  the  section  dealing  with  volumetric  analysis. 

Calcium  precipitated  as  either  carbonate  or  oxalate  may  be 
weighed  as  carbonate,  oxide  or  sulphate.  Calcium  oxalate 
decomposes  as  follows,  on  the  application  of  heat: 

CaC204->CaC03+CO,  (1) 

CaC03-*CaO+C02.  (2) 

1  Stolberg:  Z.  angew.  Chem.,  17,  269  (1904). 


80  QUANTITATIVE  ANALYSIS 

Reaction  (1)  begins  at  quite  low  temperatures.  Reaction  (2) 
begins  as  low  as  500°  but  requires  long  heating  at  this  tempera- 
ture to  complete  the  decomposition.  In  practice  the  highest 
temperature  attainable  by  the  blast  lamp  is  applied  and  heating 
is  continued  until  no  further  loss  in  weight  occurs.  Calcium 
oxide  is  not  reduced  by  hot  carbon  and  the  precipitate  may  be 
heated  without  removing  from  the  paper.  Many  chemists 
prefer  to  ignite  the  oxalate  at  a  low  temperature  and  to  weigh  as 
carbonate  but  this  procedure  is  of  doubtful  utility,  even  for  an 
experienced  analyst,  on  account  of  the  difficulty  in  stopping  the 
decomposition  at  a  point  where  all  the  oxalate  has  disappeared 
and  no  oxide  has  been  formed.  It  is  also  possible  to  add  a  few 
drops  of  sulphuric  acid  to  the  crucible  containing  the  calcium 
oxalate  and  to  weigh  the  resulting  calcium  sulphate,  after  gentle 
ignition  to  expel  the  oxalic  acid  and  the  excess  of  s\ilphuric  acid. 
The  most  important  source  of  error  in  this  procedure  comes 
from  the  loss  by  spattering,  a  loss  which  even  the  greatest  care 
can  scarcely  prevent.  There  is  also  danger  of  decomposing  cal- 
cium sulphate  by  strong  heating: 

CaS04->CaO+S03. 

It  is  much  preferable  to  heat  strongly  the  precipitate  converting 
it  quantitatively  into  calcium  oxide.  The  completion  of  the 
conversion  can  be  judged  by  the  constancy  in  the  weight  of  the 
substance  upon  further  heating.  Calcium  oxide  readily  absorbs 
moisture  and  carbon  dioxide  when  exposed  to  the  air  and  the 
resulting  change  of  weight  will  become  appreciable  if  the  process 
of  weighing  is  unduly  prolonged.  If  the  weight  of  the  crucible 
and  oxide  is  approximately  known  most  of  the  weights  may  be 
placed  on  the  balance  pan  before  the  crucible  is  removed  from 
the  desiccator  and  the  remainder  of  the  process  completed  in  a 
short  time.  It  is  a  good  plan  to  keep  a  small  piece  of  potassium 
hydroxide  in  the  desiccator  in  which  calcium  oxide  is  to  be  pre- 
served. This  lessens  the  absorption  of  carbon  dioxide  by 
keeping  the  atmosphere  free  from  that  gas. 

The  converse  of  this  method  may  be  used  for  the  determina- 
tion of  the  oxalate  radical,  precipitation  being  made  by  a  soluble 
calcium  salt  in  basic  solution.  Volumetric  methods  are,  how- 
ever, preferable. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  81 

Determination. — Fill  a  clean  dry  weighing  bottle  with  the  calcium 
compound  to  be  analyzed.  Provide  two  clean  beakers  of  Pyrex  or  other 
resistance  glass,  having  a  capacity  of  250  cc,  and  mark  them  I  and  II. 
If  the  substance  is  of  such  nature  that  it  is  altered  in  any  way  by  free 
exposure  to  air  the  sample  to  be  used  must  be  weighed  by  difference  as 
follows:  Place  the  bottle  on  the  balance  pan,  using  for  this  purpose  a 
pair  of  crucible  tongs,  having  short  pieces  of  rubber  tubing  drawn  over 
the  tips,  and  carefully  weigh.  Record  this  weight  in  the  data  book  at 
the  top  of  the  space  marked  for  sample  I,  reading  the  weights  as  directed 
on  page  62.  Carefully  remove  the  stopper,  holding  over  beaker  marked 
I,  and  pour  what  is  judged  to  be  between  0.2  gm  and  0.5  gm  into  the 
beaker.  Replace  the  stopper,  taking  great  care  that  any  falling  par- 
ticles drop  into  the  beaker  and  are  not  lost,  then  reweigh  the  bottle  and 
contents.  Record  this  weight  under  the  first,  also  at  the  top  of  the 
space  for  sample  II.  Remove  a  second  portion  to  beaker  II  and 
reweigh  the  bottle,  recording  under  the  preceding  weight.  Subtracting 
the  less  weights  from  the  greater  gives  the  weights  of  sample  used. 

If  the  substance  is  known  to  be  unaffected  by  contact  with  air  it  may 
be  poured  into  one  of  the  counterpoised  glasses  and  weighed  directly, 
being  then  brushed  into  the  beaker  by  the  small  pencil  brush  of  camel's 
hair. 

After  having  weighed  the  two  samples  for  analysis  determine,  by 
qualitative  tests  on  another  portion  of  the  substance,  whether  it  is  solu- 
ble in  water  and,  if  not,  whether  in  dilute  hydrochloric  acid.  If  soluble 
in  water  dissolve  in  about  100  cc  of  distilled  water,  add  5  cc  of  10  percent 
ammonium  chloride  solution  and  treat  each  sample  as  directed  below. 
If  insoluble  in  water  but  soluble  in  hydrochloric  acid  first  moisten 
each  sample  with  water  then  cover  the  beakers  with  glasses  and  carefully 
add  about  20  cc  of  dilute  acid.  After  effervescence  has  ceased  rinse 
down  the  cover  glass  and  the  sides  of  the  beaker  with  water  from  the 
wash  bottle,  dilute  to  about  100  cc  and  gently  boil  for  one  minute  to 
expel  dissolved  carbon  dioxide.  From  this  point  the  procedure  is  the 
same  as  for  water  soluble  salts. 

Prepare  ammonium  oxalate  solution  by  heating  to  boiling  5  gm  of 
powdered  ammonium  oxalate  and  100  cc  of  water  in  a  Pyrex  beaker. 
Part  of  the  salt  will  crystallize  when  cooled,  leaving  a  saturated  solution. 
Add  dilute  ammonium  hydroxide  (filtered  unless  already  quite  clear)  to 
the  solution  of  calcium  salt  until  the  liquid  smells  very  distinctly  of 
ammonia.  In  determining  this  point  the  ammonia  that  is  already  in  the 
air  above  the  liquid  must  be  blown  away  before  testing  the  odor.  Heat 
to  boiling  and  add,  drop  by  drop  from  a  pipette,  20  cc  of  the  recently 
prepared  ammonium  oxalate  solution,  stirring  vigorously  during  the 
addition.  If  this  is  carefully  done  the  precipitate  should  settle  readily, 


82  QUANTITATIVE  ANALYSIS 

leaving  a  clear  liquid  above.  When  this  is  the  case  add  a  few  drops 
more  of  ammonium  oxalate  solution,  observing  whether  any  precipitate 
forms.  If  so,  more  reagent  must  be  added  in  the  same  manner  as  at 
first.  When  precipitation  has  been  shown  to  be  complete  the  liquid  is 
digested  at  a  temperature  somewhat  below  the  boiling-point  over  a 
low  flame  or  on  the  steam  bath  for  one-half  hour  or  until  the  super- 
natant liquid  is  quite  clear,  when  it  is  ready  for  filtration.  The  odor 
of  ammonia  should  still  be  easily  perceptible  at  this  stage. 

Prepare  two  filters  of  extracted  paper,  marking  the  funnels  I  and  II. 
Carefully  decant  each  solution,  while  hot,  upon  the  proper  filter,  allowing 
to  run  through  into  clean  beakers.  Observe  the  directions  given  on 
page  73  for  proper  method  of  pouring  from  beakers.  Before  completing 
the  filtration  a  few  drops  of  ammonium  oxalate  solution  should  be  added 
to  the  clean  filtrate  that  has  already  run  through.  If  a  precipitate 
forms,  the  filter  must  be  well  washed;  the  filtrate  and  washings  returned 
to  the  beaker  in  which  precipitation  was  made  and  more  reagent  added 
in  the  same  manner  as  before,  until  precipitation  is  complete.  When  no 
precipitate  is  produced  in  the  filtrate,  complete  the  filtration,  washing 
the  precipitate  into  the  filter  by  a  stream  from  the  hot  water  bottle, 
rubbing  the  beakers  with  a  glass  rod  tipped  with  rubber  tubing  (a 
"policeman  ").  Wash  the  precipitate  on  the  filter  until  a  small  amount 
of  the  last  washings  fails  to  give  more  than  a  faint  precipitate  with 
silver  nitrate  and  a  drop  of  nitric  acid  showing  that  chlorides  have  been 
removed. 

Allow  the  paper  to  drain  then  remove  from  the  funnel,  fold  as  directed 
on  page  34  and  place  in  a  previously  ignited  and  weighed  porcelain  or 
platinum  crucible.  The  cover  should  have  been  weighed  with  the  cruci- 
ble because  the  closed  crucible  will  hinder  absorption  of  moisture  and 
carbon  dioxide  while  weighing.  The  crucible  is  carefully  heated  by 
the  burner  until  the  moisture  has  been  expelled  and  smoking  begins. 
The  cover  may  now  be  removed  and  placed  on  a  clean  tile  while  the 
crucible  is  heated  more  strongly  until  the  paper  is  completely  charred. 
The  crucible  is  now  placed  on  its  side,  the  cover  adjusted  and  complete 
oxidation  of  the  carbon  of  the  paper  is  accomplished  as  described  on 
page  36.  The  crucible  is  then  placed  in  an  upright  position,  is  covered 
and  subjected  to  the  hottest  flame  available  from  the  blast  lamp. 
This  ignition  is  continued  for  30  minutes,  when  the  crucible  is  placed 
in  the  proper  desiccator,  allowed  to  cool  to  the  temperature  of  the 
room  and  quickly  weighed.  It  is  ignited  for  10  minutes  longer,  cooled 
and  reweighed.  If  there  is  a  decrease  in  weight  of  more  than  0.0003 
gm  the  crucible  is  reheated  for  10  minutes  and  weighed,  the  process 
of  heating  and  weighing  being  continued  until  the  weight  remains 
constant  within  the  limit  given  above.  The  preliminary  weighings 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  83 

should  be  recorded  upon  the  back  of  the  sheet  preceding  the  one  used 
for  the  final  record,  the  final  weight  being  recorded  in  the  proper 
blank  space. 

Calculate  the  percent  of  calcium  in  the  sample,  using  the  factor 
already  calculated  and  recorded  on  page  9,  and  using  a  table  of  loga- 
rithms for  the  arithmetical  work.  Do  not  discard  the  ignited  product 
until  after  the  report  has  been  accepted.  Errors  may  have  been  made 
which  can  be  corrected  if  this  has  been  preserved.  This  is  a  principle 
that  should  be  observed,  when  possible,  in  all  analytical  work. 

SILVER 

Silver  might  be  gravimetrically  determined  as  chloride, 
bromide  or  iodide.  The  solubilities  of  these  salts  in  water, 
shown  in  the  following  table,  were  determined  by  Kohlrausch 
and  Rose,1 

Milligrams  per  liter,  soluble  at  18° 


Salt 

Silver  equivalent  to  salt 

Silver  chloride  

0  0017 

0  0013 

Silver  bromide  

0.0004 

0.00023 

Silver  iodide.  . 

0  0001 

0  00005 

From  the  comparative  solubilities  one  might  conclude  that 
silver  chloride  is  the  least  desirable  form  in  which  to  weigh  silver. 
The  bromide  and  iodide,  however,  are  much  more  sensitive  to 
the  action  of  light,  being  more  readily  decomposed  into  sub- 
halides  with  liberation  of  free  halogen.  The  stabilities  of 
these  salts  are  in  the  same  relation  to  each  other  as  are  those 
of  halides  of  other  metals  and  of  hydrogen.  On  this  account 
the  gravimetric  determination  of  silver  is  invariably  made  by 
weighing  silver  chloride,  using  hydrochloric  acid  as  the  precipi- 
tating reagent.  Conversely  the  determination  of  chloranion 
is  made  by  using  a  soluble  silver  salt  as  the  reagent.  A  small 
excess  of  either  a  soluble  silver  salt  or  a  soluble  chloride  greatly 
diminishes  the  solubility  of  silver  chloride  as  explained  in  the 
section  dealing  with  the  principles  of  precipitation.  If  more 
than  a  very  slight  excess  of  a  metal  chloride  is  present  the  solu- 

1  Z.  physik.  Chem.,  12,  234  (1893). 


84  QUANTITATIVE  ANALYSIS 

bility  of  silver  chloride  is  increased,  because  of  the  formation 
of  soluble  double  salts.  On  this  account  hydrochloric  acid 
is  used  as  the  precipitant  for  silver. 

Silver  chloride  shows  a  well  defined  tendency  toward  the  for- 
mation of  a  hydrosol  in  cold  water  and  when  this  is  formed  the 
solubility  follows  no  definite  rule.  The  sol  can  be  flocculated 
by  boiling  with  dilute  acids  or  other  electrolytes. 

The  precipitate  of  silver  chloride  is  affected  by  light  as  are 
the  other  silver  halides,  it  being  reduced  to  a  subchloride,  Ag2Cl; 
with  liberation  of  free  chlorine.  The  darkening  of  silver  chloride 
under  the  influence  of  strong  light  is  due  to  the  appearance  of 
the  subchloride  which  is  bluish  black  in  color.  While  some 
decomposition  undoubtedly  occurs  in  daylight  of  any  intensity, 
if  the  precipitation  is  performed  in  the  darker  parts  of  the  room 
ordinary  diffused  light  will  not  appreciably  affect  the  weight  of 
the  precipitate  in  a  short  time. 

The  precipitate  cannot  be  ignited  in  contact  with  filter  paper 
on  account  of  the  ease  with  which  it  is  reduced  to  metallic  silver. 
Use  can  be  made  of  any  of  the  devices  for  dealing  with  such  pre- 
cipitates, as  mentioned  in  the  general  discussion  of  the  ignition 
of  precipitates;  we  may  here  follow  either  the  method  of  removing 
the  precipitate  from  the  filter  paper  or  the  method  involving 
the  use  of  the  Gooch  crucible,  both  methods  being  described. 
Whenever  silver  chloride  is  heated,  care  must  be  taken  to  prevent 
a  rise  of  temperature  above  the  point  of  fusion,  which  is  451°, 
since  it  is  sensibly  volatile  at  high  temperatures. 

By  making  the  proper  changes  in  procedure  the  halogens  may 
be  determined,  silver  nitrate  being  used  as  the  precipitating 
reagent.  These  determinations  are  discussed  later  (page  87). 

Determination. — Fill  a  stoppered  weighing  bottle  with  the  powdered 
silver  salt  and  weigh  out  two  samples  of  about  0.3  gm  each  for  analy- 
sis, placing  in  250-cc  beakers,  which  may  be  of  ordinary  hard  glass. 
The  weighing  may  be  done  upon  counterpoised  glasses  or  from  the  weigh- 
ing bottle,  by  following  the  directions  given  for  the  determination  of 
calcium.  In  this,  as  in  all  other  cases,  care  must  be  exercised  to  avoid 
spilling  any  of  the  substance  upon  the  balance  pan,  as  this  invariably 
results  in  corrosion  of  the  pan. 

Dissolve  the  weighed  sample  in  about  100  cc  of  water,  and  heat 
nearly  to  boiling.  Add,  with  stirring,  5  cc  of  dilute  hydrochloric  acid 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  85 

and  digest  on  the  steam  bath  until  the  precipitate  is  completely  floccu- 
lated and  the  supernatant  liquid  is  quite  clear.  Test  the  clear  liquid 
with  more  hydrochloric  acid  as  soon  as  is  practicable,  to  determine 
whether  precipitation  is  complete.  When  the  precipitate  settles  com- 
pletely proceed  by  one  of  the  following  methods. 

(a)  Filtration  on  Paper. — Filter  on  extracted  paper,  with  or  without 
slight  suction,  and  wash  with  hot  water  containing  1  cc  of  dilute  nitric 
acid  in  each  100  cc  of  water,  the  acid  being  used  in  order  to  prevent  the 
silver  chloride  from  returning  to  the  condition  of  a  hydrosol  and  thus 
passing  through  the  filter.  Wash  until  free  from  chlorides,  testing  the 
washings  with  silver  nitrate.  Allow  the  precipitate  to  drain,  then 
remove  the  paper,  fold  over  the  top  and  sides  (see  page  34),  place  on  a 
watch  glass  and  dry  in  an  oven  at  100°. 

When  the  precipitate  and  paper  are  completely  dried,  place  a  piece 
of  black  glazed  paper  on  the  desk,  unfold  the  filter  paper  and  carefully 
detach  as  much  as  possible  of  the  precipitate,  using  a  spatula  for  this 
purpose  and  allowing  the  precipitate  to  fall  upon  the  central  portion  of 
the  glazed  paper.  While  it  is  desirable  to  leave  as  little  as  possible  of 
the  precipitate  upon  the  filter  paper  it  is  also  essential  that  no  paper 
fiber  be  removed  with  the  main  portion  of  the  precipitate  since  this 
portion  is  not  to  be  treated  to  reconvert  reduced  silver  into  silver  chloride. 
With  the  small  camel's  hair  brush  the  precipitate  is  now  brushed  into  a 
pile,  loosening  any  particles  that  may  have  been  caught  by  the  brush, 
and  is  covered  with  a  watch  glass.  An  ignited  and  weighed  crucible  is 
placed  upon  one  corner  of  the  glazed  paper.  The  filter  paper  is  refolded 
in  the  same  manner  as  before,  is  rolled  into  a  tight  roll  and  a  stiff  plati- 
num wire  is  coiled  around  it  in  a  manner  such  that  the  roll  can  be  held 
over  the  crucible  by  means  of  the  wire.  Being  held  in  this  position  it  is 
touched  with  the  oxidizing  flame  of  the  gas  burner  until  the  paper  ignites. 
The  gas  flame  is  to  be  used  only  often  enough  to  keep  the  paper  ignited 
and  the  outer  oxidizing  portion  of  the  flame  is  always  to  be  used  for 
this  purpose.  The  paper  is  thus  burned,  the  ash  falling  into  the  crucible, 
where  the  combustion  is  completed  at  a  low  temperature.  Some  silver 
has  been  reduced  even  with  these  precautions.  To  change  this  again 
into  silver  chloride  the  ash  is  moistened  with  a  drop  or  two  of  concentrated 
nitric  acid  and,  after  a  few  minutes,  a  drop  of  concentrated  hydrochloric 
acid  is  added.  The  reduced  silver  is  first  dissolved  by  the  nitric  acid, 
forming  silver  nitrate,  and  this  is  changed  to  silver  chloride  by  the 
hydrochloric  acid. 

Evaporate  the  acids  by  placing  the  crucible  on  a  water  bath,  then 
carefully  brush  into  the  crucible  the  main  portion  of  the  precipitate. 
Heat  gently  over  the  burner  until  the  precipitate  shows  the  first  appear- 
ance of  fusion  where  it  is  in  contact  with  the  sides  of  the  crucible.  In 


86  QUANTITATIVE  ANALYSIS 

case  the  precipitate  has  been  unduly  reduced  by  light  or  if  it  becomes 
reduced  when  heated,  on  account  of  cellulose  derived  from  the  paper, 
it  should  be  moistened  by  nitric  acid  and  hydrochloric  acid,  as  directed 
above,  and  warmed,  when  it  will  usually  become  white,  after  which 
the  stronger  heating  to  the  fusion  point  may  be  performed.  Place  the 
crucible  in  the  desiccator  and  weigh  as  soon  as  cool.  Calculate  the  per- 
cent of  silver  in  the  salt,  using  the  factor  for  silver  in  silver  chloride  as 
already  calculated  in  the  table  of  factors  on  page  9. 

(6)  Filtration  on  a  Gooch  Crucible. — A  device  which  is  similar  to  that 
shown  on  page  24  is  used  for  applying  the  suction.  Place  a  porcelain 
Gooch  crucible  in  the  holder,  apply  the  suction  and  pour  into 
the  crucible  a  suspension  of  purified  and  shredded  asbestos  until 
a  mat  about  1  mm  thick  is  obtained  on  the  bottom  of  the  crucible. 
Asbestos  to  be  used  for  this  purpose  should  have  been  prepared  by  digest- 
ing for  one  hour  with  concentrated  hydrochloric  acid  to  dissolve 
any  acid-soluble  material,  then  washing  with  distilled  water  until  free 
from  chlorides.  The  desirable  thickness  of  the  mat  in  the  crucible 
will  depend  somewhat  upon  the  character  of  the  asbestos  fiber;  if  the 
latter  has  a  fine  texture  a  closer  felt  will  result,  with  a  consequent 
increase  in  the  efficiency  of  the  filter. 

Draw  all  surplus  water  from  the  filter  by  means  of  the  pump,  then 
rinse  once  with  redistilled  alcohol.  This  is  for  the  purpose  of  promoting 
rapid  drying.  Remove  the  crucible  from  the  holder,  wipe  the  outside 
then  dry  in  an  oven  at  105°  to  110°  for  30  minutes.  Cool  in  a  desiccator 
and  weigh.  Heat  again  in  the  oven  for  30  minutes,  cool  and  weigh. 
This  weight  will  usually  be  the  same  as  the  first  but  if  there  is  a  decrease 
of  more  than  0.3  to  0.5  mg  the  heating  and  weighing  must  be  continued. 
When  the  weight  of  the  crucible  has  become  constant  replace  the 
crucible  in  the  holder  and  again  apply  suction.  Carefully  filter  the 
solution  from  which  the  silver  has  been  precipitated,  finally  transferring 
the  entire  precipitate  to  the  filter.  Wash  and  test  the  washings  as 
directed  for  the  method  of  filtering  on  paper.  Finally  rinse  once  with 
alcohol,  dry  to  constant  weight  at  105°  to  110°  and  calculate  the  per- 
cent of  silver  in  the  sample. 

This  method  is  usually  to  be  preferred  to  the  one  first  described.  The 
chief  source  of  error  is  in  the  loss  of  asbestos  during  filtration  and 
washing.  This  may  be  prevented  by  proper  preparation  of  the  asbestos 
suspension  before  using,  the  fine  material  being  removed  by  sedimenta- 
tion and  decantation.  Even  when  this  has  been  done  it  is  necessary  to 
keep  the  suction  applied  at  all  times  during  filtration  and  washing,  the 
asbestos  thus  being  held  down  in  the  felt. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  87 

CHLORIDES  (BROMIDES  AND  IODIDES) 

The  method  for  the  determination  of  the  halogen  of  halides  is 
the  converse  of.  the  one  just  described,  silver  nitrate  being  used 
as  the  precipitating  reagent.  Nitric  acid  must  be  present  to 
prevent  the  precipitation  of  salts  of  silver,  other  than  the  halide. 

Determination. — From  a  stoppered  bottle  weigh  into  250-cc  beakers 
duplicate  samples  of  about  0.2  gm  of  the  chloride,  bromide  or  iodide. 
Dissolve  in  about  100  cc  of  distilled  water  and  add  1  cc  of  dilute  nitric 
acid.  Both  distilled  water  and  nitric  acid  must  be  tested  and  found 
free  from  chlorides.  Heat  the  solution  to  near  boiling  and  add,  slowly 
from  a  pipette,  a  5  percent  solution  of  silver  nitrate  until  no  further 
precipitate  is  produced.  Digest  on  the  steam  bath  until  the  precipitate 
flocculates  readily,  leaving  a  clear  solution.  Test  the  clear  liquid  with 
another  drop  of  silver  nitrate  solution  to  insure  complete  precipitation. 

For  the  nitration  either  of  the  methods  described  for  the  determination 
of  silver  may  be  used  but  the  Gooch  crucible  is  recommended,  especially 
for  the  determination  of  halogens  other  than  chlorine.  Filter  the  solu- 
tion and  wash  free  from  silver  with  chloride-free  distilled  water  contain- 
ing nitric  acid,  testing  the  washings  at  the  last  with  a  drop  of  dilute 
hydrochloric  acid.  The  preparation  of  the  filter  and  the  treatment  of  the 
precipitate  on  the  filter  are  to  be  exactly  as  described  above  for  the 
determination  of  silver.  Calculate  the  percent  of  halogen  in  the  sample. 

The  precipitate  must  be  protected  from  the  action  of  light,  especially 
in  the  case  of  silver  bromide  or  iodide,  these  substances  being  much 
more  sensitive  to  light  than  the  chloride. 

ALUMINIUM 

The  gravimetric  determination  of  aluminium,  as  well  as  of 
iron,  chromium,  nickel,  cobalt  and  copper,  may  be  accomplished 
by  precipitating  them  as  hydroxides,  igniting  and  weighing  these 
as  oxides.  For  reasons  that  will  presently  be  discussed  all  of 
these  metals  excepting  aluminium  are  now  usually  determined 
by  volumetric  or  electrolytic  processes. 

Aluminium  is  precipitated  as  hydroxide  by  any  basic  solution, 
whether  it  be  that  of  a  pure  base  or  of  a  hydrolyzed  alkali  salt 
of  a  weak  acid,  such  as  sodium  carbonate  or  ammonium  sul- 
phide. Aluminium  hydroxide  readily  forms  hydrosols  which 
are  flocculated  by  the  addition  of  electrolytes,  which  must  for 
this  purpose  be  inorganic  salts.  Since  the  flocculated  hydroxide 
is  also  of  a  colloidal  nature  (hydrogel)  it  manifests  the  phenome- 


88  QUANTITATIVE  ANALYSIS 

non  of  adsorption  to  a  marked  degree  and  the  inorganic  salts 
are  consequently  washed  out  with  considerable  difficulty.  For 
this  reason,  as  well  as  for  other  and  more  important  ones,  am- 
monium hydroxide  is  chosen  as  the  precipitant  because  the  by- 
products of  the  reaction  will  thereby  be  ammonium  salts  and  re- 
maining traces  will  be  volatilized  when  the  precipitate  is  heated. 

Solubility  in  Bases.  —  Aluminium  hydroxide,  besides  being 
soluble  as  a  hydrosol,  also  dissolves  in  solutions  of  bases.  It 
thus  happens  that  if  an  excess  of  the  basic  precipitant  is  in- 
advertently added,  part  or  all  of  the  precipitate  returns  to  the 
solution,  the  amount  dissolved  depending  upon  the  excess  and 
ionization  of  precipitant.  The  strong  bases,  as  sodium  or 
potassium  hydroxide,  dissolve  aluminium  hydroxide  more  readily 
than  the  weaker  ones  and  this  furnishes  a  second  ^reason  for  the 
use  of  the  weaker  base,  ammonium  hydroxide,  as  the  precipitating 
reagent.  In  case  an  excess  of  this  has  been  added  it  is  possible 
to  remove  it  by  boiling. 

The  solvent  action  of  bases  has  been  explained  upon  the  sup- 
posed ability  of  aluminium  hydroxide  to  exist  in  both  acid  and 
basic  form.  When  precipitation  is  taking  place,  the  solution, 
besides  holding  more  or  less  of  the  hydrosol,  is  saturated  also 
with  the  substance  in  the  condition  of  molecular  aluminium 
hydroxide  in  equilibrium  with  two  sets  of  ions.  This  equilib- 
rium within  the  solution  might  be  expressed  simply  thus: 


+30H 
+H2AK>3 
(If  the  substance  is  an  acid  it  must  ionize  in  three  stages: 

A1(OH)3-+H+H2A1O3, 

H2A103->H+HA103, 

HA103-+H+A103. 

Also  Al(OH)3-*H+A~io2+H20. 

vSince  it  must  necessarily  be  very  weakly  ionized  the  ion  H2A103 
must  predominate,  but  the  equilibrium  represented  above  can 
be  but  an  approximate  representation  of  the  real  conditions.) 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  89 

The  addition  of  either  a  strong  base  or  a  strong  acid  to  such  a 
system  would  cause  aluminium  hydroxide  to  dissolve  if  the 
resulting  salt  were  soluble.  The  effect  of  the  strong  acid  upon 
the  acid  form  would  be  that  of  suppression  of  the  (already  small) 
ionization.  Its  effect  upon  the  basic  form  would  be  interaction 
to  form  a  salt: 

A1(OH)3+3HC1-»A1C13+3H20. 

The  disturbance  of  equilibrium  resulting  from  the  disappearance 
of  hydroxyl  ions  (due  to  the  formation  of  water)  would  cause 
more  aluminium  hydroxide  to  ionize  in  this  manner  and,  since 
nonionized  aluminium  hydroxide  is  also  in  equilibrium  with  the 
undissolved  portion,  more  would  go  into  solution. 

The  effect  of  a  base  would  be  quite  similar  to  that  of  an  acid, 
although  by  a  different  process  and  through  the  formation  of  a 
different  salt.  The  basic  ionization  of  the  aluminium  hydroxide 
would  be  suppressed  while  the  added  base  would  react  with  the 
acid  form: 

H3A103+NaOH-»NaH2A103+H20, 
or 

HA102+NaOH-*NaA102+H20. 

Here  again  a  salt  is  formed,  although  the  aluminium  appears  in 
the  anion.  The  disturbance  of  equilibrium  has  the  same  ultimate 
result  as  before,  namely,  that  the  solid  substance  passes  into 
solution. 

Adsorption. — It  has  already  been  stated  that  the  washing  of  the 
gelatinous  precipitate  is  more  or  less  difficult  on  account  of  the 
adsorption  of  dissolved  salts.  It  is  impossible  to  avoid  the 
presence  of  such  salts  when  making  separations  or  when  pre- 
cipitating aluminium  from  such  compounds  as  the  alums. 
(Ammonium  salts  are  always  present.)  There  is  also  danger 
that,  in  the  case  of  prolonged  washing  by  water  to  remove 
alkali  salts  or  other  salts,  some  of  the  hydrogel  may  return  to 
the  condition  of  the  hydrosol.  In  order  to  prevent  this  the 
customary  device  of  having  present  an  ammonium  salt  in  the 
wash  water  is  used.  Some  of  this  necessarily  remains  with  the 
precipitate  at  the  last.  If  this  salt  is  ammonium  chloride  some 
of  the  aluminium  will  be  lost  by  volatilization,  the  chloride  being, 
as  with  most  other  metals,  more  volatile  than  the  salts  of  other 


90  QUANTITATIVE  ANALYSIS 

acids.  The  chloride  will  be  formed  during  ignition  by  interaction 
of  the  aluminium  oxide  or  hydroxide  and  the  ammonium  chloride : 

A1(OH)3+3NH4C1->A1C13+3NH3+3H20. 

Ammonium  nitrate  should  therefore  be  used  in  the  wash  water. 
Aluminium  nitrate,  if  formed,  is  decomposed  at  high  temperatures 
into  aluminium  oxide  and  oxides  of  nitrogen. 

If  the  precipitate  of  aluminium  hydroxide  is  filtered  and  washed 
under  diminished  pressure,  care  should  be  exercised  that  the 
liquid  is  not,  at  any  time  before  the  completion  of  the  washing 
process,  drawn  out  so  nearly  completely  as  that  the  precipitate 
should  harden  and  crack.  In  such  a  case  the  wash  water  that 
is  subsequently  used  will  run  through  the  cracks  instead  of 
through  the  body  of  the  precipitate  and  complete  washing  will 
therefore  be  accomplished  only  after  the  use  of  much  water. 
If  it  becomes  necessary  to  allow  the  precipitate  to  remain  in 
the  funnel  from  one  day  to  the  next  and  before  the  washing  is 
completed,  the  precipitate  may  be  kept  moist  by  plugging  the 
stem  of  the  funnel,  covering  the  precipitate  with  water,  and 
placing  a  watch  glass  over  the  top. 

By  making  suitable  changes  in  the  procedure,  aluminium 
chloride  might  be  made  a  reagent  for  the  quantitative  determina- 
tion of  hydroxyl.  Volumetric  methods  are  always  used  instead. 

Determination. — Fill  a  weighing  bottle  with  the  powdered  sample 
of  an  aluminium  salt.  Choose  the  method  to  be  used  in  weighing 
according  to  the  nature  of  the  substance  and  weigh  two  samples  of  about 
1  gm  each  into  Pyrex  beakers.  Dissolve  in  100  cc  of  water  and  add 
dilute,  recently  filtered  ammonium  hydroxide,  stirring  until  the  liquid 
is  distinctly  basic,  as  shown  by  a  bit  of  litmus  paper  thrown  into  the 
beaker.  Boil  until  the  precipitate  is  coagulated  and  until  the  odor  of 
ammonia  above  the  solution  is  but  faint.  Boiling  after  the  odor  has 
disappeared  may  cause  some  of  the  precipitate  to  return  to  the  solution : 
A1(OH)3+3NH4C1-*A1C13+3NH3+3H2O. 

Allow  the  precipitate  to  settle  and  then  filter  through  paper,  using  a 
filter  pump  attached  to  a  bell  jar  or  filter  flask  and  placing  a  supporting 
cone  of  hardened  paper  or  platinum  in  the  funnel  (page  23).  Wash 
with  hot  distilled  water  containing  1  percent  of  ammonium  nitrate  until 
the  washings  are  free  from  chlorides,  shown  by  adding  a  drop  of  nitric 
acid  and  a  few  drops  of  silver  nitrate  solution  to  a  cubic  centimeter  of  the 
washings  caught  in  a  test  tube;  also  from  sulphates,  as  shown  by  adding  a 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  91 

drop  of  dilute  hydrochloric  acid  and  a  few  drops  of  barium  chloride  solu- 
tion to  another  portion  of  the  washings.  Suck  the  precipitate  as  nearly 
dry  as  possible  and  transfer  the  paper  and  precipitate  to  a  porcelain  or 
platinum  crucible  which  has  been  ignited  and  weighed,  folding  the  paper 
in  the  manner  already  learned. 

Heat  very  gently  in  the  covered  crucible  until  the  moisture  is  volatil- 
ized, then  raise  the  temperature  and  burn  the  paper,  inclining  the  cru- 
cible and  placing  the  cover  as  in  the  case  of  the  ignition  of  the  paper 
containing  calcium  oxalate.  When  all  of  the  carbon  has  been  burned, 
cover  the  crucible  and  heat  over  the  blast  lamp  for  30  minutes.  Cool 
in  the  desiccator  and  weigh.  Heat  again  for  10  minutes,  cool  and  weigh. 
If  necessary  repeat  this  process  until  the  weight  is  constant. 

Calculate  the  percent  of  aluminium  in  the  salt. 

Aluminium  oxide  absorbs  water  from  the  air,  reforming  the  hydroxide 
with  a  corresponding  gain  in  weight.  On  this  account  the  crucible  and 
oxide  should  be  weighed  rapidly. 

Copper,  cobalt  and  nickel  cannot  be  quantitatively  precipi- 
tated by  ammonium  hydroxide  because  of  the  formation  of 
soluble  complex  ammonium  salts.  Sodium  hydroxide  or  potas- 
sium hydroxide  is  used  as  the  reagent.  Adsorption  of  the 
reagent  by  the  precipitate  causes  a  large  error  and  volumetric 
or  electrolytic  methods  are  preferable. 

BARIDM 

Barium  may  be  precipitated  as  sulphate,  carbonate  or  chro- 
mate.  The  sulphate  and  chromate  are  weighed  as  such,  while 
the  carbonate  is  ignited  to  form  the  oxide,  in  which  form  it  is 
weighed.  The  solubilities  are  as  follows  (determined  by 
Kohlrausch  and  Rose1). 

Milligrams  per  liter  soluble  at  18° 


Salt 

Barium  equivalent  to  salt 

BaCO3. 

22 

15  3 

BaSO4  

2  6 

1.53 

BaCrO4   

3.8 

2.06 

The  sulphate  is  seen  to  be  the  most  suitable  compound  for 
the  separation  of  barium  from  solution.  The  precipitating  re- 
agent may  be  sulphuric  acid  or  a  soluble  alkali  sulphate.  Since 
the  latter  produces  by  the  reaction  alkali  salts  that  must  be 

*Z.  physik.  Chem.,  12,  234  (1893). 


92  QUANTITATIVE  ANALYSIS 

washed  from  the  precipitate,  while  the  former  produces  volatile 
acids,  sulphuric  acid  is  generally  used  for  the  purpose: 

Ba(NO3)2+Na2SO4-+BaSO4+2NaNO3. 
Ba(NOs)2+H2SO4->BaSO4+2HNO3. 

Barium  sulphate  easily  precipitates  in  the  form  of  fine  crystals. 
If  precipitation  takes  place  rapidly  and  from  a  somewhat  con- 
centrated solution  the  crystals  may  be  so  small  as  to  pass  through 
the  filter.  Remedies  similar  to  those  applied  to  calcium  oxalate 
may  be  used  also  with  barium  sulphate.  These  are  use  of  a 
dense  paper  for  the  filter,  precipitating  slowly  from  a  hot  solu- 
tion and  digestion  of  the  precipitate  in  the  mother  liquor  at  a 
temperature  near  the  boiling  point.  Sometimes  the  filter  paper 
is  treated  before  using  with  a  hot,  concentrated  solution  of 
ammonium  chloride  which  softens  and  swells  the  cellulose  fibers, 
making  a  less  permeable  filter.  Such  treatment  is  of  doubtful 
utility  since  the  ammonium  chloride  must  later  be  washed  out 
of  the  paper  or  cause  some  volatilization  of  barium  chloride  when 
the  precipitate  is  ignited.  No  trouble  will  be  experienced  if 
the  precipitation  is  accomplished  under  proper  conditions. 

The  converse  of  this  method  may  be  used  for  the  determina- 
tion of  the  sulphate  radical  and  for  sulphur  of  any  compound 
that  may  be  changed  to  a  sulphate.  Barium  chloride  is  then 
the  precipitating  reagent  and  the  solution  is  made  slightly  acid 
by  adding  hydrochloric  acid.  The  latter  is  necessary  to  prevent 
the  precipitation  of  barium  salts  of  certain  other  acids  whose 
salts  might  be  present.  Examples  of  such  salts  are  carbonates, 
oxalates,  phosphates  and  borates,  the  barium  salts  of  all  of  these 
being  insoluble  in  water  but  soluble  in  hydrochloric  acid. 

However,  barium  sulphate  itself  is  appreciably  soluble  in 
hydrochloric  acid  as  is  shown  in  the  following  table:1 

Solubility  of  barium  sulphate,  milligrams  per  liter 


Hydrochloric  acid 

'Barium  sulphate 

Barium  equivalent  to 
barium  sulphate 

1.82 
3.65 

7.27 
18.23 

6.7 
8.9 
10.9 
8.6 

3.9 
5.2 
6.4 
5.1 

^Banthisch:  J.  pr.  Chem.,  29,  54  (1884), 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  93 

Therefore,  if  hydrochloric  acid  is  present  in  any  considerable 
quantity  in  the  solution  it  must  be  nearly  neutralized  before 
precipitating  the  barium  sulphate,  not  only  because  of  its  solvent 
action  above  shown  but  also  because  of  its  tendency  to  increase 
the  occlusion  of  other  salts  by  barium  sulphate.1 

Such  occlusion  readily  takes  place  if  iron  salts  are  present. 
Barium  chloride  itself  is  also  readily  occluded  by  barium  sulphate. 
In  the  latter  case  the  nature  of  the  resulting  error  depends 
upon  whether  barium  or  sulphuric  acid  is  being  determined. 
If  the  former,  the  result  is  a  negative  error  because  part  of  the 
barium  is  weighed  in  combination  with  chlorine,  whose  equivalent 
weight  is  less  than  that  of  the  sulphate  radical.  If  the  sulphate 
radical  itself  is  being  determined  the  error  of  occlusion  is  positive 
because  any  barium  chloride  that  may  be  carried  down  is  simply 
a  part  of  the  precipitating  reagent,  contaminating  the  pre- 
cipitate of  barium  sulphate.  In  order  to  avoid  either  error  the 
concentration  of  hydrochloric  acid  should  be  made  as  small 
as  will  serve  to  hold  such  salts  as  the  carbonate,  oxalate,  etc., 
in  solution  and  the  precipitation  of  the  sulphate  must  be  ac- 
complished by  slow  addition  of  the  reagent  to  the  hot  solution, 
stirring  vigorously  meanwhile. 

Change  of  Weight  of  Barium  Sulphate. — Considerable  care 
must  be  exercised  in  burning  the  paper  upon  which  barium 
sulphate  has  been  filtered  and  in  subsequent  ignition  of  the 
precipitate  to  expel  traces  of  moisture.  If  the  temperature 
is  allowed  to  rise  to  too  high  a  point  barium  sulphate  will  gradu- 
ally decompose,  yielding  sulphur  trioxide  and  losing  weight 
thereby : 

BaS04-»BaO+S03.      - 

On  this  account  the  blast  lamp  should  never  be  used  for  heating 
the  precipitate  and  the  temperature  is  not  allowed  to  rise  above 
that  of  dull  redness. 

On  the  other  hand,  errors  may  occur  through  partial  reduction 
of  barium  sulphate  by  carbon  monoxide  or  organic  gases  resulting 
from  heating  of  the  filter  paper.  Barium  sulphide  is  thus 
produced  and  again  the  material  loses  weight: 

BaS04+4CO-»BaS+4C02.   ' 
1  Richards:  Z.  anorg.  Chem.,  8,  413  (1895). 


94  QUANTITATIVE  ANALYSIS 

In  order  to  avoid  this  reduction  the  temperature  should  be 
held  at  as  low  a  point  as  will  serve  to  accomplish  the  combustion 
of  the  paper  and  a  plentiful  supply  of  air  must  be  maintained 
by  inclining  the  crucible  and  cover,  as  directed  on  page  36.  Even 
with  these  precautions  some  reduction  may  occur  but  if  heating 
is  continued  for  a  few  minutes  after  the  carbon  has  disappeared, 
reoxidation  will  take  place: 

BaS+202->BaS04. 

If  it  should  be  suspected  that  either  or  both  of  the  errors  just 
discussed  has  occurred  in  any  given  analysis  a  correction  may  be 
made  by  adding  a  drop  of  dilute  sulphuric  acid  to  the  pre- 
cipitate after  the  first  weighing,  then  gently  reheating  to  expel 
the  excess  of  acid  and  water,  and  reweighing.  A  gajn  in  weight 
is  taken  as  evidence  that  sulphide  or  oxide  of  barium  was  present 
in  the  first  case.  The  second  weight  is  then  the  correct  one. 

This  addition  of  acid,  with  subsequent  heating,  also  serves 
to  correct  any  error  that  may  have  occurred  in  the  determination 
of  barium,  through  the  occlusion  of  barium  chloride  by  the 
precipitating  barium  sulphate.  It  will  be  recalled  that  such 
occlusion  occasions  a  negative  error  in  the  determination  of 
barium,  but  a  positive  one  in  the  determination  of  the  sulphuric 
acid  radical.  Then  in  the  first  case  sulphuric  acid  converts 
occluded  barium  chloride  into  barium  sulphate  and  gives  a 
precipitate  of  correct  composition.  In  the  second  case  barium 
chloride  is  an  occluded  impurity  in  the  precipitate  and  its  con- 
version to  sulphate  merely  serves  to  increase  the  error.  There- 
fore, when  barium  chloride  is  used  as  the  precipitating  reagent 
for  sulphuric  acid  it  is  highly  important  that  the  precipitation 
should  be  carried  out  very  slowly  by  adding  the  reagent  drop-wise 
and  stirring  vigorously.  This  method  serves  not  only  to  minimize 
occlusion  of  the  reagent  but  also  to  prevent  the  formation  of  a 
very  finely  divided  precipitate. 

Determination. — Weigh  about  0.2  gm  of  a  barium  salt  into  each  of 
two  beakers,  dissolve  in  water  or  the  least  possible  quantity  of  hydro- 
chloric acid,  dilute  to  100  cc  and  heat  to  boiling.  Add,  drop  by  drop, 
with  vigorous  stirring,  2  cc  of  25  percent  sulphuric  acid.  Allow  the 
precipitate  to  settle  somewhat  and  test  the  supernatant  liquid,  as  usual, 
to  determine  whether  precipitation  is  complete.  Digest  on  the  steam 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  95 

bath  for  15  minutes  or  longer,  until  the  precipitate  settles  readily. 
Filter  without  the  use  of  a  pump,  on  an  extracted  paper  and  wash  several 
times  with  hot,  distilled  water,  testing  the  washings  for  sulphates. 

Remove  the  paper  from  the  funnel,  fold  and  place  in  a  weighed  cru- 
cible. Incline  the  crucible  as  usual  for  burning  the  paper  and  heat  at 
moderate  temperature  until  white.  Cover  the  crucible  and  heat  barely 
to  dull  redness  for  15  minutes,  cool  and  weigh.  Since  no  decomposition 
of  the  precipitate  takes  place  when  it  is  heated  at  this  temperature 
there  should  be  no  change  in  weight  after  the  first  few  minutes  of 
heating  unless  washing  has  not  been  thorough,  leaving  salts  that 
slowly  volatilize. 

Calculate  the  percent  of  barium  in  the  barium  salt,  using  the  factor 
for  barium  in  barium  sulphate. 

SULPHATES 

If  a  solution  of  barium  chloride  is  used  as  the  precipitating 
reagent  the  sulphate  radical  may  be  determined  by  essentially 
the  same  process  as  that  just  described  for  barium.  A  small 
concentration  of  hydrochloric  acid  must  be  maintained  as  other 
insoluble  salts  of  barium  might  be  formed  in  a  neutral  or  basic 
solution.  Phosphate,  carbonate  and  oxalate  may  be  mentioned 
as  common  examples  of  such  salts. 

Determination. — Weigh  duplicate  samples  of  about  0.25  gm  of  the 
sulphate  into  breakers  and  dissolve  in  75  cc  of  distilled  water.  Add  1  cc 
of  dilute  hydrochloric  acid,  heat  to  boiling  and  add,  drop-wise  and  with 
constant  stirring,  a  5  percent  solution  of  barium  chloride  until  the 
sulphate  is  completely  precipitated.  Digest  on  the  steam  bath  until 
the  precipitate  settles  and  the  solution  clears,  then  filter  and  wash  with 
hot  distilled  water,  testing  finally  with  dilute  sulphuric  acid  to  insure 
removal  of  barium  chloride. 

Heat  in  a  weighed  crucible  as  directed  above  for  the  determination  of 
barium.  From  the  weight  of  sample  and  of  barium  sulphate  calculate 
the  percent  of  the  sulphate  radical  in  the  sample.  In  some  cases  it  is 
desirable  to  calculate  the  percent  of  sulphur  or  of  sulphur  trioxide. 
This  may  be  done  by  use  of  the  proper  factor. 

Free  sulphuric  acid  may  be  determined  by  the  same  process. 
However,  since  the  reaction  of  this  with  barium  chloride  produces 
free  hydrochloric  acid  it  is  unnecessary  to  add  any  of  the  latter. 


96  QUANTITATIVE  ANALYSIS 

STRONTIUM 

Strontium  is  best  determined  as  sulphate,  precipitating  from  a 
solution  containing  alcohol  and  an  excess  of  dilute  sulphuric 
acid.  Its  solubility  in  water  at  18°  is  114  mg  per  liter.1  The 
solubility  is  considerably  diminished  by  a  small  excess  of  sul- 
phuric acid  and  in  50  percent  aqueous  alcohol  the  solubility 
is  very  slight,  although  no  definite  figures  are  now  available. 

Determination. — Weigh  the  proper  quantity  of  strontium  salt  to 
produce  0.2  to  0.3  gm  of  strontium  sulphate.  If  soluble  in  water  dis- 
solve in  50  cc  of  water  and  add  to  the  solution  60  cc  of  alcohol.  If 
insoluble  in  water  dissolve  in  hydrochloric  acid,  evaporate  in  a  cas- 
serole to  expel  excess  of  acid  and  dilute  the  solution  to  50  cc,  then 
add  60  cc  of  alcohol.  Add,  slowly  and  with  stirring,  dilute  sulphuric 
acid  until  precipitation  is  complete.  An  excess  of  about  5  Cc  is  desirable. 
Stir  for  some  time  then  allow  to  stand  for  12  hours.  Filter  and  wash 
twice  with  50  percent  alcohol  containing  a  few  drops  of  dilute  sulphuric 
acid,  then  with  50  percent  alcohol  until  the  washings  fail  to  give  a  test 
for  sulphates.  Ignite  in  a  weighed  crucible  at  as  low  a  temperature 
as  possible  until  white.  Weigh  the  strontium  sulphate  and  calculate 
the  percent  of  strontium  in  the  sample. 

The  separation  of  barium,  strontium  and  calcium  is  accom- 
plished by  converting  all  of  the  metals  into  nitrates,  evaporating 
to  dryness  and  taking  up  with  a  mixture  of  alcohol  and  ether. 
Calcium  nitrate  dissolves  and  the  calcium  is  precipitated  as 
oxalate  after  evaporating  the  alcohol  and  ether  and  dissolving 
the  residue  in  water.  The  barium  and  strontium  nitrates  are 
dissolved  in  water,  barium  is  precipitated  as  chromate  and  stron- 
tium as  sulphate  as  described  above. 

POTASSIUM  AND  SODIUM 

Potassium  may  be  separated  from  sodium  and  determined  as 
perchlorate  or  as  chlorplatinate.  It  may  also  be  precipitated 
as  potassium  sodium  cobaltinitrite  and  this  determined  by  a 
volumetric  process  or  dissolved  and  the  potassium  later  deter- 
mined by  the  other  gravimetric  methods.  It  is  often  stated 
that  it  can  also  be  determined  by  weighing  as  sulphate  or  as 
chloride.  Inasmuch  as  the  latter  two  methods  involve  no  separa- 

1  Kohlrausch:  Z.  physik.  Chem.,  60,  355  (1905). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  97 

tion  by  precipitation  and  filtration  but  simply  conversion  of  the 
potassium  into  a  form  other  than  the  one  in  which  it  formerly 
existed  and  since  any  other  metals  that  might  be  present  would 
also  be  converted  into  sulphates  or  chlorides  and  weighed  with 
the  potassium,  the  value  of  these  methods  is  not  apparent.  Of 
the  first  two  methods  the  perchlorate  method  has  the  advantage 
of  cheapness,  while  the  chlorplatinate  method  is  more  convenient 
and  probably  more  accurate. 

So  far  as  is  indicated  by  present  experience  the  chlorplatinate 
method  is  the  most  reliable  and,  at  the  same  time,  the  most 
convenient  of  all  of  the  known  methods  for  the  potassium  de- 
termination. The  cobaltinitrite  method  has  not  yet  been  im- 
proved to  the  point  of  a  satisfactory  quantitative  method, 
although  useful  in  qualitative  analysis.  The  perchlorate  method 
has  also  suffered  the  drawback  of  questionable  reliability, 
as  well  as  that  of  requiring  a  reagent  which  is  more  or  less 
dangerous  to  prepare  and  handle.  The  one  serious  obstacle 
to  the  continued  use  of  the  chlorplatinate  method  is  the  very 
high  cost  of  the  reagent  itself.  The  great  scarcity  and  the  ex- 
traordinary rise  in  the  price  of  platinum  since  the  beginning  of 
the  world  war  has  made  it  increasingly  desirable,  if  not  absolutely 
essential,  that  some  method  that  does  not  involve  this  metal 
shall  be  used.  In  the  industrial  laboratory  a  considerable 
portion  of  the  used  platinum  is  systematically  recovered  but  the 
recovery  involves  considerable  work  and  expense,  while  a  certain 
fraction  of  the  metal  is  lost  in  each  recovery  operation. 

The  chlorplatinate  method  has  been  varied  in  matters  of  detail 
but  essentially  it  consists  in  precipitating  potassium  chlorplati- 
nate from  an  alcoholic  solution  by  adding  chlorplatinic  acid. 


Compounds  of  sodium  arid  magnesium  are  not  so  precipitated 
and  potassium  is  separated  from  these  by  filtration  but  other 
metals  must  be  absent  because  of  the  small  solubility  of  most 
chlorplatinates.  Potassium  chlorplatinate  may  be  either  weighed 
as  such  or  ignited  in  a  current  of  hydrogen  : 

K2PtCl6+2H2-»2KCl+Pt+4HCl. 
The  potassium  chloride  is  washed  out  of  the  reduced  platinum 

7 


98 


QUANTITATIVE  ANALYSIS 


which  is  then  weighed.     In  practice  potassium  chlorplatinate  is 
usually  weighed  without  ignition. 

Washing. — The  separation  from  alcoholic  solution  can  be 
accomplished  only  under  certain  conditions,  owing  to  the  small 
solubilities  of  certain  other  compounds  that  may  be  present. 
Sodium  chlorplatinate  is  easily  soluble  in  alcohol  but  ammonium 
chlorplatinate  is  soluble  to  a  very  slight  extent.  Ammonium 
compounds  must  therefore  be  volatilized  by  heating,  before  the 
addition  of  chlorplatinic  acid.  Sodium  chloride  and  sulphate 
are  nearly  insoluble  in  alcohol,  and  must  be  changed  into  more 
soluble  substances  before  a  separation  of  sodium  and  potassium 
can  be  accomplished.  The  following  table  will  serve  to  show  the 
nature  of  the  questions  that  must  here  be  met.  There  is  con- 
siderable disagreement  between  the  results  as  obtained  by  dif- 
ferent investigators  but  these  figures  may  be  regarded  as  at  least 
approximately  correct. 


Percent  alcohol 
in  water 

Solubility,  grams 
salt  per  liter  of 
solvent 

Sodium  sulphate1 

0.7 

127.0 

19.4 

26.0 

72.0 

<0.001 

Sodium  chloride 

80 

about  5  .  0 

Ammonium  chlorplatinate2.  .  .  . 

55 

0.150 

76 

0.067 

95 

0.0037 

Potassium  chlorplatinate3  

0 

7.742 

10 

3.72 

20 

2.18 

30 

1.34 

40 

0.76 

50 

0.491 

60 

0.265 

70 

0.128 

80 

0.085 

90 

0.025 

100 

0.009 

lde  Bruyn:  Z.  physik.  Chem.,  32,  63  (1900). 

2  Fresenius:  Z.  anal.  Chem.,  36,  322  (1897). 

3  Archibald,  Wilcox  and  Buckley:  J.  Am.  Chem.  Soc.,  30,  747  (1908). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  99 

The  Lindo  Method. — The  method  of  Lindo1  consisted  in  obtain- 
ing a  solution  containing  chlorides  of  no  other  metals  than  sodium 
and  potassium,  adding  sufficient  chlorplatinic  acid  to  convert 
all  of  the  chlorides  into  chlorplatinates,  evaporating  nearly  to 
dryness  and  adding  strong  alcohol.  Sodium  chlorplatinate 
dissolves  arid  is  separated  from  the  potassium  chlorplatinate 
by  filtration.  The  potassium  salt  is  washed  with  alcohol  and 
weighed  or  treated  as  already  described.  Lindo  also  showed 
that  the  solubility  of  very  fine  crystals  of  potassium  chlor- 
platinate is  greater  than  that  of  larger  crystals  (cf.  page  21). 

Using  this  method,  it  was  necessary  that  no  sulphate  should 
be  present  because  of  the  very  slight  solubility  of  sodium  sul- 
phate in  alcohol.  The  discussion  of  the  laws  of  precipitation 
(page  15  et  seq.)  will  make  it  clear  that  even  if  enough  chlor- 
platinic acid  were  present  to  combine  with  all  of  the  sodium 
present  to  form  sodium  chlorplatinate,  the  less  soluble  sodium 
sulphate  would  still  be  precipitated.  Even  when  all  of  the  metals 
are  present  as  chlorides  there  is  without  doubt  some  contamina- 
tion of  the  precipitated  potassium  chlorplatinate  by  sodium 
chloride.  It  was  early  observed2  that  high  results  would  be 
obtained  by  this  method  for  determining  potassium  if  the  factor 
for  potassium  in  potassium  chlorplatinate  were  calculated  by  the 
use  of  the  atomic  weight  of  platinum  as  determined  by  Seubert3 
(194.8)  or  even  195.2,  which  is  that  given  in  the  table  of  atomic 
weights  for  1918,  while  a  factor  calculated  from  the  atomic 
weight  197.2,  which  had  previously  been  accepted,  gave  correct 
results.  This  may  be  partly  due4  to  the  fact  that  the  pre- 
cipitated potassium  chlorplatinate  contains  also  some  compounds 
with  composition  as  represented  by  such  a  formula  as  H2PtCl5OH 
or  H2PtCl4O.  It  is  undoubtedly  also  partly  due  to  the  presence 
of  some  sodium  chloride  in  the  precipitate.  On  this  account 
the  method  has  been  modified  by  using  80  percent  alcohol 
instead  of  absolute  alcohol.  Reference  to  the  solubility  table 
above  will  show  that  alcohol  of  this  concentration  will  dissolve 
potassium  chlorplatinate  to  a  greater  extent  than  does  absolute 

'Chem.  News,  44,  77,  86,  97  and  129  (1881). 

2Dittmar  and  Me  Arthur:  J.  Soc.  Chem.  Ind.,  6,  799  (1887). 

3Ann.,  207,  1  (1881). 

4Dittmar  and  Me  Arthur;  loc.  cit. 


100  QUANTITATIVE  ANALYSIS 

alcohol  and  this  negative  error  seems  practically  to  balance 
the  positive  error  discussed. 

Gladding' s  Modification. — The  fact  that  volatile  acids,  organic 
compounds,  ammonium  salts,  etc.,  can  be  easily  volatilized 
by  heating  makes  it  desirable  to  obtain  the  sodium  and  potassium 
in  a  form  in  which  they  can  be  heated  to  redness  without  danger 
of  loss.  Both  chlorides  are  sensibly  volatile  at  such  temperatures 
while  the  sulphates  are  not  but,  as  already  stated,  sodium  sulphate 
must  not  be  allowed  to  form  because  of  its  small  solubility  in 
alcohol.  To  meet  this  difficulty  Gladding  suggested1  a  further 
modification  of  the  Lindo  method  in  which  the  sodium  sulphate 
was  to  be  washed  out,  after  the  removal  of  the  excess  of  chlor- 
platinic  acid,  by  a  water  solution  of  ammonium  chloride.  The 
solubility  of  sodium  sulphate  in  water  is  considerably  increased 
by  the  presence  of  ammonium  chloride.  This  follows  the  general 
law  that  the  addition  of  an  electrolyte  which  does  not  contain 
an  ion  in  common  with  the  first  electrolyte  increases  its  solubility. 
To  avoid  loss  of  potassium  chlorplatinate  the  washing  liquid  is 
previously  saturated  with  the  pure  salt.  In  using  such  a  solution 
it  is  important  that  no  great  change  in  temperature  shall  occur  in 
the  solution  after  it  is  withdrawn  from  the  bottle  and  before  it  is 
used  for  washing  the  precipitate.  This  is  because  the  solution  is 
kept  saturated  by  an  excess  of  potassium  chlorplatinate  in  the 
bottle.  If  the  temperature  should  rise  the  solution  which  was 
saturated  in  the  bottle  now  becomes  undersaturated  and  will 
dissolve  some  of  the  precipitate  on  the  filter.  On  the  other  hand, 
if  the  funnel  is  at  a  much  lower  temperature  than  the  reagent 
bottle  (due  to  working  in  a  colder  part  of  the  room)  it  will  cool 
the  solution  and  cause  a  deposition  of  potassium  chlorplatinate 
upon  the  precipitate  already  present.  This  modified  method, 
known  as  the  Lindo-Gladding  method,  is  now  used  quite  generally, 
particularly  for  the  determination  of  potassium  (and  indirectly 
of  sodium)  in  industrial  products,  minerals,  etc. 

Decomposition. — During  the  evaporation  of  sodium,  potassium 
and  ammonium  salts  with  sulphuric  acid  the  first  product  is, 
of  course,  a  mixture  of  the  acid  sulphates.  After  the  evaporation 

1  U.  S.  Dept.  of  Agr.,  Chem.  Bull.,  7,  38. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  101 

of  the  excess  of  acid  and  upon  further  heating,  the  pyrosulphates 
of  sodium  and  potassium  are  formed: 

2KHSO4-^K2S207+H20; 
2NaHSO4->Na2S2O7+H20. 

These  reactions  begin  at  about  350°.  At  a  temperature  of 
dull  redness  the  normal  sulphates  begin  to  form: 

K2S207-*K2SO4+S03; 
Na2S207-»Na2SO4+SO3. 

This  decomposition  requires  heating  for  some  time.  It  is  best 
to  test  the  completion  of  decomposition  by  repeating  the  heating 
until  the  weight  becomes  constant. 

Ammonium  acid  sulphate  is  decomposed  and  volatilized  as 
ammonia,  water  and  sulphur  trioxide: 

NH4HS04-^NH3+H20+S03. 

If  it  is  desired  to  accomplish  the  removal  of  ammonium  salts 
and  organic  matter  by  ignition  but  to  avoid  the  use  of  the  am- 
monium chloride  washing  solution  the  sulphates,  first  obtained 
by  evaporating  with  sulphuric  acid  and  igniting,  are  dissolved 
and  precipitated  by  barium  chloride  which  precipitates  barium 
sulphate  and  leaves  sodium  chloride  and  potassium  chloride  in 
solution.  The  excess  of  barium  is  then  precipitated  by  sulphuric 
acid. 

There  is  no  method  known  for  the  direct  determination  of 
sodium  if  we  exclude  the  weighing  as  sulphate  or  chloride, 
methods  of  very  limited  usefulness.  This  is  because  no  sodium 
compound  has  sufficiently  small  solubility  to  make  possible  its 
separation  from  the  corresponding  salt  of  potassium.  Sodium  is 
usually  determined  by  weighing  it  with  potassium  in  the  form  of 
sulphate  or  chloride,  determining  potassium  and  calculating 
sodium  by  difference.  It  should  be  noted  that  such  a  method 
throws  all  of  the  errors  of  the  potassium  determination  upon  that 
of  sodium,  in  addition  to  any  errors  that  may  have  occurred  in 
the  weighing  of  the  combined  chlorides  or  sulphates. 

Ammonium. — Chlorplatinic  acid  is  also  used  as  a  reagent 
for  the  quantitative  determination  of  the  ammonium  radical  but 
potassium  must  be  absent.  On  account  of  the  difficulty  ex- 


102  QUANTITATIVE  ANALYSIS 

perienced  in  the  removal  of  potassium  from  ammonium  the 
latter  is  more  conveniently  determined  by  volumetric  methods. 

Platinum. — The  converse  of  the  Lindo  method  for  potassium  is 
used  for  the  determination  of  platinum.  Either  ammonium 
chloride  or  potassium  chloride  may  be  used  as  the  reagent  but 
the  former  is  generally  used  because  of  its  greater  solubility  in 
alcohol,  which  makes  the  removal  of  the  excess  of  reagent  more 
easy. 

Determination  by  the  Lindo-Gladding  Method. — (To  be  performed 
in  an  atmosphere  which  is  free  from  ammonia).  Use  portions  of  about 
0.3  gm  of  a  sample  containing  salts  of  potassium  and  sodium  and  weigh 
into  small  weighed  evaporating  dishes.  Dissolve  in  a  small  amount  of 
hot  water,  add  0.5  cc  of  concentrated  sulphuric  acid,  evaporate  to  dry- 
ness  under  the  hood,  using  care  to  avoid  spattering,  and  ignite  at  bright 
redness  until  no  more  white  fumes  are  evolved  and  the  residue  is  white. 
The  steam  bath  should  not  be  used  for  the  evaporation  on  account  of 
appreciable  solubility  of  porcelain  in  steam,  and  consequent  loss  in 
weight.  Cool,  weigh  and  ignite  again  to  constant  weight.  Record  the 
weight  of  sulphates  of  sodium  and  potassium.  Dissolve  the  resulting 
potassium  sulphate  (mixed  with  sodium  sulphate)  in  50  cc  of  hot  water 
and  then  add  chlorplatinic  acid,  using  about  1  cc  more  than  the  theoret- 
ical amount,  calculated  upon  the  assumption  that  the  original  salt  was 
potassium  chloride.  Evaporate  on  the  steam  bath  to  a  thick  paste 
but  not  to  dryness,  cool  and  add  50  cc  of  80  percent  alcohol,  stir  up  the 
solid  matter  and  allow  to  stand,  covered,  for  30  minutes. 

If  the  liquid  is  not  visibly  colored  too  little  reagent  has  been  used. 
In  this  case  new  samples  should  be  taken  and  the  quantity  of  chlor- 
platinic acid  increased.  Filter  and  wash  the  precipitate  thoroughly 
with  80  percent  alcohol,  washing  several  times  after  the  washings  pass 
through  colorless.  The  wash  bottle  should  be  provided  with  ground- 
glass  joints  so  that  no  rubber  will  come  into  contact  with  the  alcohol. 
Remove  the  nitrate  and  washings,  pouring  these  into  the  bottle  provided 
for  platinum  waste  residues,  and  wash  the  precipitate  with  five  or  six 
portions  of  10  cc  each  of  10  percent  ammonium  chloride  solution  which 
is  saturated  with  potassium  chlorplatinate.  Wash  again,  thoroughly, 
with  80  percent  alcohol,  using  particular  care  in  washing  the  upper 
part  of  the  paper  free  from  ammonium  chloride.  Wash  until  only 
a  faint  turbidity  is  produced  by  the  addition  of  a  drop  of  silver  nitrate 
to  the  last  washings. 

Drain  most  of  the  alcohol  from  the  paper,  slip  the  latter  out  of  the 
funnel  and  dry  in  the  oven  at  100°.  Place  a  weighed  porcelain  crucible 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  103 

upon  a  piece  of  glazed  paper,  remove  most  of  the  precipitate  to  the 
crucible,  brushing  up  any  particles  that  may  have  fallen  upon  the  glazed 
paper,  and  then  replace  the  paper  in  the  funnel.  Place  the  crucible 
under  the  funnel  and  dissolve  the  remainder  of  the  precipitate  in  the 
smallest  amount  of  nearly  boiling  water,  allowing  the  solution  to  run  into 
the  crucible.  Evaporate  to  dryness  on  the  steam  bath,  carefully  wipe 
'  the  outside  of  the  crucible  with  a  clean  towel  and  dry  for  30  minutes  at 
100°.  Weigh  and  calculate  the  percent  of  potassium  in  the  salt  ana- 
lyzed. Calculate  also  the  weight  of  potassium  sulphate,  subtract  this 
weight  from  that  of  the  mixed  sulphates,  and  from  the  remainder 
calculate  the  percent  of  sodium. 

Optional  Method,  Using  a  Gooch  Crucible. — Proceed  as  above  until 
ready  to  filter  out  the  potassium  chlorplatinate.  Prepare  two  Gooch 
niters  as  directed  on  page  86,  paying  attention  to  the  precautions  sug- 
gested, and  using  strong  suction  in  forming  the  asbestos  felt.  Riiise 
the  crucible  with  alcohol,  remove,  wipe  the  outside  and  dry  at  100°  to 
105°  for  30  minutes  or  until  the  weight  is  constant.  Weigh  and  replace 
in  the  holder.  Before  the  suction  pump  is  again  turned  on  moisten  the 
asbestos  with  one  or  two  drops  of  water.  Start  the  pump  and  filter  and 
wash  the  precipitate  exactly  as  above  directed.  Remove  the  crucible, 
dry  in  the  oven  and  weigh.  Calculate  potassium  and  sodium  as  before. 

In  the  foregoing  exercise  the  procedure  is  based  upon  the  as- 
sumption that  sodium  or  ammonium  salts  or  both  may  be  pres- 
ent. The  latter  are  volatilized  by  heating  with  sulphuric  acid. 
The  former  are  removed  by  the  ammonium  chloride  solution. 

Recovery  of  Platinum  from  Waste  and  Preparation  of  Chlor- 
platinic  Acid. — The  recovery  and  purification  of  platinum  from 
miscellaneous  filtrates  and  other  waste  solutions  is  a  matter 
of  increasing  importance  on  account  of  the  extraordinary  increase 
in  the  price  of  platinum  within  recent  years.  The  following 
method,  described  by  Delong,1  has  been  found  to  serve  well 
for  this  purpose. 

Place  the  solutions  in  an  evaporating  dish  having  a  capacity  of  2  liters 
for  each  100  gm  of  platinum  and  evaporate  until  most  of  the  water 
has  been  expelled.  Make  basic  with  sodium  hydroxide  solution  and  add, 
stirring,  sodium  formate,  either  solid  or  in  concentrated  solution.  A 
quantity  of  sodium  formate  equal  to  about  half  the  weight  of  platinum 
will  be  required.  If  foaming  occurs  add  more  sodium  hydroxide. 
Heat  on  the  steam  bath  for  1  hour,  stirring  occasionally,  then  acidify 

1  Chem.  Weekblad,  10,  833  (1914). 


104  QUANTITATIVE  ANALYSIS 

with  hydrochloric  acid,  25  percent  solution,  stirring  during  the  addition 
of  acid. 

Filter  off  the  precipitated  platinum  on  a  soft  paper,  using  suction. 
Wash  twice  with  hot  2  percent  hydrochloric  acid,  then  with  hot  water 
until  free  from  acid.  Separate  the  platinum  from  the  paper,  dry,  ignite 
and  weigh.  Pour  over  the  platinum  in  a  porcelain  dish  five  times  its 
weight  of  25  percent  hydrochloric  acid,  heat  on  the  steam  bath  and  add 
slowly  50  percent  nitric  acid  until  no  more  gas  is  evolved.  About  1  cc 
of  nitric  acid  will  be  required  for  each  gram  of  platinum. 

After  the  platinum  is  in  solution  add  10  cc  of  25  percent  hydrochloric 
acid  and  evaporate  to  small  volume  and  repeat  this  process  twice. 
This  reduces  and  eliminates  nitric  acid.  Dilute  with  water  and  evapo- 
rate two  or  three  times  to  expel  hydrochloric  acid.  Finally  dilute, 
cool  and  filter  on  a  soft  filter  whose  approximate  wreight  is  known.  If 
the  filtrate  is  not  perfectly  clear  refilter.  Wash  the  paper  free  from 
platinum  stain  and  if  any  appreciable  residue  remains,  dry  and  weigh 
it  on  the  filter.  Correct  the  weight  of  platinum  for  this  weight  of  car- 
bon, etc.,  then  make  the  solution  to  the  desired  concentration.  For 
potassium  determinations  the  solution  should  contain  0.1  gm  of 
platinum  in  1  cc. 

A  method  for  the  recovery  of  platinum  from  scrap  by  elec- 
trolysis is  described  by  Weber.1  Chlorplatinic  acid  prepared  by 
this  method  is  quite  free  from  traces  of  nitric  acid. 

The  Perchlorate  Method. — The  perchlorate  method  is  based 
upon  the  fact  that  potassium  perchlorate  is  almost  insoluble 
in  97  percent  ethyl  alcohol,  while  sodium  perchlorate  dissolves 
with  greater  ease.  It  involves  the  use  of  an  aqueous  solution 
of  perchloric  acid,  the  preparation  of  which  is  somewhat  trouble- 
some and  dangerous. 

The  solubility  of  potassium  perchlorate  in  alcohol  of  various 
concentrations  is  as  follows:2 


Concentration  of 
alcohol,  percent  by 
weight 

Grams,  potassium 
perchlorate  in  one 
liter  of  solvent 

Grams,  potassium 
equivalent  to  potassium 
perchlorate 

97.2 
95.8 
90.0 

0.156 
0.20 
0.36 

0.044 
0.06 
0.10 

1  J.  Am.  Chem.  Soc.,  30,  29  (1908). 

2  Wenze:  Z.  angew.  Chem.,  6,  691  (1891). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  105 

The  solubility  is  considerably  diminished  by  an  excess  of  per- 
chloric acid.  Sodium  perchlorate  dissolves  easily  in  alcohol 
although  no  definite  data  are  on  record. 

The  perchlorate  method  has  been  improved  by  the  sub- 
stitution of  a  20  percent  solution  of  perchloric  acid  for  the  pure 
acid,  as  formerly  used.  This  solution  keeps  well  and  involves 
little  or  no  danger  of  accident  through  handling.  As  in  the 
chlorplatinate  method  it  is  necessary  to  remove  ammonium 
salts.  This  may  be  done  by  gently  heating  the  chlorides  or, 
more  safely,  by  evaporating  with  sulphuric  acid  and  heating 
the  sulphates  rather  more  strongly.  If  the  latter  method  is 
followed  it  becomes  necessary  to  reconvert  the  salts  to  chlorides 
before  precipitating  potassium  perchlorate  because  of  the  limited 
solubility  of  sodium  sulphate  in  97  percent  alcohol,  which  is  used 
for  washing  the  precipitate. 

The  analytical  method  for  materials  containing  salts  of  only 
sodium,  potassium  and  ammonium  is  described  below. 

Determination. — Weigh  1  gm  of  the  sample  in  which  sodium  and 
potassium  are  to  be  determined,  brushing  into  a  porcelain  or  platinum 
dish.  Treat  with  sulphuric  acid  and  evaporate  and  heat  to  expel 
ammonium  salts  and  excess  of  acid,  using  the 'procedure  as  described 
for  the  chlorplatinate  method.  Dissolve  the  weighed  sulphates  of 
sodium  and  potassium  in  50  cc  of  hot  water,  add  one  drop  of  concen- 
trated hydrochloric  acid,  heat  nearly  to  boiling  and  then  add,  dropwise 
and  with  constant  stirring,  a  5  percent  solution  of  barium  chloride  until 
all  sulphate  is  precipitated.  This  operation  should  be  performed  very 
carefully  in  order  to  have  the  least  possible  excess  of  barium  chloride 
at  the  end.  Digest  for  a  short  time  over  a  small  flame  or  on  the  steam 
bath,  then  filter  and  wash  the  precipitate  and  paper  well  with  hot  water. 

Evaporate  the  nitrate  and  washings  to  about  25  cc  and  add  10  cc 
of  20  percent  perchloric  acid  solution.  Evaporate  over  a  steam  bath 
in  a  hood  until  the  solution  becomes  viscous,  cool  and  dissolve  the 
residue  in  a  small  amount  of  hot  water.  Again  add  5  cc  of  perchloric 
acid  solution  and  evaporate  over  the  steam  bath  until  the  solution 
evolves  dense  white  fumes  of  perchloric  acid.  Cool  to  room  tem- 
perature and  add  25  cc  of  a  solution  made  by  mixing  1  cc  of  20  percent 
perchloric  acid  with  100  cc  of  98  percent  alcohol  (making  practically  97 
percent  alcohol).  If  the  insoluble  potassium  perchlorate  is  caked  it 
should  be  broken  with  a  stirring  rod  so  that  no  soluble  salts  will  escape 
the  action  of  the  alcohol. 


106 


QUANTITATIVE  ANALYSIS 


During  the  process  of  evaporation  of  the  various  solutions  a  Gooch 
filter  should  be  prepared,  the  asbestos  felt  being  washed  with  the 
perchloric  acid-alcohol  mixture.  The  filter  is  dried  for  1  hour  at  120° 
to  130°,  cooled  and  weighed.  Filter  the  solution  on  this  prepared  filter, 
removing  every  trace  of  the  precipitate  from  the  beaker  by  means  of  a 
policeman  and  the  prepared  washing  solution,  and  wash  four  or  five 
times  with  this  solution.  Dry  for  1  hour  at  120°  to  130°,  cool  and  weigh. 

From  the  weight  of  potassium  perchlorate  thus  obtained  calculate 
the  percent  of  potassium  in  the  sample."  Also  from  this  weight  calcu- 
late the  weight  of  potassium  sulphate  which  is  equivalent  to  it,  subtract 
this  weight  from  the  combined  weights  of  sodium  sulphate  and 
potassium  sulphate  and  from  the  remaining  sodium  sulphate  calculate 
the  percent  of  sodium  in  the  sample. 

MAGNESIUM 

The  determination  of  magnesium  is  usually  made  by  pre- 
cipitating from  a  basic  solution  as  di  magnesium  ammonium 
orthophosphate.  This  is  ignited  and  weighed  as  magnesium 
pyrophosphate.  The  reactions  may  be  expressed  thus : 

MgCl2+NH4OH+Na2HP04-^MgNH4P04+2NaCl-|-H20, 

2MgNH4P04-»Mg2P207+2NH3+H20. 

No  other  metals  than  those  of  the  alkali  group  may  be  present, 
as  the  phosphates  of  practically  all  others  are  insoluble  in  am- 
monium hydroxide.  Any  soluble  phosphate  may  be  used  as 
the  precipitating  reagent  but  the  ones  most  used  in  practice  are 
disodium  orthophosphate  and  sodium  ammonium  acid  ortho- 
phosphate  (microcosmic  salt). 

Solubility. — The  following  tabular  statement  from  the  work  of 
Ebermayer1  shows  the  solubility  of  crystallized  dimagnesium 
ammonium  orthophosphate  in  mixtures  of  ammonium  hydroxide 
and  water. 


Percent  by  volume  of 
ammonium  hydroxide 
of  sp.  gr.  0.96 

Grams  of 
MgNH4PO4.6H2O 
per  liter  of  solvent  at  15° 

Equivalent  grams  of 
Mg  per  liter  of  solvent 

0 

25 
50 
75 

0.074 
0.027 
0.023 
0.019 

0.0072 
0.0026 
0.0022 
0.0018 

J.  prakt.  Chem.,  60,  41  (1853). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  107 

This  statement  of  the  solubility  of  the  precipitate  in  solutions 
containing  various  concentrations  of  ammonium  hydroxide 
would  lead  to  the  conclusion  that  precipitation  from  the  more 
concentrated  solutions  of  ammonium  hydroxide  would  result  m 
greater  accuracy  because  of  the  small  solubility  of  magnesium 
ammonium  orthophosphate.  From  this  standpoint  alone  the 
conclusion  would  be  correct.  It  happens,  however,  that  the 
basicity  of  the  solution,  as  well  as  the  presence  of  other  salts,  has 
an  important  influence  upon  the  composition  of  the  precipitate. 
The  decrease  of  solubility  with  increasing  concentrations  of 
ammonium  hydroxide  and  also  of  ammonium  salts  is  to  be 

expected  as  a  consequence  of  the  mass  law  since  these  substances 

+ 

increase  the  concentration  of  the  ion  NEU,  a  constituent  of 
the  precipitate.  Substances  other  than  dimagnesium  am- 
monium orthophosphate  are  precipitated  to  some  extent  under 
the  following  conditions: 

Effect  of  High  Basicity. — If  the  solution  is  strongly  basic 
when  the  reagent  is  added  there  is  formed  some  trimagnesium 
orthophosphate,  Mgs(PO4)2,  and  the  quantity  of  this  substance 
is  increased  by  slow  addition  of  the  reagent.  This  is  not  decom- 
posed upon  heating  and  the  ignited  precipitate  is  therefore  not 
all  magnesium  pyrophosphate.  This  fact  makes  it  undesirable 
that  too  much  ammonium  hydroxide  should  be  present,  even 
though  the  solubility  of  the  precipitate  is  lessened  thereby. 

The  solubility  of  dimagnesium  ammonium  orthophosphate 
is  much  less  than  that  given  by  Ebermayer,  according  to  the 
work  of  Bube1  who  states  that  the  saturated  solution  in  pure 
water  contains  about  0.00014  gm  in  1000  cc.  It  is  also  stated 
that  in  such  a  solution  the  solubility  product  of  trimagnesium 
orthophosphate  is  far  exceeded  and  that  the  solubility  of  di- 
magnesium ammonium  orthophosphate  is  increased  by  large 
concentrations  of  ammonium  ions.  This  would  probably 
account  for  the  increased  precipitation  of  trimagnesium  ortho- 
phosphate  in  solutions  made  strongly  basic  by  ammonium  hydrox- 
ide, the  magnesium  ammonium  salt  changing  into  magnesium 
phosphate  and  ammonium  phosphate,  the  magnesium  salt 
precipitating : 

3MgNH4P04-»Mg3(P04)2+  (NH4)3P04. 

1  Z.  anal.  Chem.,  49,  525  (1910). 


108  QUANTITATIVE  ANALYSIS 

Effect  of  Ammonium  Salts. — -If  the  solution  contains  excessive 
quantities  of  ammonium  salts,  whether  the  precipitation  takes 
place  from  a  strongly  basic  or  weakly  basic  solution  the  dimag- 
nesium  ammonium  orthophosphate  will  contain  certain  quantities 
of  monomagnesium  ammonium  orthophosphate,  Mg(NH4)4 
(P04)2.  This  substance,  when  strongly  heated,  passes  into 
magnesium  metaphosphate,  Mg(POs)2,  a  substance  which  can 
be  converted  into  magnesium  pyrophosphate  only  after  prolonged 
heating  at  high  temperatures  (2Mg(PO3)2— »Mg2P207+P2O5). 
/  Ammonium  salts  should  thus  be  nearly  or  entirely  absent, 
with  the  exception  of  a  certain  amount  of  ammonium  chloride, 
which  must  be  present  to  prevent  the  precipitation  of  magnesium 
hydroxide.  If  they  have  accumulated  in  the  solution  as  a  result 
of  the  use  of  ammonium  hydroxide  in  the  separation  of  other 
metals,  they  should  be  removed  before  precipitation,  by  (a) 
evaporating  to  dryness  and  heating  strongly  or  (6)  evaporating 
to  small  volume  and  heating  with  concentrated  nitric  acid  or  (c) 
performing  a  double  precipitation,  dissolving  the  first  impure 
precipitate  in  hydrochloric  acid  and  reprecipitating.  Method 
(a)  or  (c)  is  to  be  preferred. 

Temperature  and  Rate  of  Precipitation. — Most  chemists 
prefer  to  precipitate  magnesium  from  a  cold  solution  although 
Gibbs1  recommends  a  boiling  solution.  Whether  the  cold  or 
hot  solution  is  used  one  of  two  procedures  may  be  followed  in 
order  to  conform  to  the  principles  outlined  above.  The  entire 
amount  of  disodium  phosphate  solution  may  be  added  at  once 
to  an  acid  solution  and  then  dilute  ammonium  hydroxide  slowly 
added  until  the  solution  is  basic.  After  standing  a  short  time 
most  of  the  precipitate  will  form  and  the  remaining  magnesium 
can  be  precipitated  by  the  addition  of  concentrated  ammonium 
hydroxide.  Instead  of  following  this  method  the  solution  may 
be  made  neutral  or  faintly  basic  and  disodium  phosphate  added 
slowly,  thus  precipitating  nearly  all  of  the  substance,  when 
concentrated  ammonium  hydroxide  may  be  added  as  before. 
The  second  method  is  recommended. 

Decomposition  upon  Heating. — The  reason  for  the  difference 
in  the  mode  of  decomposition  of  the  precipitate  containing  more 
of  the  ammonium  radical  from  that  of  the  dimagnesium  am- 

1  Am.  J.  Sci.,  [3]  6,  114. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  109 

rnonium  salt  is  apparent  when  the  properties  of  the  three  phos- 
phoric acids  and  of  the  salts  are  examined.  Phosphorus  pent- 
oxide,  by  combining  with  different  proportions  of  water,  gives 
rise  to  three  different  acids  : 

P205+  H20—  >2HP03,  metaphosphoric  acid, 
P205+2H2O—  >H4P2O7,  pyrophosphoric  acid. 
P205-f-3H20-»2H3P04,  orthophosphoric  acid. 

Either  metaphosphoric  or  pyrophosphoric  acid  will  be  trans- 
formed into  the  one  containing  more  water  if  allowed  to  stand  in 
solution.  Also  the  acids  may  be  changed  in  the  opposite  sense 
by  heating.  At  about  213°  orthophosphoric  acid  loses  water 
and  yields  pyrophosphoric  acid. 

2H3P04^H4P207+H20. 

At  about  400°  pyrophosphoric  acid  loses  one  molecule  of  water 
and  yields  metaphosphoric  acid. 


When  heated  to  higher  temperatures  the  remaining  molecule  of 
water  is  lost  and  phosphorus  pentoxide  remains 

2HP03-»P205+H20. 

Phosphorus  pentoxide  is  thus  seen  to  be  the  final  product  of  any 
of  the  three  acids  when  the  acid  is  heated  to  a  high  temperature 
and  this  is  because  a  volatile  substance  (water)  is  produced  by 
heating.  Just  as  the  acids  are  compounds  of  phosphorus  pent- 
oxide  and  water,  so  the  salts  may  be  regarded  as  compounds 
of  phosphorus  pentoxide  and  metallic  oxide  (which  is  analogous 
to  hydrogen  oxide).  Consequently  the  extent  to  which  the 
salts  may  be  decomposed  by  heating  will  be  conditioned  by  the 
nature  of  the  metallic  oxide  or,  in  other  words,  by  its  degree  of 
volatility.  Thus  the  normal  phosphates  of  sodium,  potassium, 
magnesium,  calcium,  etc.,  are  not  decomposable  at  all,  except 
at  extremely  high  temperatures  where  phosphorus  pentoxide 
begins  to  be  volatile,  while  the  acid  phosphates  of  these  metals 
are  decomposable  to  whatever  extent  is  denoted  by  the  propor- 
tion of  water  that  may  be  formed.  Ammonium  salts  are  con- 
verted completely  into  phosphorus  pentoxide  because,  instead 
of  the  hypothetical  metallic  oxide,  (NH4)20,  there  are  formed 


110  QUANTITATIVE  ANALYSIS 

ammonia  and  water  and  both  of  these  substances  are  volatile. 
Orthophosphoric  and  pyrophosphoric  acids  are  polybasic  and  a 
considerable  variety  of  salts  may  be  prepared,  containing  varying 
amounts  of  metals,  ammonium  and  hydrogen,  so  that  they  may 
be  regarded  as  containing  varying  amounts  of  metallic  oxide, 
ammonia,  water  and  phosphorus  pentoxide. 

Examples. — The  composition  and  decomposition  of  the  three 
phosphoric  acids  and  typical  examples  of  their  salts  are  shown  in 
the  following  statement: 

Composition  |  Decomposition  by  Heat 

Acids 


2HP08 

H4P2O7  =0=  2H2O.P20S 

2H,P04=0=3H2O.P2O6 


2HPO3->P20 
H4P207-»P2Os 
2HsPO4->P2O5+ 3H2O 


Normal  Potassium  Salts 


2KPOS  OK 
K4P207  0=  2K2O.P2O6 
2K,P0403K2O.P2Os 


Not   decomposed  except  by  slow  loss  of 
P2O»  at  high  temperatures 


Potassium  Acid  Salts 


No  acid  metaphosphate  possible 
K2H2P2O7=c=  K2O.H2O.P2OB 
2K2HPO4  =^  2K2O.H2O.P2OS 
2KH2PO4  O  K2O.2H2O.P2OB 


K2H2P207->2KPO3  +  H2O 
2K2HP04->K4P2O7  +  H2O 
2KH2PO4->2KP03  +  2H2O 


Potassium  Ammonium  Salts 


No  double  metaphosphate  possible 
K2(NH4)2P2070  K20.(NH4)2O.P2O8 
2K2NH4PO4      =C=  2K2O.(NH4)2O.P2O6 
2K(NH4)2PO4  OrKz 


2K2NH4PO4-»K4P2O74-2NH3  +  H2O 
2K(NH4-)  2PO4->  2KPO3  +  4NH  j -f- 2H2O 


Monomagnesium  ammonium  orthophosphate,  Mg(NH4)4(PO4)2, 
is  analogous  to  monopotassium  ammonium  orthophosphate, 
K(NH4)2P04,  as  may  be  seen  from  the  structural  formulae: 


x  NH4 

4-,T04    and  N 

4/  Mg 


4 
NH4/ 

Its  decomposition  can  therefore  proceed  as  far  as  magnesium 
metaphosphate  : 

Mg(NH4)4(P04)2-^Mg(P03)2+4NH3+2H20. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  111 

while  dimagnesium  ammonium  orthophosphate  can  decompose 
only  as  far  as  the  pyrophosphate : 

2MgNH4P04-^Mg2P2O7+2NH34-H2O. 

This  makes  it  necessary  that  such  conditions  shall  be  maintained 
as  will  make  possible  the  formation  of  but  one  double  salt,  in 
order  that  the  composition  of  the  ignited  precipitate  may  be 
definite  and  constant. 

Rate  of  Crystallization. — The  complete  precipitation  of 
dimagnesium  ammonium  orthophosphate  takes  place  only 
after  standing  for  some  time.  Formerly  it  was  considered  neces- 
sary to  allow  24  hours  for  the  action  to  proceed.  It  is  now  gener- 
ally considered  that  from  2  to  3  hours  is  sufficient  for  the  pre- 
cipitation of  all  but  a  minute  amount,  negligible  under  ordinary 
circumstances.  The  usual  method  of  testing  with  excess  of 
reagent,  to  determine  whether  precipitation  is  complete,  is 
rendered  useless  because  of  the  slow  crystallization  of  the  pre- 
cipitate unless  several  hours  are  allowed  for  the  possible  pre- 
cipitation of  small  amounts.  The  crystalline  precipitate  may  be 
readily  filtered  and  washed  by  a  dilute  solution  of  ammonium 
hydroxide  or  ammonium  nitrate.  The  precipitate  is  appreciably 
soluble  in  distilled  water.  An  application  of  the  laws  of  solubility, 
discussed  under  the  head  of  " Precipitation"  (page  15)  would 
lead  to  the  conclusion  that  any  one  of  the  three  classes  of  soluble 
compounds:  phosphates,  ammonium  salts  or  magnesium  salts, 

will  lessen  the  solubility  of  ammonium  magnesium  phosphate 

+         ++ 
since  the  latter  dissociates  into  the  three  ions  NH4,  Mg  and  PO^ 

The  addition  of  magnesium  salts  to  the  washing  fluid  is  clearly 
out  of  the  question  if  magnesium  is  to  be  determined. 
Phosphates  must  themselves  be  removed  by  washing  because 
only  the  ammonium  phosphates  are  entirely  volatile  and  these 
only  with  some  difficulty.  Either  ammonium  hydroxide  or 
ammonium  nitrate  is  suitable  for  the  purpose,  excess  of  either 
being  driven  off  during  drying  and  ignition  of  the  precipitate. 

Ignition. — Considerable  difficulty  is  often  experienced  in 
obtaining  pure,  white  magnesium  pyrophosphate  by  igniting  the 
magnesium  ammonium  orthophosphate.  This  is  usually  due 
to  imperfect  washing,  sodium  phosphate  being  left  in  the  pre- 


112  QUANTITATIVE  ANALYSIS 

cipitate.  Upon  heating,  traces  of  the  salt  cause  partial  fusion, 
particles  of  carbon  being  enclosed  and  oxidation  made  difficult. 
Thorough  washing  followed  by  long  heating  at  high  temperatures 
is  the  only  remedy. 

A  similar  method  may  also  be  used  for  the  determination  of 
arsenic  acid,  the  precipitate  of  magnesium  ammonium  arsenate 
being  heated  at  a  moderate  temperature  until  it  forms  magnesium 
pyroarsenate  : 


Determination.  —  Weigh  into  Pyrex  beakers  portions  of  about  0.3  gm 
of  a  magnesium  salt.  If  the  salt  is  soluble  in  water  dissolve  in  about 
100  cc  of  distilled  water  and  add  a  drop  of  concentrated  hydrochloric 
acid.  If  not  soluble  in  water  dissolve  in  hydrochloric  acid  (1  part  of 
concentrated  acid  to  1  part  of  water),  warming  if  necessary.  Cool  and 
drop  in  a  very  small  piece  of  litmus  paper  and  then  add,  slowly  and  with 
stirring,  dilute  ammonium  hydroxide  until  the  solution  is  faintly  basic. 
Now  add  from  a  pipette,  slowly  and  with  stirring,  15  cc  of  a  clear 
10  percent  solution  of  disodium  orthophosphate.  Allow  to  stand  for 
15  minutes  until  a  considerable  part  of  the  precipitate  has  appeared, 
then  add  concentrated  ammonium  hydroxide  solution  (sp.  gr.  0.90),  in 
such  quantity  that  the  solution  shall  finally  contain  ammonium  hydrox- 
ide equivalent  to  one-ninth  of  its  total  volume.  Cover  and  allow  to 
stand  for  three  hours  or  stir  continuously  for  30  minutes. 

Filter  the  precipitate  on  a  filter  of  extracted  paper,  in  a  weighed 
platinum  Gooch  crucible  or  in  an  ignited  and  weighed  alundum  crucible 
and  wash  until  free  from  chlorides  with  a  solution  containing  2  percent 
of  ammonia  or  5  percent  of  ammonium  nitrate,  finally  testing  the 
washings  with  silver  nitrate  after  acidifying  with  nitric  acid.  If  a 
Gooch  crucible  has  been  used  place  the  cap  on  the  bottom  and  heat 
over  the  burner  until  dry,  then  over  the  blast  lamp  for  20  minutes. 
An  alundum  crucible  is  treated  similarly.  If  a  paper  filter  was  used 
remove  the  paper  from  the  funnel  and,  if  sufficient  precipitate  is  present 
to  make  its  removal  from  the  paper  feasible,  dry  and  remove  most  of 
the  precipitate  to  a  sheet  of  glazed  paper,  refold  the  paper  and  place 
in  a  weighed  porcelain  or  platinum  crucible.  Incline  the  crucible  with 
the  cover  leaned  against  it  and  heat  gently  over  the  burner  until  the 
paper  is  completely  burned  and  the  precipitate  is  nearly  white.  After 
the  precipitate  is  white  or  gray  the  main  portion  is  added  and  the  cruci- 
ble is  heated  for  20  minutes  over  the  blast  lamp,  cooled  in  the  desiccator 
and  weighed.  From  the  weight  of  magnesium  pyrophosphate  calculate 
that  of  magnesium  and  the  percent  of  magnesium  in  the  original  sample. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  113 

PHOSPHATES 

For  the  precipitation  of  the  phosphate  radical  as  magnesium 
ammonium  phosphate  it  is  necessary  that  no  metal  that  can_ 
form  an  insoluble  phosphate  shall  be  present.  In  case  this 
condition  is  not  fulfilled  a  preliminary  separation  of  the  phosphate 
radical  is  made  by  precipitating  ammonium  phosphomolybdate 
from  a  solution  containing  free  nitric  acid.  This  operation  is 
described  on  page  453  and  following,  in  the  discussion  of  the 
determination  of  phosphorus  in  steel.  A  sample  containing 
no  metals  other  than  those  of  the  alkali  group  does  not  require 
this  treatment.  This  simpler  determination  will  now  be 
described. 

Determination. — Prepare  a  solution  of  "magnesia  mixture"  as  follows: 
Dissolve  55  gm  of  crystallized  magnesium  chloride  and  140  gm  of 
ammonium  chloride  in  water,  add  130  cc  of  ammonium  hydroxide 
(specific  gravity  0.90)  and  dilute  to  1000  cc.  If  this  solution  is  kept 
in  stock  for  any  considerable  time  it  will  acquire  a  flocculent  precipitate 
of  hydrated  silica,  derived  from  solution  of  the  glass  by  the  base.  The 
solution  must  be  clear  when  used.  This  condition  may  be  insured  by 
filtering  the  solution  or  by  preparing  only  enough  of  the  reagent  to  last 
a  short  time. 

Weigh  duplicate  samples  of  0.2  to  0.4  gm  of  the  phosphate  into  beakers 
of  resistance  glass,  dissolve  and  dilute  to  75  cc.  Drop  in  a  very  small 
bit  of  litmus  paper  and  if  a  basic  reaction  is  not  shown  add  dilute 
ammonium  hydroxide  until  the  paper  becomes  blue.  Add  10  cc  of 
a  10  percent  solution  of  ammonium  chloride,  mix  and  then  add, 
very  slowly,  "magnesia  mixture"  sufficient  in  quantity  to  precipitate 
all  of  the  phosphate.  As  the  precipitate  does  not  form  rapidly  in  a 
barely  basic  solution  it  is  not  always  easy  to  determine  when  enough 
of  the  reagent  has  been  added.  It  is  then  best  to  use  what  is  thought 
to  be  a  good  excess  and  to  rely  upon  testing  the  filtrate  which  is  obtained 
later. 

The  rest  of  the  procedure  is  exactly  the  same  as  in  the  determination  of 
magnesium,  with  the  single  exception  that  "magnesia  mixture"  instead 
of  sodium  phosphate  solution  is  used  in  testing  for  the  completion  of 
precipitation.  From  the  weight  of  magnesium  pyrophosphate  finally 
obtained  calculate  the  percent  of  the  phosphate  radical,  of  phosphorus 
pentoxide  or  of  phosphorus,  the  report  depending  upon  the  nature  of  the 
sample  examined. 


114  QUANTITATIVE  ANALYSIS 

MANGANESE 

The  method  of  Gibbs  for  manganese  depends  upon  the  same 
chemical  principles  as  are  involved  in  the  determination  of 
magnesium.  A  soluble  orthophosphate  is  added  to  the  solution 
of  the  manganese  salt  and  the  solution  is  then  made  basic  with 
ammonium  hydroxide.  Dimanganese  ammonium  orthophos- 
phate is  precipitated  and  this,  when  ignited,  gives  manganese 
pyrophosphate. 


2MnNH4P04->Mn2P207+2NH3+H2O. 

If  the  manganese  is  already  in  its  lowest  state  of  oxidation, 
precipitation  is  accomplished  without  further  change.  If  it 
is  in  the  form  of  a  manganate  or  permanganate  or  of  manganese 
dioxide  it  is  first  reduced  by  sulphurous  acid: 


Mn02+H2S03-»MnSO4+H2O. 

Determination.  —  Weigh  enough  of  the  sample  to  contain  about  0.1 
gm  of  manganese.  If  the  sample  is  a  permanganate  or  manganese  di- 
oxide dissolve  in  50  cc  of  a  saturated  solution  of  sulphurous  acid,  con- 
taining also  1  percent  of  hydrochloric  acid,  filter  if  necessary  and  boil 
to  expel  the  excess  of  sulphur  dioxide. 

If  the  sample  is  a  soluble  manganese  salt  omit  the  treatment  with 
sulphurous  acid.  In  either  case  proceed  as  follows  : 

Add  to  the  solution  in  a  Pyrex  beaker,  3  percent  more  than  the  quan- 
tity of  10  percent  disodium  phosphate  solution  calculated  to  be  neces- 
sary for  complete  precipitation  of  the  manganese.  Heat  to  boiling  and 
add  dilute  ammonium  hydroxide  solution,  drop  by  drop  with  constant 
stirring,  until  a  precipitate  begins  to  form.  Boil  and  stir  until  this  pre- 
cipitate becomes  crystalline,  then  add  another  drop  of  ammonium  hy- 
droxide and  stir  and  boil  until  the  additional  amorphous  precipitate 
becomes  crystalline.  Continue  this  process  until  further  addition  of 
ammonium  hydroxide  produces  no  precipitate.  All  of  the  precipitate 
should  now  be  in  the  crystalline  condition.  Add  0.5  cc  excess  of  ammo- 
nium hydroxide,  then  cool  the  solution  by  placing  the  beaker  in  ice-water. 
Filter  and  wash  with  a  clear,  slightly  basic,  10  percent  solution  of  am- 
monium nitrate  or  a  2  percent  solution  of  ammonium  hydroxide  until 
free  from  chlorides,  then  ignite  with  the  same  precautions  as  were  ob- 
served in  the  ignition  of  magnesium  ammonium  phosphate.  From  the 
weight  of  manganese  pyrophosphate  obtained  calculate  the  percent  of 
manganese  in  the  sample. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  115 

CHLORINE,  BROMINE  AND  IODINE 

The  members  of  the  halogen  group  may  occur  in  different 
forms,  requiring  different  methods  of  procedure.  This  occur- 
rence may  be  as  free  halogens,  as  oxyacids  or  salts,  as  hydracids 
or  salts,  or  as  organic  compounds.  The  gravimetric  determina- 
tion of  the  negative  radical  of  the  halogen  hydracids  is  invariably 
made  by  precipitating  and  weighing  the  silver  salt.  The 
solubilities  of  the  latter,  as  well  as  the  principles  involved  in 
the  precipitation,  washing  and  ignition,  were  discussed  under  the 
description  of  the  determination  of  silver.  The  procedure  is  also 
similar  to  that  involved  in  the  determination  of  silver,  the  silver 
salt  (silver  nitrate)  being,  in  this  case,  the  precipitant  while  the 
chloride,  bromide  or  iodide  is  the  substance  being  investigated. 
Separation  of  Chlorine  and  Iodine.  —  If  chlorides  and  iodides 
occur  together  the  iodine  may  be  precipitated  as  palladious  iodide 
by  a  solution  of  palladious  chloride,  PdCl2.  In  another  portion 
the  total  halogen  may  be  precipitated  by  silver  nitrate  and 
weighed  as  a  mixture  of  silver  iodide  and  chloride.  The  proper 
weight  of  silver  iodide,  as  calculated  from  the  weight  of  pal- 
ladious iodide  found,  is  subtracted  and  chlorine  calculated  from 
the  remainder. 

Indirect  Method.  —  The  chlorine  and  iodine  may  be  determined 
indirectly  by  precipitating  by  excess  of  silver  nitrate,  weighing  the 
mixed  chloride  and  iodide,  then  converting  the  silver  iodide  into 
chloride  by  heating  in  an  atmosphere  of  chlorine  and  reweighing. 
If  x  =  weight  of  chlorine,  a  =  weight  of  silver  chloride  and 
iodide,  b  =  weight  after  conversion  of  silver  iodide  into  chloride 

and  y  =  weight  of  iodine,  then  -^-^  -  x  =  weight  of  silver  chloride 

oO.4o 

234.80 
and  12Q92  y  =  we^t  of  silver  iodide,  therefore: 

143.34      .  234.80 

'  ^  ^-^ 


35.46          126.  92 
143.34        143.34 


,0. 


Subtracting  (2)  from  (1),  ~       y  =  a-b. 

Then  2/  =  1.3877(a-6),  (3) 

z  =  0.63506-0.3877a.  (4) 


116  QUANTITATIVE  ANALYSIS 

Problems 

In  a  manner  similar  to  that  shown  above  for  chlorine  and  iodine,  derive 
the  following  formulae: 

2.  x  =2.7855  b -1.7004  a, 
?/=2.7004a-3.53806, 

where  x  =  weight  of  bromine,  y=  weight  of  iodine,  a  =  weight  of  silver 
bromide  and  iodide  and  6=  weight  of  silver  chloride  after  chlorination. 

3.  x  =  1.0451  6-0.7976  a, 
y- 1.7974  (0-6), 

where  x  =  weight  of  chlorine,  y=  weight  of  bromine,  a  =  weight  of  silver 
chloride  and  bromide  and  b=  weight  of  silver  chloride  after  chlorination. 

Determination  of  Two  Halogens  in  Mixed  Halides,  Indirect  Method- 

— Use  about  0.5  gm  of  sample.  Dissolve  in  75  cc  of  water,  add  0.5  cc 
of  dilute  nitric  acid  and  then,  drop  by  drop  and  with  constant  stirring, 
a  slight  excess  of  5  percent  silver  nitrate  solution.  10  to  30  cc  will 
be  sufficient.  Digest  at  near  the  boiling  temperature  until  the  precipi- 
tate settles  readily,  leaving  a  clear  supernatant  solution.  Test  for 
completion  of  the  precipitation  then  filter  in  a  prepared  and  weighed 
Gooch  crucible  and  wash  free  from  excess  of  silver  nitrate.  Dry  at  105° 
to  constant  weight. 

Ignite  a  small  porcelain  boat  and  cool  in  a  desiccator.  Place  this  on  a 
sheet  of  black  glazed  paper  and  carefully  remove  the  asbestos  filter 
and  every  particle  of  the  silver  halides  from  the  filter,  placing  these  in  the 
boat.  Allow  this  to  remain  in  the  desiccator  for  15  minutes,  then  weigh. 
From  this  weight  subtract  that  of  the  silver  halides,  already  found. 
This  gives  as  the  remainder  the  weight  of  the  boat  plus  asbestos. 

Prepare  a  chlorinating  apparatus  according  to  Fig.  34. 

A  is  a  distilling  flask  of  100  cc  capacity,  fitted  with  a  funnel  tube  which 
reaches  to  the  bottom  of  the  flask.  The  latter  is  connected  at  the  side 
with  a  glass  combustion  tube,  B,  of  12  to  15  mm  internal  diameter 
and  40  cm  length.  Corks  are  used  in  the  ends  of  the  combustion  tube. 

Place  the  entire  apparatus  under  a  hood.  Insert  the  boat  containing 
the  silver  halides  and  asbestos  into  the  middle  of  the  tube.  Place 
10  gm  of  potassium  permanganate  in  the  flask  and  then  pour  into  the 
tube  5  cc  of  concentrated  hydrochloric  acid.  Warm,  if  necessary,  to 
start  the  reaction  and  when  the  tube  is  filled  with  chlorine  carefully  heat 
directly  under  the  boat,  using  a  wing  burner.  Add  more  acid  to  the 
flask  as  it  may  become  necessary,  in  order  to  maintain  a  slow  evolution 
of  chlorine,  and  heat  the  boat  for  15  minutes  to  a  temperature  just  under 
the  fusing  point  of  the  silver  chloride.  Finally  raise  the  temperature 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS 


117 


until  fusion  barely  begins,  then  remove  the  flame.  Disconnect  the  tube 
from  the  flask,  attach  a  tube  of  calcium  chloride  to  one  end  and  slowly 
draw  air  through  until  the  tube  is  quite  cool.  Remove  the  boat  from 
the  tube  and  test  the  odor  to  determine  whether  all  free  chlorine  has 
been  removed.  Dry  for  30  minutes  at  105°,  cool  in  the  desiccator  and 
weigh.  From  this  weight  subtract  that  of  the  boat  plus  asbestos.  The 
remainder  is  the  weight  of  silver  chloride.  From  this  and  the  weight  of 
the  mixed  silver  halides  and  of  the  sample  calculate  the  percent  of  each 
halogen  in  the  sample,  using  one  of  the  formulas  derived  above. 
The  entire  experiment  should  be  conducted  away  from  bright  light. 


B 


FIG.  34. — Apparatus  for  chlorination  of  mixed  halides  of  silver. 

Chlorine  and  iodine  may  also  be  separated  by  the  method  of 
graded  oxidation,  to  be  described  in  connection  with  the  separa- 
tion of  the  three  halogens. 

Separation  of  Bromine  and  Iodine. — Mixtures  of  bromides 
and  iodides  may  be  analyzed  by  methods  similar  to  those  de- 
scribed above.  Palladious  bromide  is  sufficiently  soluble  to 
make  possible  the  precipitation  of  palladious  iodide  in  the  pres- 
ence of  the  bromide.  Also  either  the  indirect  analysis  or  the 
method  of  graded  oxidation  may  be  used. 

Separation  of  Chlorine  and  Bromine. — For  mixtures  of 
chlorides  and  bromides  either  the  method  of  graded  oxidation 
or  that  of  indirect  analysis  may  be  used.  For  the  latter  silver 


118  QUANTITATIVE  ANALYSIS 

chloride  and  bromide  are  weighed  together  and  the  bromide  is 
then  converted  into  silver  chloride  and  re  weighed. 

Separation  of  Chlorine,  Bromine,  and  Iodine,  by  Graded  Oxi- 
dation.— In  the  discussion  of  the  decomposition  voltages  of 
electrolytes  (page  141)  it  is  shown  that  any  electrically  neutral 
element  that  is  capable  of  ion  formation  will,  when  placed  in 
contact  with  a  solution  containing  its  ions,  generate  a  definite 
potential  difference  whose  magnitude  depends  upon  the  solution 
tension  of  the  element  and  the  concentration  of  its  ions  already  in 
solution.  Two  such  systems  will  generate  a  definite  electro- 
motive force  if  external  connection  is  made  between  the  non- 
ionized  elements  and  if  the  two  solutions  are  brought  into  contact. 
This  electromotive  force  is  always  in  the  direction  that  would 
cause  a  current  to  flow  externally  from  the  element  having  the 
less  solution  tension  (if  its  ions  are  positive,  or  the  greater  if 
its  ions  are  negative)  to  the  one  having  the  greater  solution 
tension.  When  a  metal  passes  into  solution  and  forms  positive 
ions  it  is  thereby  oxidized.  When  an  element  capable  of  negative 
ion  formation  passes  into  solution  and  forms  ions  it  is  thereby 
reduced.  Conversely,  when  metallic  ions  are  converted  into  mas- 
sive, uncharged  metal  they  are  reduced  and  when  non-metallic  or 
negative  ions  are  discharged  they  are  thereby  oxidized.  Ac- 
cording to  this  view  oxidation  consists  in  the  addition  of  positive 
charges  or  the  removal  of  negative  ones,  while  reduction  is  the 

addition  of  negative  charges  or  the  removal  of  positive  ones. 

++     +++ 
Thus  the  change :  Fe— >Fe  (change  of  the  ferrous  ion  to  the  ferric 

ion)  is  oxidation,  while  the  reverse  is  reduction  and  MnO4— > 

MnO4  (change  of  the  permanganate  ion  to  the  manganate  ion) 
is  reduction,  while  the  reverse  is  oxidation.  It  follows  from 
these  statements  that  the  metal  having  the  greater  solution 
tension  is  the  stronger  reducing  agent  while  the  non-metallic 
element  having  the  greater  solution  tension  is  the  better  oxidizing 
agent.  It  thus  becomes  •  possible  to  compare  the  activities  of 
two  oxidizing  or  reducing  agents  by  measuring  the  magnitude 
and  direction  of  the  electromotive  force  produced  by  combining 
two  systems  made  up  of  these  agents  in  contact  with  solutions 
containing  the  respective  products  of  reduction  or  oxidation. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  119 

If  the  oxidizing  or  reducing  agents  are  solids  the  electrodes 
are  composed  of  these  solids.  If  gases  the  electrode  is  of  some 
material  that  will  superficially  dissolve  the  gases,  while  if  the 
agents  are  solutions  they  are  merely  brought  into  contact  with 
electrodes  made  of  indifferent  metals,  such  as  platinum.  Thus 
silver  as  a  reducing  agent  would  itself  be  made  to  form  the 
electrode  material,  in  contact  with  an  ionized  silver  salt.  Oxygen 
as  an  oxidizing  agent  would  be  caused  to  bubble  into  the  solution 
in  contact  with  an  electrode  of  platinum  which  is  coated  with 
platinum  black,  in  which  oxygen  dissolves,  and  this  would  be 
immersed  in  a  solution  containing  hydroxyl  ions.  Potassium 
permanganate  would  be  used  in  simple  contact  with  a  platinum 

electrode   and    the   solution   would   also    contain   the   positive 

++  — 

manganese  ion,  Mn,  the  product  of  reduction  of  the  ion  Mn04. 
In  the  last  case  the  force,  solution  tension,  is  replaced  by  the 
tendency  of  the  permanganate  anion,  already  in  solution,  to 
become  reduced. 

Oxidation  Potential. — All  of  the  elementary  halogens  are 
oxidizing  agents  because  they  exhibit  a  tendency  toward  negative 
ion  formation: 

C1-»C1,  Br-»Br,  etc. 

Conversely  the  change  of  halogen  ions  into  neutral  elements  is  an 
oxidation  of  these  ions.  In  order  to  bring  about  this  oxidation 
it  is  necessary  to  apply  another  oxidizing  agent  whose  "  oxida- 
tion potential, "  F  (potential  of  the  nonionized  electrode  minus 
that  of  the  electrolyte)  is  greater.  This  is  analogous  to  the 
decomposition  of  a  halide  by  means  of  the  current,  which  is  an 
oxidizing  agent  for  the  negative  ion  and  a  reducing  agent  for 
the  positive  ion. 

Selective  Oxidation. — If  an  oxidizing  agent  can  be  found, 
having  an  oxidation  potential  greater  than  that  of  one  of  the 
halogens  and  less  than  that  of  another  this  agent  may  be  used 
for  separating  the  two  halogens  by  oxidation  of  the  salt  (or  acid) 
of  one,  removing  the  liberated  element  by  distillation  and  leaving 
the  other,  which  was  incapable  of  being  oxidized  by  this  agent. 
This  is  analogous  to  the  electrolytic  separation  of  metals  by 
grading  the  electromotive  force  which  is  applied  to  two  electrodes 


120  QUANTITATIVE  ANALYSIS 

in  the  solution.  The  measurement  of  oxidation  potentials 
should  therefore  furnish  valuable  information  for  assisting  in 
the  selection  of  oxidizing  agents  suitable  for  graded  oxidation 
of  the  anions  of  the  halogen  hydracids.  Bancroft  has  shown1 
that  the  following  differences  exist  between  the  oxidation  po- 
tentials of  chlorine,  bromine  and  iodine  in  solutions  of  salts 
of  their  respective  hydracids: 

Chlorine  in  potassium  chloride  —  bromine  in  potassium  bro- 
mide =0.241  volt. 

Bromine  in  potassium  bromide  —  iodine  in  potassium  iodide  = 
0.535  volt. 

These  differences  vary  somewhat  if  the  concentrations  are 
altered. 

As  examples  of  oxidizing  agents  which  will  serve  for  the  iodine 
anion  without  thereby  oxidizing  bromine  or  ohlorine  anions, 
may  be  mentioned  monopotassium  arsenate  and  nitrous  acid. 
These  were  suggested  by  Gooch.2  The  oxidation  potential  of 
nitrous  acid  (or  potassium  nitrite  and  sulphuric  acid),  according 
to  the  measurements  of  Bancroft,  is  0.249  volt  higher  than  that 
of  iodine  in  potassium  iodide,  and  0.285  volt  lower  than  that 
of  bromine  in  potassium  bromide. 

For  the  selective  oxidation  of  the  bromine  anion  in  presence 
of  the  chlorine  anion  the  following  substances  have  been  used: 
potassium  permanganate  in  acid  solution,  lead  peroxide  in  acid 
solution,  potassium  dichromate  in  acid  solution,  ammonium 
persulphate  in  neutral  solution  and  potassium  iodate  in  acid 
solution.  With  the  exception  of  the  last  named  oxidizing  agent 
all  of  these  substances  possess  oxidation  potentials  higher  than 
that  of  chlorine  in  potassium  chloride  and  could  therefore  be 
made  to  serve  for  a  quantitative  separation  of  bromine  and 
chlorine  only  by  carefully  regulating  the  concentrations  of  oxi- 
dizing and  reducing  agents  and  by  stopping  the  distillation  at 
exactly  the  correct  time.  Attempts  have  been  made  to  regulate 
the  speed  of  oxidation  by  acidifying  with  substances  that  can 
furnish  only  a  small  concentration  of  hydrogen  ions,  since  it  is 
only  in  presence  of  hydrogen  ions  that  the  reactions  can  proceed. 
Weakly  acid  substances  that  have  been  used  for  this  purpose 

1  Z.  physik.  Chem.,  10,  387  (1892). 

2  Chem.  News,  61,  235  (1890). 


EXPERIMENTAL  GRAVIMETRIC-  ANALYSIS  121 

are  acetic  acid,  potassium  acid  sulphate,  ferrous  sulphate  and 
aluminium  sulphate.  The  last  two  are  weakly  acid  through 
hydrolysis. 

Potassium  permanganate  is  the  oxidizing  agent  used  by  Jan- 
nasch  and  Aschoff1  and  the  oxidation  of  hydrochloric  acid  is 
prevented  by  acidifying  with  an  acid  no  stronger  than  acetic 
acid  and  by  employing  a  large  dilution.  The  oxidation  potential 
of  potassium  permanganate  with  sulphuric  acid  is  0.097  volt 
higher  than  that  of  chlorine  with  potassium  chloride. 

The  reduction  of  potassium  permanganate  is  really  a  reduction 
of  manganese  itself,  being  a  change  of  heptavalent  into  bivalent 
manganese.  The  complete  equation  is 


5Br+4H20. 
The  ionic  change  involving  manganese  is 

Mn04-4in-j-2O2. 

The  electrical  change  of  manganese  itself  is  not  oxidation,  as 
would  appear  from  the  last  equation,  but  reduction,  because  the 
univalent  anion,  MnO4,  is  composed  of  one  atom  of  heptavalent 
positive  manganese  and  four  atoms  of  bivalent  negative  oxygen, 
BO  that  the  change  of  manganese  is  really 

V+V      +  + 
Mn—  KMn. 

If  the  substance  being  analyzed  is  known  to  be  a  pure  mixture 
of  only  two  of  the  halides,  one  of  the  halogens  may  be  liberated 
and  removed  by  distillation  without  subsequent  absorption,  the 
other  being  determined  in  the  residual  solution.  If  it  is  not  a 
pure  mixture  or  if  it  contains  salts  of  three  halogens  it  is  necessary 
to  absorb  at  least  one  of  these  and  make  a  direct  determination 
of  it  in  the  absorbing  solutions. 

Bugarszky2  used  potassium  iodate  and  sulphuric  acid  for 
separating  bromine  and  chlorine,  distilling  the  bromine  without 
absorption.  The  oxidation  potential  of  acidified  potassium 
iodate  was  found  by  Bancroft  to  be  0.064  volt  higher  than  that 

1  Z.  anorg.  Chem.,  1,  144  and  245  (1892);  5,  8  (1894). 

2  Ibid.,  10,  387  (1895). 


122  QUANTITATIVE  ANALYSIS 

of  bromine  and  0.178  volt  lower  than  that  of  chlorine.     The 
reaction  is  as  follows: 

KI03+5KBr+3H2S04->3K2SO4+5Br+I+3H2O. 
Both  bromine  and  iodine  are  liberated  and  distilled  and  account 
of  this  must  be  taken  if  the  free  halogens  are  absorbed  and  subse- 
quently determined.  Chlorine  is  determined  in  the  residual 
solution,  after  reducing  the  excess  of  iodic  acid  to  hydriodic  acid 
by  means  of  sulphurous  acid,  then  oxidizing  by  nitrous  acid  and 
distilling. 

Andrews1  modified  the  method  of  Bugarszky  by  substituting 
nitric  acid  for  sulphuric  acid  and  by  reducing  the  excess  of  iodic 
acid  by  means  of  phosphorous  acid.  His  method  was  not  tested, 
however,  except  for  the  determination  of  chlorides  in  crude 
bromides  and  of  chlorine  in  crude  bromine  In  both  cases  the 
chlorine  was  present  in  relatively  small  quantities  (less  than  10 
percent)  and  it  was  not  adapted  to  the  determination  of  both 
bromine  and  chlorine.  For  the  determination  of  chlorine, 
bromine  and  iodine  by  direct  means;  the  method  of  Jannasch 
and  Aschoff  is  probably  the  best  of  all  methods  yet  proposed, 
even  though  permanganic  acid  is  not  an  ideal  oxidizing  agent 
for  the  separation  of  chlorine  and  bromine.  In  this  method 
the  solution  of  mixed  chlorides,  bromides  and  iodides  is  first 
acidified  with  sulphuric  acid  and  potassium  nitrite  is  added: 

KI+KNO2+2H£S04-+2KHSO4+NO+H2O+L 
The  liberated  iodine  is  distilled,  absorbed,  and  subsequently 
determined.  The  sulphuric  acid  is  then  neutralized  by  sodium 
hydroxide,  acetic  acid  and  potassium  permanganate  are  added 
and  the  bromine  is  distilled,  absorbed  and  determined.  In  the 
residual  solution  the  excess  of  permanganate  is  reduced  and  the 
chlorine  is  determined  gravimetrically. 

The  absorbent  which  best  serves  for  iodine  and  bromine  is  a 
solution  containing  sodium  hydroxide  and  hydrogen  peroxide. 
Bromine  and  iodine  react  with  sodium  hydroxide  to  form  sodium 
bromide  and  sodium  hypobromite  in  the  one  case  and  sodium 
iodide  and  sodium  hypoiodite  in  the  other: 

2NaOH+2Br-»NaBrO+NaBr+H20, 
2NaOH+2L-»NaIO-fNaI-fH20. 

1  J.  Am.  Ghem.  Soc.,  29,  275  (1907). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS 


123 


If  the  solutions  are  allowed  to  stand  for  some  time  bromates  and 
iodates  are  formed: 

3NaBrO-»NaBrO3-f2NaBr, 
3NaIO->NaIO,-f2NaI. 

The  last  change  does  not  take  place  if  hydrogen  peroxide  is 
present  and  the  solution  is  kept  cold,  the  hypobromite  and 
hypoiodite  being  reduced  as  fast  as  formed: 


NaBrO+H2O2- 
NaIO+H202- 


>NaBr+H2O+O2, 
>NaI+H20+02. 


In  the  resulting  solutions  bromine  and  iodine  may  be  determined 
as  the  silver  salts  in  the  usual  manner  after  acidifying  with 
sulphuric  acid. 


•"  ^ V  \ I  I i 

FIG.  35. — Apparatus  for  the  separation  of  the  halogens. 


If  the  absorbing  solution  has  been  allowed  to  become  warm 
some  iodate  or  bromate  will  be  formed.  In  this  case  acidification 
will  cause  the  liberation  of  free  halogen  which  will  escape  precipi- 
tation. If  silver  nitrate  is  added  before  acidifying,  any  iodate 
or  bromate  will  remain  in  solution  as  the  silver  salt.  This  inter- 
ference of  oxysalts  may  also  be  prevented  by  the  addition  of  a 
sulphite  before  the  addition  of  acid.  The  resulting  sulphurous 
acid  then  reduces  the  iodate  or  bromate  to  iodide  or  bromide. 

During  the  distillation  of  bromine  and  iodine  it  is  essential 
that  contact  with  cork  or  rubber  be  avoided,  since  the  halogens 
are  thereby  reduced  and  absorbed.  Ground-glass  stoppers  are 
necessary  in  all  parts  of  the  apparatus  where  such  contact  would 


124  QUANTITATIVE  ANALYSIS 

occur  and,  where  rubber  connections  are  used,  the  glass  tubes 
inside  must  be  pushed  together  so  as  to  expose  as  little  of  the 
rubber  tubing  as  is  possible.  All  reagents  must  be  tested  and 
found  free  from  the  halogens. 

Determination. — Weigh  about  1  gm  of  the  mixture  of  halides,  placing 
the  sample  in  a  round  bottomed,  glass  stoppered  distilling  flask,  having 
a  capacity  of  1000  cc,  and  having  an  inlet  tube  sealed  into  the  side 
of  the  neck  and  reaching  to  the  bottom  of  the  flask.  Connect  the  appa- 
ratus as  shown  in  Fig.  35.  A  is  a  vessel  in  which  steam  may  be  generated, 
B  is  the  distilling  flask,  C  and  D  are  bubble  tubes  having  a  capacity 
of  150  cc.  The  tube  a  should  reach  to  the  bottom  of  the  steam  generator 
and  should  extend  about  18  inches  above.  This  tube  provides  an  inlet 
for  air,  in  case  there  is  any  tendency  toward  drawing  liquid  back  from  B. 

Each  of  the  absorption  tubes  C  and  D  contains  50  cc  of  5  percent 
sodium  hydroxide  and  50  cc  of  hydrogen  peroxide.  The  union  between 
B  and  C  and  between  C  and  D  should  be  made  by  bringing  the  glass 
tubes  quite  together  inside  the  rubber  connections. 

Dissolve  the  weighed  sample  in  about  600  cc  of  water,  add  5  cc  of 
25  percent  sulphuric  acid  and  2  gm  of  sodium  nitrite  Heat  the  solution 
nearly  to  boiling  and  pass  steam  through  the  flask  for  twenty  minutes 
after  the  solution  is  colorless.  During  this  time  the  tubes  C  and  D 
must  be  kept  cool  by  immersion  in  ice  water. 

When  all  of  the  iodine  has  been  distilled  the  boiling  is  interrupted, 
the  absorption  tubes  are  disconnected  and  their  contents  washed  into 
a  300  cc  beaker.  The  tubes  are  then  returned  to  the  apparatus  and 
are  refilled  with  sodium  hydroxide  and  hydrogen  peroxide  as  before. 
The  solution  in  the  flask  is  barely  neutralized  with  sodium  hydroxide 
solution  and  evaporated  to  a  volume  of  about  500  cc.  1.5  gm  of 
potassium  permanganate  and  60  cc  of  33  percent  acetic  acid  are  added 
and  the  bromine  thus  liberated  is  distilled  and  absorbed  in  hydrogen 
peroxide  and  sodium  hydroxide  in  the  cooled  tubes.  When  steam  has 
been  passed  through  the  solution  for  some  time  after  the  latter  has 
become  colorless  the  distillation  is  again  stopped  and  the  contents  of 
the  tubes  washed  into  another  beaker. 

The  solutions  now  containing  the  iodine  and  bromine  are  boiled  until 
the  excess  of  hydrogen  peroxide  is  completely  decomposed.  0.5  gm 
of  sodium  sulphite  is  added  and  then  dilute  sulphuric  acid  until  the  solu- 
tion is  slightly  acid  in  character.  If  any  color  appears  at  this  point  it  is 
due  to  the  presence  of  iodine  or  bromine  produced  by  iodate  or  bromate, 
showing  that  insufficient  sodium  sulphite  has  been  added.  In  this  case 
0.5  gm  more  is  at  once  added  to  reduce  the  free  halogen. 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  125 

When  the  solutions  are  acid  and  colorless  a  5  percent  solution  of  silver 
nitrate  is  added,  drop  by  drop  from  a  pipette,  stirring  vigorously  until 
no  further  precipitation  occurs.  The  liquid  is  digested  at  near  the  boil- 
ing temperature  until  the  precipitate  settles  readily,  after  which  it  is 
filtered  on  a  Gooch  crucible,  as  directed  on  page  86,  and  the  precipitates 
are  washed  free  from  silver  nitrate,  testing  the  washings  with  dilute 
hydrochloric  acid.  The  crucibles  are  finally  washed  once  with  alcohol 
to  promote  rapid  drying  and  are  then  dried  at  110°  for  one-half  hour  or 
until  the  weight  is  constant.  The  percent  of  iodine  and  of  bromine  is 
calculated. 

The  solution  in  the  large  distilling  flask  is  boiled  with  alcohol  to  reduce 
the  excess  of  potassium  permanganate  and  is  then  poured  into  a  beaker 
or  evaporating  dish  and  evaporated  to  a  volume  of  not  more  than  150cc. 
5  cc  of  dilute  nitric  acid  is  added  and  the  chlorine  is  precipitated  and 
weighed  exactly  as  directed  in  the  case  of  iodine  and  bromine. 

Halogen  Oxyacids. — The  oxyacids  of  the  halogens  (or  their 
salts)  may  be  reduced  to  the  hydracids  by  warming  with  hydrogen 
peroxide,  after  which  the  separation  and  determination  may  be 
accomplished  as  above  directed. 

Free  Halogens  existing  in  solution  may  be  converted  into 
oxysalts  by  treatment  with  alkali  bases, 'after  which  their  separa- 
tion and  determination  may  be  carried  out  by  methods  already 
discussed.  Their  determination  is  more  conveniently  made  by 
volumetric  methods  which  will  be  discussed  later.  Chlorine  in 
gaseous  mixtures  is  also  determined  by  absorption  followed  by  a 
volumetric  process. 

Organic  Halogen  Compounds. — Compounds  of  the  halogens 
with  organic  residues  cannot  be  analyzed  by  the  usual  methods 
because  such  compounds  do  not,  as  a  rule,  ionize  to  form  the 
anions  of  the  halogen  acids.  The  compound  must  be  decomposed 
in  such  a  manner  as  to  leave  the  halogen  in  the  form  of  an  in- 
organic compound  of  one  of  the  well-defined  acids.  In  such 
cases  either  the  lime  method  or  the  method  of  Carius  may  be  used. 

In  the  lime  method  the  material  is  mixed  with  pulverized  lime, 
free  from  halogens,  and  is  placed  in  a  hard  glass  tube,  closed  at 
one  end.  Lime  is  placed  in  the  open  end  of  the  tube,  which  is  then 
heated  in  a  combustion  furnace.  The  organic  compound  is 
decomposed  and  the  halogen  unites  with  the  calcium  oxide  to 
form  calcium  halide,  from  the  acid  solution  of  which  the  halogen 
may  be  precipitated  by  silver  nitrate.  If  more  than  one  halogen 


126  QUANTITATIVE  ANALYSIS 

is  present  the  separation  may  be  made,  after  the  heating  is 
finished,  by  methods  already  outlined. 

In  the  Carius1  method  the  material  is  heated  in  a  closed  tube 
in  contact  with  fuming  nitric  acid  and  silver  nitrate.  The  organic 
compound  is  oxidized  and  the  free  halogen  thus  produced  is 
converted  into  the  hydracid.  The  halogen  hydracid  at  once 
reacts  with  silver  nitrate  and  the  silver  halide  is  later  weighed. 
The  method  is  not  well  adapted  to  separation  of  the  halogens, 
since  a  mixture  of  silver  salts  is  obtained  in  the  tube. 

Determination. — Carius  tubes  of  hard  glass  may  be  obtained  with 
one  end  already  closed.  The  tube  should  be  approximately  50  cm 
long  and  2  cm  in  diameter.  If  such  a  tube  is  not  at  hand  a  good  grade 
of  combustion  tubing  may  be  used.  One  end  is  closed  as  follows:  The 
tube  is  carefully  heated  at  a  point  about  10  cm  from  one  end  by  rotating 
in  the  flame  of  the  blast  lamp.  When  the  glass  has  softened  the  tube  is 
quickly  drawn  out,  until  half  closed.  It  is  allowed  to  cool  and  is  then 
removed  from  the  flame  and  cut  at  the  narrow  part.  The  nearly  closed 
end  is  then  fused  together  until  a  well-rounded  end  is  produced.  This 
must  be  annealed  with  great  care  or  disastrous  breaks  will  occur  later. 

Having  prepared  a  tube  that  is  clean  and  dry,  another  small  tube 
about  4  cm  long  is  closed  at  one  end  to  serve  as  a  weighing  tube.  About 
0.2  gm  of  the  organic  material  is  weighed  into  the  latter.  Into  the 
Carius  tube  is  carefully  placed  about  1.5  gm  of  powdered  silver  nitrate 
and  2  cc  of  fuming  nitric  acid,  free  from  halogens.  The  acid  is  intro- 
duced through  a  funnel  with  a  long  stem  which  reaches  at  least  half 
way  to  the  bottom  of  the  tube,  thus  keeping  the  upper  half  dry.  The 
weigl  ing  tube  containing  the  substance  to  be  analyzed  is  inserted  into 
the  end  of  the  Carius  tube,  the  latter  being  placed  in  a  slanting  position. 
Mixing  of  the  contents  of  the  weighing  tube  with  the  acid  should  not 
occur  until  after  the  Carius  tube  is  sealed.  The  latter  is  now  heated 
about  10  cm  from  the  open  end,  the  tube  is  drawn  out  while  in  the  flame 
and  the  walls  are  sealed  together.  A  more  or  less  blunt  point  should  be 
left  here  as  shown  in  Fig.  36. 

Since  a  high  pressure  will  be  generated  within  the  tube  when  heating 
begins  it  is  necessary  to  place  the  tube  inside  an  iron  tube  having  caps 
screwed  over  the  ends.  The  glass  tubes  frequently  break  on  account  of 
high  pressure.  The  iron  tube  is  now  placed  in  a  suitable  furnace  in  which 
it  may  be  gradually  and  uniformly  heated.  The  temperature  and 
time  necessary  for  heating  will  vary  with  the  nature  of  the  substance 
under  examination.  Most  organic  compounds  will  be  completely 

*Z.  anal.  Chem.,  1,  240  (1861);  4,  451  (1864);  10,  103  (1871). 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  127 

decomposed  by  heating  for  three  hours  at  300°,  while  many  aliphatic 
campounds  will  require  a  temperature  no  higher  than  150°. 

After  the  decomposition  is  completed,  as  shown  by  the  disappearance 
of  carbon  the  furnace  is  allowed  to  cool,  the  iron  pipe  containing  the  tube 
is  carefully  removed,  the  cap  unscrewed  and  the  glass  tube  taken  out. 
The  latter  is  wrapped  in  a  towel,  to  minimize  the  danger  due  to  possible 
explosions,  and  the  point  of  the  tube,  where  it  was  last  sealed  off,  is 
held  in  a  flame  until  softened.  The  internal  pressure  causes  the  glass  to 
blow  out  and  the  gas  escapes,  after  which  the  tube  may  be  handled 
without  risk  of  injury.  A  scratch  is  made  near  the  blown-out  end,  but 


FIG.  36. — Sealed  end  of  Carius  tube. 

on  the  wide  part,  and  this  end  is  broken  off  by  touching  the  scratch  with 
a  hot  glass  rod.  The  contents  of  the  tube  are  rinsed  into  a  beaker, 
diluted  with  water  and  filtered;  the  precipitate  is  washed  and  weighed 
by  the  ordinary  process,  using  either  a  paper  filter  or  a  Gooch  crucible. 
From  the  weight  of  silver  halide  found  the  percent  of  halogen  in  the 
organic  compound  is  calculated. 

If  the  fuming  nitric  acid  contains  halogens,  blank  determinations 
must  be  made  and  corrections  applied. 

CARBONIC  ACID  AND  CARBON  DIOXIDE 

The  following  cases  are  to  be  considered:  Carbon  dioxide  in 
gaseous  mixtures,  solutions  of  carbonic  acid  and  salts  of  car- 
bonic acid. 

Carbon  Dioxide  in  Gaseous  Mixtures  (air,  chimney  gases, 
etc.) — This-  determination  is  best  made  by  gasometric  methods 
which  will  be  considered  in  a  later  section  (pages  333  and  341). 

Carbonic  Acid  in  Solution. — The  most  frequently  occurring 
case  is  that  of  underground  waters.  Such  waters,  coming  from 
regions  of  low  temperature  and  high  pressure,  often  contain  con- 
siderable quantities  of  carbonic  acid.  When  the  water  reaches 
the  surface,  diminished  pressure  and  rise  in  temperature  cause 
the  release  of  more  or  less  carbon  dioxide,  so  that  a  determina- 
tion is  always  subject  to  some  uncertainty  regarding  the  rela- 
tion of  the  original  concentration  of  carbonic  acid  to  that  in  the 
water  as  the  analyst  receives  it.  Determinations  are  also  re- 


128 


QUANTITATIVE  ANALYSIS 


quired  of  carbonic  acid  in  carbonated  drinks.  In  such  cases 
provision  must  be  made  for  transferring  the  solution  from  the 
pressure  bottle  to  the  apparatus  in  which  the  determination  is 
to  be  made  without  loss  of  carbon  dioxide. 

The  procedure  for  the  determination  of  carbonic  acid  in  water 
is  given  on  page  401. 

Carbon  Dioxide  in  Carbonates. — Determinations  of  this  class 
are  by  far  the  most  common  in  general  analytical  practic.  The 


FIG.  37. — Rohrbeck's  appara- 
tus for  determination  of  carbon 
dioxide  by  loss. 


FIG.  38. — Moor's  apparatus  for 
determination  of  carbon  dioxide  by 
loss. 


carbonate  is  decomposed  by  means  of  a  stronger  acid  than  car- 
bonic acid  and  the  carbon  dioxide  determined  in  one  of  three 
ways:  (1)  by  a  determination  of  loss  in  weight,  (2)  by  measuring 
the  gas  disengaged,  or  (3)  by  weighing  this  gas  after  absorbing 
by  reagents  in  a  suitable  apparatus. 

Determination  by  Loss. — Many  forms  of  apparatus  may  be 
obtained  for  the  determination  of  carbonic  acid  by  loss.     Three 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS 


129 


of  these  are  shown  in  Figs.  37,  38,  and  39.  Any  such  apparatus 
must  include  means  for  drying  incoming  air  and  outgoing  gas. 
It  must  also  be  compact  and  not  too  heavy  to  be  weighed  on  the 
analytical  balance.  In  using  such  apparatus  the  sample  is 
weighed  and  brushed  into  the  lower  generating  vessel.  Hydro- 
chloric or  sulphuric  acid  is  placed  in  the  upper  bulb  and  the 
bubble  tubes  are  partly  filled  with  concentrated  sulphuric 
acid.  The  whole  apparatus  is  accu- 
rately weighed,  after  which  the  cock  is 
carefully  opened  so  that  acid  drops 
upon  the  carbonate,  evolving  carbon 
dioxide  at  a  moderate  rate.  This 
carbon  dioxide  passes  out  through  the 
sulphuric  acid  in  the  bubble  tube, 
being  freed  from  moisture  by  so  doing. 
The  apparatus  is  finally  heated  and 
air  is  drawn  through  to  displace  the 
remaining  carbon  dioxide.  The  loss 
in  weight  is  taken  to  represent  carbon 
dioxide.  This  is  not  accurately  the 
case  unless  the  air  that  is  drawn 
through  the  apparatus  is  first  dried. 
The  determination  by  means  of  such 
apparatus  is  quickly  made  but  is  sub- 
ject to  a  rather  large  error  on  account 
of  the  large  weight  of  the  apparatus, 
because  of  the  large  surface  and  largely 
because  of  the  difficulty  encountered 
in  the  drying  and  purification  of  the 
outgoing  gases  unless  unduly  large 
quantities  of  sulphuric  acid  are  used,  as  well  as  an  absorbent  for 
acid  vapors. 

Determination  by  Absorption. — The  direct  determination  by  a 
somewhat  more  elaborate  apparatus  is  to  be  preferred  if  accuracy 
is  an  object.  In  such  a  method  the  purification  of  the  carbon 
dioxide  is  rendered  complete  by  elaborating  that  part  in  which 
the  purification  is  accomplished,  providing  better  contact  of 
the  gases  with  drying  agents  and  acid  absorbents.  Instead  of 
weighing  the  entire  apparatus  before  and  after  expulsion  of 


FIG.  39. — Schrotter's  ap- 
paratus for  determination  of 
carbon  dioxide  by  loss. 


130 


QUANTITATIVE  ANALYSIS 


carbon  dioxide  from  the  carbonate  the  carbon  dioxide  is  ab- 
sorbed in  a  weighed  amount  of  potassium  hydroxide,  which  is 
again  weighed  after  the  absorption.  Many  variations  in  the 
apparatus  have  been  employed  but  the  apparatus  here  described 
embodies  the  essential  features  of  most  of  these. 

In  Fig.  40,  A  is  a  generating  flask  into  which  the  weighed 
sample  of  carbonate  is  placed.  B  is  a  dropping  funnel  having  a 
capacity  of  50  cc,  and  having  the  lower  end  drawn  out  to  a  point 
and  turned  upward.  This  part  should  extend  to  the  bottom  of 
the  flask.  At  the  top  of  the  dropping  funnel  a  drying  tube  C  is 
connected  by  means  of  a  rubber  stopper  and  a  bent  glass  tube. 


FIG'.  40. — Assembled    apparatus    for    determination    of    carbon    dioxide    by 

absorption. 

The  drying  tube  is  filled  with  soda  lime  for  the  absorption  of 
carbon  dioxide  from  the  air  that  is  later  to  be  drawn  through. 
Following  the  generating  flask  is  a  short  condenser  D  and  then 
U-tubes  E,  F  and  G.  The  first  U-tube  is  omitted  if  sulphuric  acid 
is  to  be  used  for  decomposing  the  carbonate,  or  is  filled  with  an  ab- 
sorbent for  hydrochloric  acid  vapors  if  this  acid  is  used.  The 
U-tubes  F  and  G  are  filled  with  granular  calcium  chloride  which 
absorbs  moisture  from  the  gas  mixture.  Following  these  is  the 
apparatus  H  in  which  potassium  hydroxide  is  placed  for  the  ab- 
sorption of  carbon  dioxide.  This  apparatus  also  carries  a  small 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  131 

tube  filled  with  calcium  chloride  to  prevent  the  removal  of  mois- 
ture from  the  apparatus,  which  would  occur  if  the  dry  entering 
gases  were  allowed  to  leave  the  apparatus  saturated  with  mois- 
ture. To  provide  a  means  for  drawing  air  through  the  whole 
apparatus  the  aspirator  J  is  placed  at  the  end  of  the  series,  while 
to  prevent  moisture  from  diffusing  backward  into  the  absorption 
apparatus  the  calcium  chloride  tube  /  is  interposed. 

Choice  of  Acid. — The  choice  of  acid  to  be  used  in  decomposing 
the  carbonate  will  depend  upon  the  nature  of  the  latter.  Sul- 
phuric acid  is  to  be  preferred  where  it  can  be  used,  because  it  is 
non-volatile  and  thus  needs  no  absorbent  in  the  purifying 
apparatus.  If,  however,  the  carbonate  is  one  of  a  metal  which 
forms  a  sulphate  of  small  solubility  (e.g.,  calcium  carbonate  or 
barium  carbonate)  sulphuric  acid  soon  coats  the  particles  with 
insoluble  sulphate  which  hinders  the  decomposition  of  the 
interior  of  the  particles.  Decomposition  is  slow  and  uncertain 
and  for  this  reason  hydrochloric  acid  is  used  instead  of  sulphuric 
acid.  A  preliminary  test  should  be  made  to  ascertain  whether 
sulphuric  acid  forms  a  complete  solution  of  the  carbonate  to  be 
analyzed. 

Absorbent  for  Hydrochloric  Acid. — If  hydrochloric  acid  must 
be  used  a  suitable  absorbent  is  placed  in  the  U-tube  E,  following 
the  generating  flask.  Absorbents  which  serve  best  for  this 
purpose  are  silver  sulphate  and  anhydrous  copper  sulphate.  For 
such  a  purpose  the  copper  sulphate  is  prepared  by  first  dropping 
red  hot  pieces  of  pumice  stone  into  a  concentrated  solution  of 
copper  sulphate,  removing  the  pumice  stone,  allowing  to  drain 
and  then  drying  at  200°.  A  supply  of  the  dry  pieces  is  kept  in  a 
desiccator  and  fresh  pieces  placed  in  the  U-tubes  for  each  deter- 
mination. A  more  satisfactory  absorbent  is  a  saturated  solution 
of  silver  sulphate  in  concentrated  sulphuric  acid.  This  may  be 
absorbed  by  pieces  of  pumice  and  used  in  the  same  manner  as 
copper  sulphate.  When  sulphuric  acid  is  used  in  this  manner  it 
must  not  be  allowed  to  come  into  contact  with  corks,  cotton  or 
any  other  organic  matter.  The  evidence  of  such  contact  is 
blackening.  The  result  is  the  formation  of  both  carbon  dioxide 
and  sulphur  dioxide.  Both  oxides  are  absorbed  in  the  potas- 
sium hydroxide  and  give  rise  to  errors  in  the  determination. 
A  U-tube  with  glass  stoppers  should  be  used.  Silver  nitrate 


132  QUANTITATIVE  ANALYSIS 

cannot  be  used  because  its  reaction  with  hydrochloric  acid  pro- 
duces nitric  acid,  which  is  nearly  as  volatile  as  hydrochloric 
acid,  and  also  chlorine. 

Soda  Lime. — The  soda  lime  which  is  used  for  the  removal  of 
carbon  dioxide  from  the  entering  air  should  be  fresh  and  in  the 
form  of  lumps.  A  powdered  condition  is  evidence  of  having 
been  air-slaked,  in  which  case  it  is  unfit  for  use  since  it  is  already 
saturated  with  carbon  dioxide. 

Calcium  Chloride. — The  calcium  chloride  used  for  the  absorp- 
tion of  moisture  should  be  the  granular  form  which  has  been 
fused.  Fusion  is  necessary  in  order  to  produce  an  anhydrous 
material.  This  fusion  always  produces  a  certain  amount  of 
calcium  oxide  which,  if  allowed  to  remain  as  such,  will  absorb  a 
certain  quantity  of  carbon  dioxide  as  well  as  of  water.  It  is 
best  to  treat  the  material  directly  in  the  bottle  by  passing  dried 
carbon  dioxide  through  for  several  hours,  then  displacing  the 
carbon  dioxide  by  drawing  through  dried  air.  In  filling  U-tubes 
only  lumps  should  be  used.  The  tube  is  filled  to  just  below  the 
side  branches  and  then  a  loose  plug  of  cotton  or  glass  wool  is 
placed  on  top  in  each  side  to  prevent  drawing  out  of  any  grains 
of  powder  that  may  subsequently  be  produced.  For  sealing 
the  tubes  a  cork  is  pressed  in  until  it  begins  to  fit  closely.  It  is 
then  cut  off  even  with  the  top  and  the  smaller  part  is  pressed 
into  the  tube  about  one-eighth  inch  farther.  The  shallow  cup 
thus  formed  is  poured  full  of  melted  paraffin  or  sealing  wax. 
When  this  solidifies  an  air-tight  seal  should  result,  unless  bubbles 
have  formed  in  the  sealing  material.  In  the  latter  case  a  flame 
may  be  lightly  touched  to  the  surface  of  the  solid  paraffin  or 
wax,  which  will  cause  the  bubbles  to  break. 

Phosphorus  Pentoxide. — Passing  gases  are  dried  to  a  greater 
extent  by  phosphorus  pentoxide  than  by  calcium  chloride.  The 
former  absorbs  moisture  so  rapidly  as  to  make  the  charging  of 
tubes  difficult  unless  the  humidity  of  the  atmosphere  is  low. 
The  oxide  is  usually  obtained  as  a  fine  powder,  formed  by  sub- 
limation. As  this  combines  with  water  it  forms  a  sticky  mass  of 
phosphoric  acid,  which  soon  clogs  the  tube  unless  some  device 
is  employed  to  prevent  this.  The  best  method  for  charging 
drying  tubes  with  phosphorus  pentoxide  is  to  arrange  a  ribbon 
of  glass  wool,  over  which  the  oxide  is  sifted.  The  glass  wool  is 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  133 

then  quickly  folded  into  a  narrow  strip  which  is  placed  in  the 
tube. 

Choice  of  Drying  Agent. — It  might  seem  at  first  sight  that 
practically  perfect  drying  of  carbon  dioxide  before  absorption 
would  be  necessary  for  accurate  determinations  and  that  phos- 
phorus pentoxide  would  therefore  be  the  ideal  drying  agent. 
An  inspection  of  the  conditions  will  show  that  this  is  not  the  case. 
Of  course  the  increase  in  the  weight  of  the  absorption  bulbs  must 
accurately  represent  the  weight  of  carbon  dioxide  absorbed. 
But  in  order  that  this  may  be  the  case  it  is  only  necessary  that 
the  unabsorbed  air  shall  pass  out  of  the  bulbs  with  the  same  degree 
of  hydration  that  it  possesses  when  entering.  Therefore  any 
fairly  good  dehydrating  agent  will  serve  provided  that  the  same 
agent  that  is  used  preceding  the  bulb  in  the  train  is  also  used 
in  the  tube  that  is  attached  to  the  bulbs  for  drying  outgoing 
gases.  Either  calcium  chloride  or  phosphorus  pentoxide,  but 
not  both,  may  be  used  in  the  same  train.  Similar  reasoning  will 
apply  to  the  use  of  concentrated  sulphuric  acid  with  other  drying 
agents. 

Absorbent  for  Carbon  Dioxide. — Potassium  hydroxide  is 
generally  used  for  the  absorption  of  carbon  dioxide,  a  solution 
33  percent  by  weight  being  commonly  employed.  In  practice 
it  is  found  that  absorption  becomes  so  slow  as  to  be  uncertain 
before  the  point  of  complete  saturation  is  reached.  Tke  prac- 
tical limit  is  reached  when  0.10  gm  of  carbon  dioxide  has  been 
absorbed  by  each  cubic  centimeter  of  potassium  hydroxide 
solution.  To  determine  the  amount  of  gas  that  can  be  absorbed 
by  the  solution  in  the  apparatus  the  latter  is  first  filled  with 
water  to  the  height  at  which  the  liquid  is  to  stand.  This  is 
emptied  out  and  measured.  The  number  of  cubic  centimeters 
times  0.1  gm  is  the  weight  of  carbon  dioxide  which  can  be 
absorbed  before  the  solution  becomes  inefficient.  By  adding 
together  the  weights  of  gas  absorbed  in  successive  experiments  it 
is  easy  to  determine  when  the  bulbs  need  refilling.  The  bulbs 
in  which  the  absorption  is  to  take  place  furnish  the  greatest 
source  of  error  to  be  encountered  in  this  method.  Inaccuracies 
are  due  to  the  large  weight,  the  large  surface  and  the  possibility 
of  moisture  being  carried  out  by  the  outgoing  air.  The  effect 
of  the  comparatively  large  weight  of  the  bulbs  and  their  contents 


134  QUANTITATIVE  ANALYSIS 

is  to  decrease  the  sensibility  of  the  balance.  The  surface  gives 
rise  to  a  possible  error  because  of  the  variable  amount  of  moisture 
which  is  always  dissolved  in  the  surface  of  the  glass.  This 
error  may  be  considerable  if  the  two  weights  (before  and  after 
the  absorption)  are  observed  under  different  atmospheric 
conditions  of  humidity.  For  this  reason  it  is  necessary  that 
the  two  readings  of  weights  shall  be  made  on  the  same  day  and 
as  near  to  each  other  in  point  of  time  as  possible. 

The  danger  of  loss  of  moisture  from  the  potassium  hydroxide 
solution  to  the  dry  air  which  enters  is  magnified  by  the  necessary 
limit  which  the  already  large  weight  of  the  bulbs  places  upon 
the  tube  which  carries  the  calcium  chloride  for  drying  the 
outgoing  air.  For  this  reason  many  analysts  prefer  to  separate 
this  drying  tube  from  the  bulbs,  using  a  small  U-tube  or  even 
two  such  tubes,  and  weighing  the  apparatus  in  the  two  or  three 
parts.  Sometimes  there  is  placed  in  the  first  half  of  the  U-tube 
so  used,  or  in  the  first  U-tube  if  two  are  used,  solid  potassium 
hydroxide  to  insure  complete  absorption  of  carbon  dioxide. 
While  this  procedure  may  make  more  certain  the  complete  drying 
of  the  air  and  thus  prevent  a  loss  of  weight  from  this  cause,  an 
added  uncertainty  is  introduced  due  to  the  accumulation  of  the 
errors  of  four  weighings.  It  is  possible  to  insure  complete 
detention  of  the  moisture  by  passing  the  gas  at  a  regular,  specified 
rate,  not  exceeding  a  maximum  found  by  experience. 

Determination. — Procure  the  following  parts  for  assembling: 

1  dropping  funnel,  50  cc,  with  1-hole  rubber  stopper, 

1  short,  wide  flask,  75  cc,  such  as  is  used  for  fat  extractions,  with 
2-hole  rubber  stopper, 

1  condenser  with  body  not  more  than  6  inches  long, 

3  U-tubes  with  corks  to  fit, 

1  U-tube  with  glass  stoppers, 

1  straight  drying  tube  with  1-hole  rubber  stopper, 

1  set  "potash  bulbs"  of  some  approved  form, 

1  aspirator  bottle,  tubulated  near  bottom,  with  1-hole  rubber  stoppers 
to  fit, 

1  piece  glass  tubing,  about  2  feet  X  |  inch,  for  supporting  apparatus, 

2  clamps, 

2  pinch  cocks, 

1  small  screw  clamp  (Hoffman  screw), 

2  retort  stands, 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  135 

Glass  and  rubber  tubing  for  connections. 

Fill  and  connect  the  apparatus  in  the  manner  previously  described. 
Measure  the  capacity  of  the  absorption  bulbs  by  drawing  in  distilled 
water,  then  blowing  out  and  measuring  the  water.  This  volume  will  be 
used  as  a  basis  for  the  calculation  of  absorbing  power  as  already  directed. 
When  filling  the  absorption  bulbs  with  potassium  hydroxide  solution 
the  latter  should  not  be  warmer  than  the  air  of  the  room.  The  bulbs 
are  detached  from  the  apparatus  and  the  solution  is  drawn  in  through 
a  tube  attached  at  a,  suction  being  applied  at  6.  The  solution  should 
about  half  fill  the  bulb  c  when  air  is  bubbling  through.  The  ground- 
glass  joint  between  the  drying  tube  6  and  the  bulbs  should  be  lightly 
coated  with  vaseline  and  the  tube  then  twisted  on  until  it  fits  closely 
enough  that  there  will  be  no  danger  of  loosening  during  the  course  of  an 
experiment.  Any  surplus  vaseline  is  removed  from  the  outside  of  the 
joint. 

Place  the  bulbs  in  position,  close  the  cock  of  the  dropping  funnel 
and  open  the  pinch  cock  at  e  to  allow  water  to  flow  from  the  aspirator. 
Bubbles  of  air  will  at  first  pass  through  the  bulbs  but  this  action  will 
finally  cease  unless  there  is  a  leak  in  the  apparatus,  in  which  case  it 
must  be  found  and  closed  It  is  important  that  all  glass  tubes  be  brought 
entirely  together  inside  the  rubber  connections  since  rubber  is  slightly 
permeable  to  gases. 

After  the  apparatus  has  been  shown  to  be  free  from  leaks  the  pinch 
cock  at/  is  closed,  the  cock  of  the  separatory  funnel  slowly  opened  and, 
after  equilibrium  is  established,  the  clamp  k  is  so  adjusted  that  when 
clamp  /  is  opened  air  will  pass  through  the  bulbs  at  a  rate  not  greater 
than  3  bubbles  per  second.  Clamp  k  is  not  thereafter  changed.  This 
provides  against  too  rapid  flow  of  gas  under  any  conditions.  Clamp  / 
is  now  closed,  the  bulbs  are  removed,  the  inlet  and  outlet  tubes  are 
closed  by  short  rubber  tubes  containing  glass  plugs  and  the  bulbs  are 
wiped  clean  and  placed  in  the  balance  case.  A  short  glass  tube  is 
inserted  to  bridge  the  gap  made  by  removing  the  bulbs.  The  bulbs 
should  be  allowed  to  stand  for  15  minutes  before  weighing.  In  the 
meantime  about  1  gm  of  the  carbonate  is  weighed  and  brushed  into  the 
generating  flask  and  a  small  amount  of  water  is  added  to  moisten  the 
sample.  The  stopcock  of  the  funnel  B  and  the  clamp  e  are  now  opened 
and  500  cc  of  air  is  drawn  through  the  apparatus,  measured  by  the 
outflowing  water  from  the  aspirator.  This  frees  the  apparatus  from 
carbon  dioxide.  After  the  absorption  bulbs  have  stood  for  15  minutes 
the  tubes  carrying  the  plugs  are  removed  and  the  bulbs  are  weighed. 
The  plugs  are  then  replaced  and  left  so  until  the  bulbs  can  be  connected 
in  the  apparatus.  50  cc  of  dilute  sulphuric  acid  or  hydrochloric  acid  is 
placed  in  the  dropping  funnel,  a  test  having  previously  been  made  to 


136  QUANTITATIVE  ANALYSIS 

determine  whether  sulphuric  acid  will  form  a  clear  solution  with  the 
carbonate.  If  such  a  solution  is  not  produced,  of  course  hydrochloric 
acid  must  be  used  and  silver  sulphate  and  pumice  must  be  placed  in  tube 
E.  Reconnect  the  apparatus  and  open  all  cocks  except  the  stop-cock 
in  the  dropping  funnel,  leaving  the  clamp  k  set  for  the  proper  rate  of  gas 
flow,  as  previously  determined.  Slowly  open  the  cock  of  the  dropping 
funnel,  allowing  acid  to  drop  just  fast  enough  to  evolve  carbon  dioxide  at 
the  prescribed  rate.  The  constant  attention  of  the  operator  is  necessary 
at  this  point,  for  by  causing  too  rapid  evolution  of  gas  some  moisture 
may  escape  absorption  in  the  small  tube  of  the  absorption  bulbs  and  the 
experiment  be  rendered  worthless. 

The  acid  should  be  allowed  to  run  in  until  about  1  cc  is  left  above 
the  stopcock,  this  acting  as  a  seal  during  the  subsequent  boiling.  After 
the  decomposition  of  the  carbonate  is  complete  the  solution  in  the  flask  is 
slowly  heated  until  it  boils,  always  with  due  regard  to  the  rate  at  which 
the  gas  is  made  to  flow  through  the  absorption  bulbs.  The  boiling  is 
continued  for  one  minute,  when  the  flame  is  withdrawn,  the  cock  of  the 
dropping  funnel  being  opened  at  the  same  time  to  allow  air  to  enter  so 
that  no  back  suction  occurs,  due  to  the  cooling  effect.  Air  is  now  drawn 
through  the  apparatus  until  1000  cc  of  water  has  flowed  from  the  aspira- 
tor. This  amount  of  air  should  be  sufficient  to  sweep  all  of  the  carbon 
dioxide  into  the  absorption  bulbs. 

The  clamp  /  is  now  closed,  and  the  absorption  bulbs  are  removed, 
plugged  and  placed  in  the  balance  case.  After  15  minutes  they  are 
weighed  without  the  plugs,  the  increase  in  weight  being  the  weight 
of  carbon  dioxide.  From  this  and  the  weight  of  sample  the  percent  of 
carbonic  anhydride  (combined  carbon  dioxide)  is  calculated. 

For  the  duplicate  or  any  subsequent  determination  the  generating 
flask  and  the  dropping  funnel  are  washed  absolutely  free  from  acid,  so 
that  no  decomposition  of  the  next  carbonate  sample  may  occur  before 
the  bulbs  are  in  place.  The  first  U-tube  should  also  be  emptied  and 
recharged  with  absorbent,  if  such  is  to  be  used  for  the  next  determination. 

If  a  large  number  of  determinations  is  to  be  made  with  the  same 
apparatus  much  time  will  be  saved  by  providing  two  decomposition 
flasks  and  two  absorption  bulbs.  While  one  determination  is  being 
made  another  sample  may  be  weighed  into  the  duplicate  flask  and  the 
second  absorption  bulb  may  be  weighed.  The  next  determination  may 
then  be  started  while  the  first  bulbs  are  standing  in  the  balance  case,  pre- 
liminary to  the  final  weighing.  It  is  also  necessary  to  determine  when 
the  various  absorbents  have  become  so  saturated  as  to  be  inefficient 
for  further  work.  Soda  lime  in  the  tube  C  is  good  until  the  lumps  have 
fallen  into  a  powder.  Silver  sulphate  in  the  pumice  of  tube  E  may 
become  inefficient  through  absorption  of  hydrochloric  acid  or  through  the 


EXPERIMENTAL  GRAVIMETRIC  ANALYSIS  137 

accumulation  of  water  in  the  tube.  The  solubility  of  silver  sulphate  in 
water  is  much  less  than  in  concentrated  sulphuric  acid.  If  the  acid 
solution  becomes  diluted  the  silver  salt  crystallizes  and  will  not  there- 
after readily  absorb  hydrochloric  acid.  As  the  silver  sulphate  becomes 
saturated  with  hydrochloric  acid  it  darkens,  on  account  of  the  action  of 
light.  When  the  darkening  effect  has  proceeded  as  far  as  the  middle 
of  the  tube  the  material  should  be  replaced.  Calcium  chloride  must  be 
replaced  when  it  becomes  visibly  moist  for  the  first  third  of  any  absorbing 
tube. 

A  method  has  already  been  given  for  the  determination  of  the  amount 
of  carbon  dioxide  which  can  be  absorbed  by  the  solution  in  the  bulbs. 


CHAPTER  IV 
ELECTRO-ANALYSIS 

We  have  here  to  deal  with  a  class  of  work  that,  while  also 
gravimetric  in  most  cases,  is  sufficiently  different  from  what  has 
already  been  considered  to  be  treated  as  a  separate  division.  In 
all  of  the  preceding  exercises  the  element  or  radical  to  be  deter- 
mined was  precipitated  from  a  solution  by  chemical  reactions 
produced  by  other  substances  which  were  added  for  the  purpose. 
In  the  cases  now  to  be  considered  the  precipitation  will  be  brought 
about  by  electrical  action,  the  passage  of  a  current  through  the 
solution  causing  the  deposition  of  a  metal  upon  a  cathode  in 
such  a  form  that  it  can  be  weighed,  or  the  accomplishment  of 
some  change  which  makes  possible  the  determination  of  a  sub- 
stance not  a  metal.  The  electrolysis  of  silver  sulphate  will  serve 
as  an  example.  When  a  solution  of  this  salt  is  electrolyzed  at 
platinum  electrodes  the  metal  is  plated  on  the  cathode  and 
sulphuric  acid  is  produced  at  the  anode,  thus: 

2Ag+S04-*2Ag+S04, 
2SO4+2H20->2H2SO4+O2. 

The  silver  can  then  be  weighed  and  the  sulphuric  acid  deter- 
mined volumetrically. 

While  the  electrolysis  of  a  simple  salt  is  frequently  a  tolerably 
simple  and  well  understood  process,  the  practical  accomplish- 
ment of  such  a  process  for  the  purpose  of  a  quantitative  analysis 
is  usually  possible  only  when  a  certain  set  of  conditions  is  main- 
tained. The  principal  reasons  for  failure  to  attain  accuracy  are 
three:  (1)  Deposition  may  not  occur  upon  passage  of  a  current. 
(2)  The  deposit  may  be  contaminated  by  other  products  of 
electrolysis.  (3)  The  deposit  may  not  have  the  proper  physical 
character,  so  that  it  will  not  adhere  to  the  electrode  but  crumbles 
off  during  the  electrolysis  or  during  the  process  of  washing.  We 
have  thus  to  consider  the  nature  of  salt  to  be  used,  solvents,  tem- 

138 


ELECTRO-ANAL  YSIS  139 

perature,   electrolytic  pressure    (voltage),   current   density  and 
nature  and  kind  of  electrode. 

Nature  of  Electrolyte. — Electrolytic  methods  are  more  fre- 
quently applied  to  the  determination  of  metals  than  of  non- 
metals,  although  methods  have  lately  been  perfected  for  the 
determination  of  the  latter.  If  the  metal  alone  is  to  be  deter- 
mined it  will  usually  be  possible  to  obtain  it  in  the  form  of  what- 
ever salt  gives  the  best  results.  Certain  anions  must  be  excluded 
in  specific  cases,  either  because  they  yield  substances  that  at- 
tack the  anode  or  because  the  acids  that  are  produced  by  their 
electrical  discharge  cause  the  metal  to  deposit  in  an  undesirable 
physical  form.  As  an  example  of  corrosive  action  upon  the  anode 
it  is  sufficient  to  mention  here  the  formation  of  nascent  chlorine 
at  a  platinum  anode  when  a  chloride  is  electrolyzed.  With  regard 
to  the  effect  of  the  acid  that  accumulates  in  the  solution  as 
electrolysis  proceeds  it  may  be  stated  that  there  is  little  known, 
at  present,  of  the  reasons  for  the  effect  of  acids,  bases  and  other 
substances  that  may  be  in  the  solution,  upon  the  nature  of  the 
deposit.  Experiment  shows,  however,  that  such  substances 
often  exert  a  very  important  influence  upon  the  physical  character 
of  a  deposited  metal  and  they  are  often  added  for  this  reason, 
although  they  may  be  objectionable  for  other  reasons.  A  solu- 
tion of  copper  sulphate,  if  electrolyzed  without  the  addition  of 
another  substance,  usually  gives  a  dark  red  or  brown  deposit  of 
finely  divided  copper  which  is  liable  to  powder  and  be  lost  during 
washing.  If  a  small  amount  of  sulphuric  acid  is  first  added 
the  deposit  is  improved,  while  nitric  acid  causes  a  still  better 
deposit  of  bright  red,  firm  and  adherent  metal.  For  this  reason 
nitric  acid  is  usually  added  although  it  gives  rise  to  more  or 
less  danger  of  resolution  when  the  cathode  copper  is  being  washed. 
On  the  other  hand,  a  silver  salt  is  best  electrolyzed  in  the  absence 
of  nitric  acid.  If  silver  nitrate  is  electrolyzed  from  water  solution 
with  or  without  the  addition  of  nitric  acid  (the  latter  is  formed 
by  the  electrolysis)  the  plate  of  silver  on  the  cathode  is  so  decidedly 
crystalline  that  it  is  very  easily  detached.  The  addition  of 
potassium  cyanide  in  quantity  sufficient  to  redissolve  the  pre- 
cipitate of  silver  cyanide  first  formed  gives  a  solution  from  which 
silver  will  deposit  as  a  white  firm  plate.  The  solution  in  potas- 
sium cyanide  has  a  comparatively  high  electrical  resistance 


140  QUANTITATIVE  ANALYSIS 

so  that  more  energy  is  consumed  in  the  accomplishment  of  its 
decomposition,  nevertheless  potassium  cyanide  is  generally 
added. 

Other  examples  of  similar  effects  will  appear  in  the  exercises. 
It  is  desirable  to  note  that  little  is  known  of  the  cause  of  such 
effects,  also  to  guard  against  a  very  common  misconception 
regarding  the  purpose  of  adding  other  electrolytes  to  solutions 
that  are  to  be  electrolyzed.  It  is  frequently  stated  that  such 
substances  are  added  in  order  to  increase  the  conductivity  of  the 
solution.  If  such  substances  could  increase  the  ionization  of  the 
salt  that  is  to  be  electrolyzed,  or  in  any  manner  diminish  the  fric- 
tional  resistance  to  the  passage  of  the  ions,  such  an  effect  would  be 
desirable.  It  is  evident,  however,  that  the  addition  of  a  foreign 
electrolyte  can  usually  increase  the  conductivity  only  by  itself 
acting  as  a  carrier  of  current,  in  which  case  it  has  accomplished 
no  desirable  effect  since  the  prime  object  is  not  to  use  a  large 
current  but  to  make  the  minimum  current  do  the  maximum 
work  in  discharging  an  ion  already  in  solution. 

Solvent. — Very  little  work  has  been  done  in  any  solvents  other 
than  water.  The  use  of  organic  solvents  may,  in  some  cases, 
prove  advantageous  in  producing  good  deposits  where  other  con- 
ditions fail  to  do  so. 

Temperature. — Conductivity  of  solutions  usually  increases 
with  rise  in  temperature.  This  is  not  due  to  increased  ionization 
(ionization  usually  decreases  with  rise  in  temperature)  but  to  re- 
duced viscosity  and  consequent  reduction  in  frictional  resistance 
to  ionic  migration.  If  the  complete  electro-decomposition  of  a 
substance  requires  considerable  time  it  is  not  convenient  to  heat 
to  any  definite  elevated  temperature.  In  most  cases,  therefore, 
the  temperature  of  the  solution  is  not  raised  above  that  of  the 
laboratory  except  at  the  beginning. 

Decomposition  Voltage. — For  every  electrolyte  in  solution 
there  is  a  definite  minimum  voltage,  below  which  no  decomposi- 
tion will  take  place.  If  but  one  electrolyte  is  present  and  the 
voltage  lies  below  this  minimum,  a  continuous  current  cannot  flow. 
The  minimum  voltage  necessary  to  produce  a  continuous  flow 
of  current  is  called  the  "decomposition  voltage"  for  the  sub- 
stance in  question.  If  salts  of  more  than  one  metal  are  present 
in  solution,  the  deposit  on  the  cathode  will  consist  of  any  metals, 


ELECTRO-ANALYSIS  141 

the  decomposition  voltage  of  whose  salts  has  been  exceeded. 
If  there  is  sufficient  difference  in  the  values  of  decomposition 
voltage  for  the  different  salts,  separation  may  be  made.  It  is 
only  necessary  to  adjust  the  voltage  so  that  it  shall  exceed  the 
decomposition  voltage  of  the  metal  that  is  most  easily  discharged. 
After  this  metal  has  been  completely  removed  from  solution  and 
weighed  the  voltage  is  raised  until  it  exceeds  the  decomposition 
voltage  of  the  metal  next  in  order  and  so  on.  While  this  consti- 
tutes the  general  procedure  for  electrolytic  separations  it  is 
necessary  to  make  certain  changes  in  the  nature  of  the  solution 
after  each  metal  is  removed  in  turn  as  will  be  understood  from 
the  discussion  of  the  solutions  to  be  employed. 

In  order  to  understand  the  origin  of  the  decomposition  voltage 
it  will  be  necessary  to  briefly  consider  the  underlying  principles 
of  electrolysis.  If  a  metal  is  placed  in  contact  with  a  solution  of 
one  of  its  salts  it  will  be  found  that  a  difference  in  potential  exists 
between  the  metal  and  solution.  This  difference  may  be  either 
positive  or  negative,  i.e.,  the  metal  may  be  at  a  higher  or  lower 
potential  than  that  of  the  solution.  The  difference  may  be 
zero  in  certain  cases  but  such  cases  are  special.  The  conception 
of  Helmholtz1  regarding  the  cause  of  this  potential  difference 
may  be  thus  stated:  Whenever  two  dissimilar  substances  are 
in  contact  a  potential  difference  is  established  because  of  the 
passage  of  one  into  the  other.  In  the  case  of  metals  and  their 
salt  solutions,  one  of  two  things  may  happen :  either  some  metal 
atoms  pass  into  solution  and  become  charged  ions  or  some  ions 
are  discharged  by  the  mass  of  metal  and  themselves  become 
elementary  In  the  first  case  the  solution  assumes  a  higher 
potential  than  the  metal  because  positive  charges  have  been 
transferred  from  metal  to  solution.  In  the  second  case  the 
solution  is  at  a  potential  lower  than  that  of  the  metal  because 
positive  electricity  has  passe.d  from  solution  to  metal.  The 
direction  of  the  change  is  determined  by  the  relative  magnitude 
of  two  opposing  forces.  The  metal  shows  a  tendency  to  pass 
into  solution  in  the  ionic  condition  in  obedience  to  a  force  called 
by  Nernst2  " electrolytic  solution  tension."  This  force  varies 
with  different  elements,  but  is  constant  for  a  given  element  and 

1Wied.  Ann.,  7,  337  (1879). 

» Z.  physik,  Chem.,  4,  129  (1889), 


142 


QUANTITATIVE  ANALYSIS 


may  be  relatively  large  or  small.  When  positive  ions  have  been 
thrown  into  the  solution  the  potential  difference  thus  established 
gives  rise  to  an  attraction  of  an  electrostatic  nature  between 
the  positively  charged  solution  and  the  negatively  charged  metal. 
This  may  be  represented  diagrammatically  as  in  Fig.  41. 

Double  Layer. — Helmholtz  considered  that  a  "  double  layer " 
was  thus  formed,  composed  of  positive  and  negative  charges, 
and  that  the  components  of  this  layer  were  very  close  together. 
The  attraction  existing  between  the  components  of  the  double 
layer  increases  as  more  ions  are  formed  and  finally  reaches  equilib- 
rium with  the  solution  pressure.  If  ions  of  the  metal  in  question 


Fio.  41. — Diagram  illustrating  the  "double  layer." 

were  already  in  the  solution,  then  osmotic  pressure  would  oppose 
the  entrance  of  more  ions  into  the  solution  and  would  thus  act  in 
conjunction  with  the  electrostatic  attraction,  so  that  equilibrium 
would  be  reached  with  a  smaller  potential  difference.  Evidently, 
then,  the  potential  difference  between  the  electrode  and  elec- 
trolyte, in  the  case  of  a  given  metal  and  its  salt  solution,  will 
be  numerically  greatest  when  the  initial  ion  concentration  is 
least,  and  least  when  the  latter  is  greatest.  If  the  solution 
pressure  is  small  and  the  ion  concentration  large,  equilibrium 
may  be  reached  only  by  the  actual  deposition  of  ions  upon  the 
metal.  In  this  case  the  potential  difference  will  be  positive. 

If  the  different  elements  are  compared,  with  regard  to  the 
potential  difference  established  between  them  and  their  ion  solu- 


ELECTRO-ANALYSIS 


143 


tions,  the  ion  concentration  being  the  same  in  all  cases,  a  series 
of  different  values  is  obtained.  It  is  to  be  noted  that  if  an 
element  takes  a  negative  charge  upon  becoming  ionized  the 
potential  difference  is  reversed  in  sign.  In  the  following  table1 
several  of  the  elements  are  given  with  values  for  the  potential 
difference.  These  differences  are  measured  by  a  method  that 
need  not  be  here  discussed,  involving  the  use  of  an  arbitrary 
standard,  so  that  only  the  relative  values  are  important.  These 
values  are  for  solutions  which  contain  1  gm-ion  per  liter. 


Element 

Potential  difference 
electrode-electrolyte 

Element 

Potential  difference 
electrode-electrolyte 

Potassium  

—  3  20 

Hydrogen 

0  000 

Sodium  

—  2  82 

Arsenic.  .  . 

<+0  293 

Barium  

-2  82 

Copper  . 

+  0  329 

Strontium  

-2.77 

Bismuth  .  .  . 

<  +  0  391 

Calcium  
Magnesium 

-2.56 
—  2  54 

Antimony  

<+0.466 
+  0  750 

Aluminium  

-1.276 

Silver  

+  0  771 

Manganese  
Zinc 

-1.075 
—  0  770 

Palladium  
Platinum 

<+0.789 
<  +  0  863 

CadnrmiTn  

—  0  420 

Gold 

<  +  l  079 

Iron 

—  0   340 

+  1   96 

Thallium  

-0.322 

Chlorine 

+  1  417 

Cobalt 

—  0  232 

+  0  993 

Nickel...   . 

—  0  228 

+  0  520 

Tin 

<     0  192 

+  1   119 

Lead  

-0.148 

In  the  electrolysis  of  a  salt  solution  both  positive  and  negative 
ions  are  discharged  at  the  electrodes,  both  electrodes  become 
coated  with  the  products  of  'decomposition  (polarized),  and  both 
then  become  essentially  electrodes  of  the  respective  elements, 
no  matter  what  the  original  substance  might  have  been.  Hence 
such  a  system  as  that  just  discussed  may  be  considered  to  exist 
at  each  electrode.  Since,  after  electrolytic  decomposition  has 
begun,  electrolysis  consists  of  electrical  discharge  of  ions  it  is 
evident  that  it  must  act  in  opposition  to  solution  pressure  and  in 
conjunction  with  osmotic  pressure.  That  is,  in  order  to  produce 
continuous  electrolysis  a  voltage  must  be  applied  to  the  electrodes 
that  is  at  least  as  great  as  the  algebraic  difference  between  the 
single  potential  differences  normally  established  at  the  cathode 
and  anode.  This  difference  constitutes  the  theoretical  "decom- 

1  Wilsmore:  Z.  physik.  Chem.,  35,  291  (1900). 


144  QUANTITATIVE  ANALYSIS 

position  voltage"  of  a  given  salt  solution,  and  no  appreciable 
electrolysis  can  take  place  as  a  result  of  the  application  of  a 
lower  voltage.  The  attempt  to  calculate  the  decomposition 
voltage  of  a  solution  from  the  single  potential  differences,  ex- 
perimentally determined,  does  not  always  give  results  that  agree 
with  those  found  by  experiment.  This  is  because  the  ion  con- 
centration is  not  necessarily  equal  to  1  gm-ion  per  liter  and  it 
changes  as  electrolysis  proceeds.  Also  where  gases  are  evolved 
at  the  anode  (a  condition  generally  noticed)  the  phenomenon 
of  "  overvoltage "  exerts  a  very  important  influence  upon  the 
practical  decomposition  voltage. 

Importance  of  this  Principle. — Agreement  is  not  at  all  satis- 
factory in  the  case  of  salts  of  oxyacids.  This  is  partly  because 
oxysalts  are  not  normal  with  respect  to  the  oxygen  or  hydroxyl 
ion,  also  because  oxygen  shows  overvoltage  to  a  marked  degree, 
giving  a  much  larger  decomposition  voltage  than  would  be 
calculated.  The  matter  that  is  here  important  is  not  the  possibility 
of  calculating  decomposition  voltages  from  known  single  potential 
differences  but  the  recognition  of  the  reasons  for  the  fact  that  a 
minimum  decomposition  voltage  must  exist  for  every  compound, 
under  definite  conditions,  and  that  if  this  value  can  be  determined 
a  method  for  electrolytic  separations  is  available.  We  shall  under- 
stand that  " decomposition  voltage"  refers  to  the  fall  in  potential, 
measured  across  the  electrodes,  below  which  electrolysis  cannot 
take  place  continuously.  So  far  as  we  now  know  there  is  no 
upper  limit  to  the  voltage  that  should  be  used,  excepting  that  set 
by  the  current  density  that  should  be  employed,  unless 
separations  are  to  be  made. 

Because  the  variable  factors  which  influence  the  practical  de- 
composition voltage  (such  as  overvoltage  of  the  anion,  varying 
temperature  and  added  electrolytes)  and  the  consequent  diffi- 
culty that  is  experienced  in  its  calculation,  Sand1  proposed  to 
measure  merely  the  cathode  potential  difference  and  to  make 
metal  separations  by  properly  grading  this  potential.  This  is, 
no  doubt,  the  ideal  method.  For  making  such  measurements, 
however,  more  elaborate  apparatus  is  required  and  much  care 
must  be  exercised.  For  most  purposes  such  refinement  is  entirely 
unnecessary  because  the  practical  decomposition  voltage  is 

*  J.  Chem.  Soc.,  91,  374  (1906). 


ELECTRO-ANALYSIS  145 

usually  fairly  accurately  known  as  the  result  of  experiments  with 
a  given  salt,  electrolyzed  under  specified  conditions. 

Current  Density. — The  relation  between  the  amount  of  current 
flowing  through  a  solution  of  an  electrolyte  and  the  amount  of 
substance  decomposed  is  stated  in  the  law  of  Faraday:1  (a) 
For  any  given  electrolyte  the  amount  of  decomposition  is  directly 
proportional  to  the  amount  of  current;  (6)  the  amounts  of  different 
substances  decomposed  by  the  same  current  are  proportional  to  the 
combining  weights  of  the  substances.  According  to  the  first  part 
of  this  law  the  rate  of  decomposition  and  electro-deposition  in 
any  experiment  will  depend  upon  the  current  strength,  except- 
ing the  part  played  by  other  electrolytes  that  may  be  present. 
Being  thus  able  to  limit  and  control  the  rate  of  deposition  the 
question  arises  as  to  whether  there  is  any  suitable  current  strength, 
above  or  below  which  good  results  will  not  be  attained.  If  it 
were  not  possible  for  any  current  to  pass  through  a  solution  ex- 
cept that  carried  by  the  ions  of  the  salt  which  we  desire  to  de- 
compose there  would  probably  be  no  definite  limit  to  the  prac- 
tical current  strength  to  be  employed.  The  ions  encounter  a 
large  frictional  resistance  in  their  migration  toward  the  elec- 
trodes. In  order  to  overcome  this  resistance  the  pressure  (volt- 
age) is  raised,  often  considerably  above  the  decomposition  voltage 
of  the  metal  salt  being  analyzed,  in  order  to  hasten  the  action. 
If  no  other  cation  is  present  there  is  no  upper  limit  to  the  prac- 
tical voltage.  There  is,  however,  another  positive  ion  that  is 
present  in  all  aqueous  solutions  and  particularly  when  acids  are 
present.  The  ion  referred  to  is  that  of  hydrogen.  If  the  pres- 
sure is  raised  above  its  discharge  potential  it  can  discharge  at 
the  cathode  and  will  do  so  unless  the  current  strength,  concen- 
tration of  hydrogen  ion  and  concentration  of  metal  ion  are  so 
related  that  the  current  can  easily  be  carried  from  solution  to 
cathode  by  the  metal  ion  without  the  necessity  for  discharge 
of  the  hydrogen  ion.  This  relation  will  evidently  be  such  that 
there  is  a  relatively  small  current,  large  metal  ion  concentration 
and  small  hydrogen  ion  concentration.  The  objection  to  the 
deposition  of  hydrogen  on  the  cathode  is  based  upon  the  fact 
that  minute  bubbles  of  gas  prevent  the  proper  coherence  of  the 
deposited  metal.  Apparently,  then,  the  upper  limit  of  current 

1  Pogg   Ann.,  33,  301  and  481  (1834). 

10 


146  QUANTITATIVE  ANALYSIS 

will  be  fixed  by  the  point  at  which  noticeable  evolution  of  hy- 
drogen (or  other  gas)  occurs.  This  limit  cannot  well  be  calcu- 
lated but  is  determined  by  experiment.  It  should  be  noted  that 
the  value  to  be  measured  is  not  that  of  total  current  flowing 
across  the  solution  but  is  that  of  the  current  flowing  into  unit 
area  of  the  electrode  where  the  desired  deposition  is  taking 
place,  usually  the  cathode.  This  gives  rise  to  the  term  "  cur- 
rent density/'  abbreviated  to  CD.  In  stating  the  conditions 
to  be  observed  in  electro-depositions  the  current  density  may  be 
more  specifically  defined  as  the  current  in  amperes  flowing  into 
each  100  sq  cm  of  cathode  surface.  This  is  denoted  by  CZ>10o. 
Evidently 

„ total  amperes ^ 

~  square  decimeters  cathode  surface 

The  proper  current  density  to  be  employed  in  a  given  case  is  called 
the  " normal  density"  for  that  experiment.  This  is  indicated 
by  NDiQQ.  The  normal  density  is  fixed  by  the  conditions  already 
discussed.  There  is  considerable  variation,  however,  based 
also  upon  the  form  of  the  electrodes.  This  will  be  taken  up 
in  the  next  paragraph. 

Nature  of  Electrodes. — Electrodes  must  possess  certain  proper- 
ties in  order  to  be  suitable  for  use  in  quantitative  analysis.  The 
electrode  material  must  be  insoluble  in  the  solution  of  electro- 
lyte, with  or  without  current  action.  In  the  process  of  metal 
plating  as  used  in  the  arts  it  is  customary  to  make  the  anode  of 
the  metal  being  plated,  so  that  the  rate  of  deposition  at  the  cath- 
ode is  equal  to  the  rate  of  solution  at  the  anode,  the  mean  con- 
centration of  the  metal  in  the  solution  remaining  constant. 
This  is  obviously  out  of  the  question  in  quantitative  analysis, 
where  the  total  metal  in  solution,  and  no  more,  is  to  be  deposited. 

The  material  most  used  for  electrodes  is  platinum.  The  con- 
tinued advance  in  the  cost  of  platinum  has  led  to  a  search  for 
less  expensive  materials.  However,  the  combination  of  high 
electrical  conductivity  and  low  solubility  is  a  rare  one.  Other 
metals  could  be  used  for  cathodes  because  the  current  action 
prevents  their  resolution,  but  when  the  deposited  metal  is  to  be 
removed  after  the  process  is  finished  the  solvent  used  will  gener- 
ally dissolve  some  of  the  electrode  also.  So  long  as  the  cost 
of  platinum  is  sufficiently  low  to  make  it  possible  to  provide  an 


ELECTRO-ANALYSIS  147 

adequate  supply  of  platinum  electrodes  it  is  doubtful  whether 
any  other  material  will  supplant  it  to  any  great  extent. 

Gooch  and  Burdick1  have  perfected  a  method  for  making 
electrodes,  by  which  a  very  small  amount  of  platinum  is  spread 
over  a  relatively  large  surface  of  glass.  A  mixture  of  glycerine 
and  chlorplatinic  acid  is  spread  over  the  glass  surface,  which 
is  then  heated.  The  glycerine  is  evaporated  and  the  chlor- 
platinic acid  is  decomposed,  elementary  platinum  fusing  into  the 
glass  surface. 

Mercury  is  used  as  a  cathode  for  a  certain  class  of  work  and 
this  will  be  discussed  in  a  later  paragraph. 

The  electrodes  must  also  be  chemically  unaltered  by  the 
passage  of  a  current  or  else  altered  in  a  definite  manner.  The 
first  condition  is  more  often  realized  but  there  are  cases  where 
one  electrode  is  altered  in  a  definite  manner,  as  a  silver  anode, 
used  for  the  determination  of  chlorine,  bromine  or  iodine  anion 
becomes  coated  with  chloride,  bromide  or  iodide  of  silver. 

The  electrode  that  is  to  be  weighed  and  is  to  receive  the  deposit 
(usually  the  cathode)  should  present  the  maximum  surface 
for  the  minimum  weight  of  electrode  material  Since  a  practical 
limit  is  placed  upon  the  current  density  the  duration  of  the  process 
of  deposition  will  be  inversely  proportional  to  the  total  electrode 
surface  exposed.  The  weight  of  the  electrode  must  not  be  too 
large  for  accurate  work  and  these  considerations  naturally  lead 
us  to  consider  the  form  of  material  where  the  ratio  of  surface 
to  weight  will  be  as  large  as  possible.  In  general,  any  piece 
of  platinum,  small  enough  to  be  weighed,  may  be  used  as  a 
cathode  for  metal  deposition.  Any  chemist  who  has  a  dish 
or  crucible  may  make  at  least  an  occasional  analysis  by  the  use 
of  such  an  article  as  cathode.  The  dish  is  simpler  because  the 
solution  may  be  placed  directly  in  it  and  a  coil  of  platinum  wire 
used  as  an  anode.  The  ratio  of  surface  to  weight  is  not  large  in 
this  case,  especially  as  only  one  surface  is  effective.  Moreover 
if  there  is  any  sediment  in  the  solution  this  may  be  partly  caught 
by  the  depositing  metal  and  weighed  along  with  the  latter.  The 
cathode  dish  designed  by  Classen  is  quite  thin  and  presents  a 
larger  surface.  A  dish  of  this  kind  weighing  about  40  gm  has 
a  capacity  of  250  cc  and  presents  an  inner  surface  of  about  150 

1  Z.  anorg.  Chem.,  78,  213  (1912). 


148 


QUANTITATIVE  ANALYSIS 


sq  cm  to  the  solution.  A  crucible  may  be  used  as  a  cathode  by 
connecting  as  in  Fig.  42.  A  rubber  stopper  is  used  to  help 
support  the  crucible,  a  metal  rod  passing  through  and  connecting 
with  the  negative  of  the  current  source.  A  small  platinum  wire 
serves  to  complete  the  connection  of  crucible 
with  rod.  This  form  of  cathode  also  possesses 
a  small  relative  surface. 

Other  forms  of  cathodes  are  open  cylinders 
and  cones  of  foil,  and  gauze  cylinders  and 
plates,  made  from  gauze  of  small  mesh  and 
fine  wire.  These  forms  are  shown  in  the 
illustrations  (Figs.  43  and  44).  Of  all  of  these 
forms  the  gauze  electrode  is  most  efficient,  not 
only  because  the  relative  surface  is  greatly 
increased  by  constructing  of  fine  wire  but  also 
because  practically  all  parts  of  the  surface  are 
equally  effective.  The  latter  condition  does 
not  obtain  for  foil  electrodes  of  any  form,  the 
surface  farthest  from  the  anode  being  in  a 
relatively  weak  electrolytic  field.  Gauze  elec- 
trodes also  permit  better  mixing  of  the  solu- 
tion and  very  much  higher  current  densities 
may  be  used.  Special  forms  of  electrodes  for  rapid  rotation  will 
be  discussed  later. 

Other  Apparatus. — The  necessary  apparatus  for  electro- 
analysis  will  include,  besides  the  electrodes,  a  generator  of  direct 
current,  variable  resistance,  voltmeter  and  ammeter.  The  source 
of  current  may  be  a  dynamo,  any  of  the  forms  of  primary  cells, 
secondary  or  storage  cells  or  thermoelements.  The  thermoele- 
ment is  not  a  practical  source  of  current,  being  both  inefficient 
and  unreliable.  The  direct  current  from  a  dynamo  may  be 
used  and  is  better  than  primary  cells,  the  latter  being  trouble- 
some in  the  matter  of  maintenance.  The  chief  objection  to 
the  dynamo  current  lies  in  the  fluctuations  usually  resulting 
from  a  variable  load  on  the  line  from  the  generator.  The  best 
and  most  satisfactory  current  producer  for  this  class  of  work  is 
the  secondary  or  storage  element.  Any  of  the  various  forms  of 
accumulators  will  prove  satisfactory,  the  lead-lead  peroxide  cell 
being  the  best  known.  The  great  merit  of  the  secondary  cell 


FIG.  42. — Crucible 
cathode. 


ELECTRO-ANALYSIS 


149 


is  its  constancy  and  reliability.  The  E.  M.  F.  of  the  lead  cell 
is  about  2  volts  and  the  necessary  voltage  for  the  work  may  be 
obtained  by  connecting  several  cells  in  series. 

Any  rheostat  will  do  for  this  work,  provided  that  the  range 
in  resistance  is  properly  related  to  the  other  factors  entering 
into  the  determination  of  current  strength.  In  the  absence 


FIG.  43. — Platinum  foil  cathodes. 


FIG.  44. — Platinum 
gauze  cathode. 


of  such  a  rheostat  carbon  lamps  may  be  used  for  the  current 
control,  if  the  line  voltage  is  high.  The  resistance  of  a  16  c.p. 
carbon  lamp  is  about  220  ohms  and  by  arranging  several 
in  parallel  a  fairly  satisfactory  regulation  of  current  may  be 
provided. 

Voltmeters  and  ammeters  should  have  the  scales  graduated 
with  a  range  as  limited  as  is  consistent  with  the  current  conditions 


150 


QUANTITATIVE  ANALYSIS 


to  be  employed,  so  that  each  subdivision  may  represent  a  small 
fraction  of  a  unit.  A  satisfactory  plan  is  to  have  double  scale 
instruments,  the  range  of  one  scale  being  ten  times  that  of  the 
other. 

The  necessary  connections  for  the  apparatus  used  for  electro- 
analysis  are  shown  in  figure  45.  B  represents  the  source  of 
current.  In  series  with  this  are  connected  A,  the  ammeter  for 
measuring  the  current  strength,  R,  a  rheostat  for  varying  the 
resistance  in  the  circuit  and  thus  providing  current  control, 
c  and  a,  cathode  and  anode.  The  voltmeter  V,  for  measuring 
the  pressure,  is  given  a  shunt  connection  across  the  electrodes. 


FIQ.  45. — Diagram  of  connections  for  electro-deposition  of  metals. 

The  actual  current  flowing  through  V  is  very  small  because  of 
the  high  resistance  possessed  by  the  winding  of  the  voltmeter. 
On  this  account  the  indications  of  the  ammeter  are  practically 
correct  for  the  current  flowing  through  the  electrolyte  between 
the  electrodes,  although  not  absolutely  so. 

The  following  is  a  description  of  the  apparatus  now  in  use  in 
the  Purdue  laboratory. 

Purdue  University  Laboratory  for  Electro-Analysis. — Current 
is  furnished  by  storage  cells  of  the  lead  type,  each  having  a 
capacity  of  48  amp-hours  and  a  maximum  charge  and  discharge 
rate  of  6  amp.  They  are  placed  in  a  closed  battery  case  which  is 
outside  the  room  for  electro-analysis.  The  cells  are  provided 
with  sand  trays  and  insulators,  and  also  with  glass  covers  which 


ELECTRO-ANALYSIS 


151 


almost  entirely  prevent  the  annoyance  due  to  acid  spray  during 
charging,  and  the  case  ventilator  makes  charging  absolutely 
inoffensive.  The  interior  of  the  case  is  protected  against  the 
attack  of  acid  by  asphaltum  paint. 

Wires  from  the  cells  run  "into  the  special  laboratory  where 
they  are  connected  with  the  distributing  switchboard.     This  is 


FIG.  46. — Distributing    switchboard     of     the    Purdue    University   Laboratory 

for  electro-analysis. 

a  28  X  72-inch  board  of  black  oiled  slate,  providing  switches 
and  plug  receptacles  for  the  control  of  all  current  which  is 
used  for  any  purpose  within  this  room.  The  cells  are  con- 
nected in  series  groups  of  three  each,  the  outside  terminals  of 
such  group  having  double  receptacles.  This  arrangement  en- 


152  QUANTITATIVE  ANALYSIS 

ables  the  operator  to  connect  his  cells  with  any  number  of 
groups  in  multiple,  thus  giving  greater  latitude  in  the  selection 
of  voltage  and  current  strength  than  is  possible  with  the  usual 
series  connections. 

The  110-volt  charging  current  enters  the  board  through  a 
switch  which  can  be  connected  by  plug  connectors  with  any 
cell  or  combination  of  cells,  and  a  slide-wire  rheostat  on  the 
back  of  the  board  makes  it  possible  to  charge  any  number  of 
cells  at  a  time.  The  cells  are  protected  during  charging  by  an 
under-load  circuit-breaker,  and  also  during  both  charge  and 
discharge  by  a  fuse  panel  which  is  placed  on  the  back  of  the 
board.  An  ammeter  with  a  range  of  20  amperes  and  a  double 
scale  voltmeter,  with  ranges  of  150  volts  and  15  volts,  are  pro- 
vided for  proper  control  of  the  charging  process.  ,The  15-volt 
scale  is  used  for  testing  the  voltage  of  the  cells  when  they  are  not 
in  use. 

A  switch  on  the  distributing  board  controls  the  110-volt 
alternating  current  which  is  used  for  the  lights  and  motors. 
Finally,  on  this  board  are  the  terminals  for  all  of  the  desks,  so 
that  any  operator  may  connect  with  his  desk  any  of  the  cells 
not  then  in  use,  and  in  almost  any  combination.  It  will  be  seen 
that  the  connections  on  this  board  make  the  different  cells  and 
desks  practically  independent  of  each  other;  for  instance,  a  part 
of  the  cells  may  be  charging  while  the  remainder  may  be  dis- 
tributed to  the  various  desks  as  wanted. 

At  each  working  desk  is  a  24  X  36-inch  slate  panel  which 
carries  all  of  the  apparatus  that  will  be  needed  by  the  analyst, 
making  each  board  an  independent  working  unit.  The  volt- 
meter and  ammeter  on  each  board  are  double-scale  instruments 
with  ranges  of  2  volts  and  20  volts  and  amperes,  respectively. 
The  multiplier  for  the  voltmeter  is  controlled  by  a  small  knife 
switch,  and  the  shunts  for  the  ammeter  are  joined  to  plug 
receptacles. 

The  current  to  each  desk  panel  is  controlled  by  a  slide-wire 
taper  rheostat.  These  rheostats  sre  wound  to  give  a  total  re- 
sistance of  130  ohms,  in  254  steps.  The  carrying  capacity  of 
the  first  step  is  1.3  amperes  and  that  of  the  last  step  25  amperes, 
on  continuous  work. 

For  working  with  rotating  electrodes,  1/30  h.p.  alternating- 


ELECTRO-ANALYSIS 


153 


current,  series  motors  of  the  commutator  type  are  mounted  on 
the  board  and  are  controlled  by  a  switch  and  5-step  rheostat. 
Induction  motors  are  not  used,  because  it  is  desirable  to  make 
variation  in  speed  possible.  These  motors  have  a  maximum 
speed  of  2200  r.p.m.  on  110  volts  and  are  provided  with  three 
pulleys  of  different  sizes. 


FIG.  47. — A  single  desk  panel  and  rotator,  Purdue  University  Laboratory. 

In  many  laboratories  it  is  the  practice  to  mount  the  motor 
directly  on  the  electrolyzing  stand,  thus  avoiding  all  belting  and 
making  possible  direct  connection  with  the  rotating  electrode. 
On  the  other  hand,  the  application  of  a  small  belt  is  a  simple 


154  QUANTITATIVE  ANALYSIS 

operation  and  not  only  very  much  decreases  the  vibration  of 
the  stand,  and  consequently  the  danger  of  dust  particles  fall- 
ing into  the  bath,  but  also  removes  the  motor  from  the  region 
of  the  bath,  which  in  many  cases  contains  acids  and  which  is 
frequently  hot.  Corrosion  of  the  motor  parts  is  in  this  way 
largely  prevented. 

The  stand  for  holding  the  electrodes  is  of  iron,  is  quite  heavy, 
to  prevent  vibration,  and  is  fitted  with  rubber  feet.  Every 
portion  of  the  base  and  vertical  rod  is  heavily  enameled  and  the 


FIG.  48. — Group  of  five  desk  panels,  Purdue  University  Laboratory. 

electrode  supports  are  clamped  to  the  rod  by  means  of  heavy 
thumb  screws,  but  in  such  a  manner  that  the  screw  does  not  come 
into  contact  with  the  rod,  so  that  the  enamel  is  not  injured  by 
the  grip.  Each  clamp  is  insulated  from  the  rod  by  a  fiber 
bushing  and  each  carries  a  binding  post.  There  is  no  glass 
about  the  stand,  perfect  insulation  of  the  electrode  clamp 
being  secured  by  the  fiber  bushings.  The  supporting  ring  for 
a  dish  cathode  has  three  brass  screw  contacts  which  are  ad- 
justable for  dishes  of  different  sizes.  Finally,  for  stationary 
electrodes,  simpler  clamps  are  provided  to  take  the  place  of  the 


ELECTRO-ANAL  YSIS 


155 


rotator.  The  rotator,  which  carries  three  pulleys  of  different 
sizes,  is  a  vertical  shaft,  the  lower  end  of  which  carries  a  universal 
chuck,  electrical  contact  being  insured  by  a  brass  brush. 


SAMPLE:  OF_ 


MARKED 


ELECTRO -ANALYSIS 


SOJISTANOE 
DETERMINED 

EXPEBUJEST 

2iUMBSB 

I 

II 

J 

II 

AMOUNT  or 
SAMPLE   TAKE* 

FRACTION  UBSD 

QUANTITY  or 
•OTHER 
ELECTROLYTES 
PRESENT 

VOLTS 

TOTAL  AMPERES 

CATHODE  SURFACE 

*%, 

TEMPERATURE 
RANGE 

TOTAL  TiME'or 
ELECTROLYSIS 

DESCUIEIIOM  ojf 
ELECTRODES 

AND  BEBtD,:iIi 

ROX&TED 

TOEUHTO* 
JtfEictJFocMf) 

CH«»AOTER  or 

DEPOSIT 

PERCENT. 

AVERAGE 

FIG.  49. — Blank  for  reporting  results  of  electro-analysis. 

Records. — Systematic  records  should  be  kept  of  all  of  the  data 
obtained  during  the  experiment.  A  satisfactory  blank  for  this 
purpose  is  shown  in  figure  49. 


156  QUANTITATIVE  ANALYSIS 

COPPER 

If  a  copper  salt  is  to  be  used  a  nitrate  or  sulphate  is  best  suited. 
Other  salts  of  volatile  acids  may  be  converted  into  the  sulphate 
by  evaporating  with  sulphuric  acid,  stopping  the  evaporation 
before  any  decomposition  into  copper  oxide  occurs.  Copper 
deposits  in  a  coherent  form  from  solutions  containing  sulphuric 
acid,  nitric  acid,  oxalic  acid  and  ammonium  oxalate,  potassium 
cyanide,  phosphoric  acid,  formic  acid  or  ammonium  hydroxide. 
Of  all  of  these,  nitric  acid  produces  the  best  results  and  probably 
sulphuric  acid  is  next  best.  Chlorides  should  not  be  present. 
If  metallic  copper  is  to  be  analyzed  it  may  be  dissolved  in  nitric 
acid  and  the  undesired  excess  of  acid  removed  by  evaporation. 

Determination  of  Copper  in  Soluble  Salts. — Use  enough  sample  to 
yield  0.25  to  0.50  gm  of  copper.  Dissolve  in  such  a  manner  that  200 
cc  of  solution  will  contain  about  2  cc  concentrated  nitric  acid.  If  the 
sample  contains  chlorides  it  must  be  treated  with  the  least  possible 
excess  of  sulphuric  acid  and  heated  until  fumes  of  sulphur  trioxide 
appear.  It  is  sometimes  desirable  to  make  a  larger  quantity  of  solution, 
as  250  cc,  and  to  use  an  aliquot  part  for  each  determination.  In  this 
case  the  acid  may  be  added  to  the  solution  as  used.  The  electrolysis 
may  be  begun  and  finished  at  the  temperature  of  the  room,  but  it  will 
be  hastened  by  warming  the  solution  to  about  70°  at  the  beginning. 
Connect  the  weighed  electrodes  and  add  enough  water  to  cover  the 
cathode,  which  must  extend  entirely  to  the  bottom  of  the  beaker,  place 
split  cover  glasses  on  the  beaker  and  electrolyze  with  a  pressure  not 
below  1.7  volts  and  not  above  2.0  volts  unless  other  metals  are  known  to 
be  absent.  In  the  latter  case  there  is  no  upper  limit  to  the  voltage  that 
may  be  used,  except  that  fixed  by  the  current  density  desired  to  give 
good  deposits.  The  current  density  that  may  be  used  will  depend  upon 
the  kind  of  electrodes.  For  foil  cones  or  cylinders  or  for  dishes,  NDioo  = 
about  0.1  amp.  For  gauze  electrodes  NDiOQ  may  sometimes  be  as  high 
as  5  amp.  In  any  case  the  analyst  must  use  his  judgment,  watching  the 
deposited  metal  to  discover  its  character.  The  copper  should  appear  as 
a  bright  red  metal  with  no  spots  of  brown  and  no  tendency  to  crumble 
off  the  cathode.  When  the  disappearance  of  color  indicates  that  the 
metal  is  deposited  remove  a  few  drops  to  a  white  test  plate  by  means  of  a 
pipette  and  test  for  traces  of  copper  by  adding  a  drop  of  concentrated 
ammonium  hydroxide  or  of  potassium  ferrocyanide  solution.  The  for- 
mer is  preferable  for  the  first  test  because  if  copper  is  found  the  solution 
can  be  neutralized  with  nitric  acid  and  returned  to  the  beaker.  If  no 
copper  is  indicated  make  another  test,  using  four  or  five  drops  taken 
from  the  bottom  of  the  beaker,  and  applying  the  ferrocyanide  test. 


ELECTRO-ANALYSIS  157 

When  all  of  the  metal  is  found  to  be  deposited  arrange  a  small  siphon 
tube  in  such  a  manner  that  the  solution  may  be  drawn  from  the  bottom 
of  the  beaker  without  interrupting  the  current.  As  the  solution  is 
removed  wash  down  the  exposed  portion  of  the  cathode  removing  every 
trace  of  acid  before  the  E.  M.  F.  is  removed  from  the  system.  It  is  best 
to  add  water  fast  enough  to  keep  the  bottoms  of  both  electrodes  covered 
until  the  acid  has  become  so  diluted  that  no  further  action  is  to  be  feared. 
Siphon  out  the  remaining  liquid  and  perform  the  next  operations  as 
quickly  as  possible.  Instead  of  siphoning,  another  plan  is  to  have  the 
beaker  supported  on  blocks,  removing  these  and  lowering  the  beaker 
gradually  and  washing  the  cathode  as  it  becomes  exposed. 

Lower  the  beaker  and  remove  the  cathode,  taking  care  to  avoid 
touching  cathode  to  anode  and  thus  making  a  short  circuit,  and  quickly 
wash  with  much  water.  Set  aside  until  the  duplicate  cathode  has  been 
treated  in  the  same  way,  then  wash  both  cathodes  with  redistilled  alcohol 
and  dry  at  100°.  The  alcohol  washing  may  be  omitted,  but  drying  is 
hastened  by  this  means.  Weigh  and  calculate  the  percent  of  copper  in 
the  sample. 

Remove  the  copper  from  the  cathode  by  dipping  into  warm  dilute 
nitric  acid. 

Determination  of  Copper  in  Brass  and  Similar  Alloys.1 — Dissolve 
0.5  gm  of  the  drillings  in  a  mixture  of  5  cc  each  of  concentrated  nitric 
acid  and  water,  in  a  covered  casserole,  heating  finally  to  expel  all  brown 
oxides  of  nitrogen.  Dilute  to  75  cc  and  filter  through  a  paper  of  close 
texture.  Wash  the  residue  of  metastannic  acid  with  hot  water,  pre- 
serving the  washings  with  the  filtrate.  (The  precipitate  may  be  ignited 
in  a  porcelain  crucible  to  stannic  oxide  for  an  approximate  determination 
of  tin.) 

To  the  filtrate  and  washings  in  a  casserole  add  5  cc  of  concentrated 
sulphuric  acid.  Evaporate  under  a  hood  until  copious  fumes  of  sul- 
phuric acid  appear.  The  nitric  acid  is  thus  driven  off.  Cool,  add  35  cc 
of  water  and  boil  for  1  minute  in  order  to  dissolve  all  soluble  sulphates. 
Filter  on  a  Gooch  crucible  and  wash  with  dilute  sulphuric  acid  until  a 
few  drops  of  the  washings  show  no  blue  color  with  a  slight  excess  of 
ammonium  hydroxide.  (In  case  the  Gooch  crucible  has  been  ignited 
and  weighed  before  filtering  the  solution,  the  lead  sulphate  may  be  used 
for  a  determination  of  lead.  It  is  washed  with  50  percent  alcohol  to 
remove  sulphuric  acid  and  is  then  dried  and  heated,  cautiously  at  first, 
to  dull  redness  for  15  minutes.  After  cooling  it  is  weighed  as  lead  sul- 
phate, from  which  the  percent  of  lead  is  calculated.) 

1  The  complete  analysis  of  brass  is  described  on  page  505.  The  brief 
outline  here  given  is  with  reference  to  the  electrolytic  determination  of 
copper. 


158  QUANTITATIVE  ANALYSIS 

The  solution  and  washings  (not  including  alcohol  washings  from  the 
lead  sulphate)  now  contain  copper,  zinc  and  sulphuric  acid.  In  pres- 
ence of  so  much  sulphuric  acid  it  is  not  desirable  to  add  more  nitric  acid, 
even  though  the  latter  would  improve  the  character  of  the  deposit  of 
copper  to  be  obtained.  Instead,  add  1  gm  of  ammonium  nitrate,  which 
results  in  the  substitution  of  nitric  for  sulphuric  acid.  Rinse  the  solu- 
tion into  the  beaker  in  which  electrolysis  is  to  take  place,  arrange  the 
electrodes  and  add  water  until  the  cathode  is  covered,  mixing  well. 
Conduct  the  electrolysis  and  subsequent  treatment  as  directed  on  page 
156,  keeping  the  voltage  below  2.5  to  avoid  the  deposition  of  zinc. 

Determination  of  Copper  in  Ores. — The  ore  will  always  contain  more 
or  less  insoluble  gangue.  It  may  or  may  not  contain  metals  whose 
compounds  are  soluble  in  aqua  regia  and  whose  decomposition  voltages 
are  near  to  that  of  copper  (bismuth,  antimony,  arsenic,  mercury  or 
silver) .  In  case  such  metals  are  absent  or  are  present  in  quantities  so 
small  that  they  may  be  disregarded,  proceed  as  follows : 

Weigh  0.5  gm  of  the  powdered  ore  and  place  in  a  casserole.  Add 
10  cc  of  concentrated  hydrochloric  acid  and  5  cc  of  concentrated  nitric 
acid,  cover  and  boil  slowly  to  aid  in  dissolving.  Digest  on  the  steam 
bath  until  action  seems  to  be  complete  then  add  7  cc  of  concentrated 
sulphuric  acid  and  boil  under  a  hood  until  volatile  acids  are  expelled 
and  fumes  of  sulphuric  acid  appear.  Cool,  add  25  cc  of  water,  boil  1 
minute  and  filter  to  remove  lead  sulphate  and  gangue,  receiving  the 
filtrate  in  the  beaker  in  which  the  solution  is  to  be  electrolyzed.  Wash 
well  with  hot  water,  preserving  the  washings  with  the  filtrate.  Add  1 
gm  of  dry  ammonium  nitrate,  connect  the  electrodes  in  place,  add  water 
until  the  cathode  is  covered  and  mix  well.  Electrolyze  as  directed  on 
page  156. 

If  any  of  the  metals  named  above  are  present  in  appreciable  quantities 
•the  procedure  is  the  same  as  that  just  described  for  other  ores,  until 
after  the  filtrate  from  lead  sulphate  and  gangue  is  obtained.  This 
filtrate  and  the  washings  are  received  in  a  casserole.  Dilute,  if  neces- 
sary, to  75  cc.  Cut  a  strip  of  sheet  aluminium  about  2.5  cm  wide  and 
14  cm  long,  bend  into  a  triangle  and  place  in  the  casserole.  Cover  the 
solution  and  boil  gently  until  all  of  the  copper  is  precipitated,  leaving 
the  solution  colorless  or  green  from  ferrous  sulphate.  (If  this  condition 
cannot  be  obtained  it  is  because  nitric  acid  has  not  been  expelled  com- 
pletely when  evaporating  with  sulphuric  acid.) 

When  all  copper  is  precipitated  wash  down  the  sides  of  the  casserole 
with  a  stream  of  hydrogen  sulphide  solution  and  pour  the  solution  and 
copper  into  a  filter  paper.  Filter  rapidly,  to  minimize  oxidation  and 
resolution  of  copper,  and  wash  the  aluminium  and  copper  with  hydrogen 
sulphide  solution.  The  latter  will  cause  the  precipitation  of  traces  of 


ELECTRO-ANALYSIS  159 

copper  that  may  have  redissolved  but  this  precipitation  should  take 
place  before  the  solution  has  passed  through  the  filter.  If  the  filtrate 
appears  brown  this  indicates  a  loss  of  copper  and  the  solution  must  be 
refiltered  and  washed  until  a  clear  filtrate  is  obtained.  Place  under 
the  filter  the  beaker  which  is  to  be  used  for  the  electrolysis,  then  pour 
over  the  aluminium  in  the  casserole  a  mixture  of  4  cc  of  concentrated 
nitric  acid  with  the  same  volume  of  water.  Heat  to  dissolve  all  adher- 
ing copper  then  pour  the  acid  slowly  over  the  copper  in  the  filter.  When 
all  copper  is  dissolved  wash  the  paper  very  thoroughly  with  hot  water. 
Boil  the  filtrate  and  washings  to  expel  oxides  of  nitrogen,  then  electro- 
lyze  without  further  addition  of  acid. 

SILVER 

The  deposition  of  silver  from  solutions  containing  free  acids 
may  be  accomplished,  but  the  deposit  is  usually  either  spongy  or 
crystalline  so  that  it  cannot  be  washed  without  loss.  When 
nitric  acid  is  present  there  is  also  a  deposit  of  silver  peroxide  at 
the  anode  if  high  voltage  is  applied.  The  addition  of  potassium 
cyanide,  although  materially  lowering  the  concentration  of  silver 
cations,  is  desirable  because  it  entirely  prevents  the  formation 
of  silver  peroxide  and  also  yields  a  coherent  deposit  of  silver  at 
the  cathode.  The  deposit  is  without  lustre  and  should  be  white. 

The  deposition  of  silver  peroxide  upon  the  anode,  when  high 
voltage  is  applied  to  solutions  containing  nitric  acid,  is  probably 

due  to  the  existence  of  the  anion  of  a  silver  oxyacid,  H2AgO2—  * 

+ 

2H+AgO2.  This  is  a  theoretical  derivative  of  the  dihydroxide 
of  silver,  Ag(OH)2.  The  decomposition  potential  of  this  anion 
is  higher  than  that  of  the  univalent  silver  cation  and  its  concentra- 
tion is  always  small,  yet  it  will  discharge  to  some  extent  at  the 
same  time  that  silver  is  depositing  upon  the  cathode,  if  high  vol- 

tage and  current  density  are  used:  Ag02—  »AgO+0.  Such  a 
deposit  having  formed  upon  the  anode  it  will  finally  redissolve 
as  the  silver  becomes  more  dilute  in  the  solution  because  the 
equilibrium 


is  disturbed  by  the  removal  of  silver  cations,  Ag,  which  are  dis- 
charging at  a  much  greater  rate  on  account  of  their  lower  decom- 


160  QUANTITATIVE  ANALYSIS 

position  potential.  The  discharge  and  deposition  of  silver 
peroxide  is  entirely  prevented  by  potassium  cyanide.  When 
this  is  added  to  a  solution  of  silver  nitrate  a  precipitate  of  silver 
cyanide  is  first  formed: 

AgNO3+'KCN->AgCN+KN03. 

An  excess  of  potassium  cyanide  redissolves  this  precipitate, 
forming  a  salt  of  a  complex  anion: 

AgCN+KCN-»KAg(CN)2. 

In  this  way  the  equilibrium, 

+ 
Ag<=±AgO2 

is  also  disturbed  by  the  conversion  of  silver  mto  the  new  anion 

Ag(CN)2,  which,  presumably,  has  a  high  decomposition  potential. 
The  double  cyanide  of  potassium  and  silver  cannot  all  be  in  the 
form  represented  by  the  formula  KAg(CN)2  as  in  this  case  no 
silver  could  be  discharged  at  the  cathode,  but  only  hydrogen. 
There  must,  therefore,  be  equilibrium  between  the  salt  having  the 
composition  represented  above  and  the  single  cyanides: 

KAg(CN)2^KCN+AgCN, 
or  the  ionic  equilibrium 

Ag(CN)2^Ag+2CN. 
It   is   to   be   supposed   that  the  relatively  high  decomposition 

potential  of  the  anion  Ag(CN)2  prevents  its  discharge  under 
ordinary  conditions. 

Determination. — Use  sufficient  sample  to  give  about  0.3  gm  of  silver. 
Dissolve  this  in  a  small  amount  of  water  and  add  just  double  the  quan- 
tity of  a  solution  containing  3  to  5  gm  of  potassium  cyanide  in  25  cc  of 
water  that  is  necessary  to  redissolve  the  precipitated  silver  cyanide. 
Fasten  the  weighed  electrodes  in  place  and  dilute  with  water  until  the 
cathode  is  covered.  The  E.M.F.  required  will  be  about  2  volts  and  NDiQO 
=  0.04  to  0.10  amp  for  electrodes  not  of  gauze  or  as  high  as  2  amp  if 
gauze  electrodes  are  used.  If  the  current  density  is  too  large  the  potas- 
sium cyanide  will  be  decomposed  around  the  anode,  giving  a  dark  solu- 


ELECTRO-ANAL  YS2S  161 

tion  of  organic  matter  that  will  eventually  reach  the  cathode  and  darken 
the  silver.  Such  darkening  will  occur  also  if  the  potassium  cyanide  is 
impure.  A  cyanide  of  high  grade  is  required  for  this  purpose.  The 
deposition  will  require  30  minutes  to  5  hours,  depending  upon  the 
current  density  used. 

When  the  deposition  is  thought  to  be  complete  a  few  drops  of  the 
solution  may  be  tested  for  silver  by  adding  dilute  nitric  acid  in  sufficient 
quantity  to  decompose  the  potassium  cyanide.  Hydrocyanic  acid  is 
removed  by  boiling  and  then  ammonia  is  added  to  make  basic,  this 
being  followed  by  ammonium  sulphide.  When  the  deposition  has  been 
shown  to  be  complete  the  solution  is  removed  and  the  cathode  washed, 
dried  and  weighed  as  in  the  preceding  exercise.  Remove  the  silver  from 
the  electrode  by  dipping  into  dilute  nitric  acid. 

Caution. — Avoid  inhaling  the  vapors  that  arise  during  the  progress 
of  electrolysis.  Do  not  use  pipettes  and  do  not  fill  the  siphon  by  suction. 

Determination  of  Silver  and  Copper  in  an  Alloy. — (Other  metals,  with 
the  exception  of  gold,  are  assumed  to  be  absent.) 

Clean  the  alloy  by  polishing,  followed  by  wiping  with  filter  paper. 
Cut  into  pieces  or  drill  so  that  about  0.5  gm  may  be  used  for  each 
analysis.  Dissolve  in  a  covered  casserole  in  a  mixture  of  5  cc  each  of 
concentrated  nitric  acid  and  water.  Rinse  down  the  cover  glass  and 
evaporate  the  solution  nearly  to  dryness  on  the  steam  bath.  Do  not 
heat  the  dry  mass  as  this  would  cause  the  decomposition  of  some  of  the 
nitrates.  Redissolve  the  salts  in  about  10  cc  of  water. 

Silver. — Place  3  gm  of  potassium  cyanide  in  the  electrolyzing  beaker, 
dissolve  in  50  cc  of  water  and  rinse  into  this  the  solution  of  copper  and 
silver  nitrates,  stirring.  Insert  the  electrodes,  dilute  until  the  cathode 
is  covered  with  solution,  mix  and  electrolyze  as  directed  on  page  160, 
keeping  the  voltage  below  1.5. 

Copper. — To  the  solution  from  which  silver  has  been  deposited, 
including  the  washings  from  the  latter,  add  5  cc  of  concentrated  nitric 
acid.  This  should  be  done  under  a  hood  as  the  vapors  of  hydrocyanic 
acid  are  extremely  poisonous.  Evaporate  to  75  cc  or  less,  to  expel  all 
of  the  hydrocyanic  acid.  Determine  the  copper  as  directed  on  page  156. 

IRON 

Iron  does  not  deposit  well  from  solutions  containing  nitrates, 
chlorides  or  strong  inorganic  acids.  If  the  iron  salt  is  derived 
from  such  an  acid  this  acid  will  be  formed  as  electrolysis  proceeds 
and  iron  will  redissolve  from  the  cathode.  To  prevent  its  forma- 
tion a  salt  of  a  weak  inorganic  or  organic  acid  may  be  added. 
11 


162  QUANTITATIVE  ANALYSIS 

Examples  of  salts  that  are  used  for  this  purpose  are  ammonium 
oxalate,  ammonium  tartrate  and  sodium  citrate.  There  is  always 
a  possibility  of  depositing  some  carbon  from  such  solutions, 
and  this  is  least  likely  to  occur  when  the  oxalate  is  used.  The 
iron  or  iron  salt  should  be  converted  into  the  sulphate  before 
electrolyzing. 

Determination. — Calculate  the  weight  of  sample  that  will  be  required 
to  give  about  0.3  gm  of  iron.  If  the  sample  is  an  iron  salt,  soluble 
in  water,  dissolve  in  water,  using  no  more  than  is  necessary.  If  the 
sample  is  iron  or  steel,  dissolve  in  the  proper  quantity  of  dilute  sul- 
phuric acid,  avoiding  an  excess.  In  either  case,  pour  the  solution 
slowly  into  a  solution  of  5  to  10  gm  of  ammonium  oxalate  in  as  little  water 
as  possible,  stirring  until  the  precipitate  of  iron  oxalate  is  redissolved. 
Dilute,  after  the  electrodes  are  in  place,  until  the  solution  covers  the 
cathode.  The  decomposition  voltage  for  iron  in  such  a  solution  is  about  2 
andNDioo=  0.1.  The  deposited  iron  should  be  bright.  It  does  not 
easily  redissolve  in  the  solution  when  the  current  is  interrupted  and  may 
be  readily  washed.  The  end  of  the  process  is  tested  by  the  use  of  potas- 
sium ferricyanide  or  potassium  thiocyanate.  If  the  latter  is  used  the 
solution  should  be  previously  warmed  with  two  or  three  drops  of  nitric 
acid,  since  the  iron  (if  any  is  present)  has  been  reduced  to  the  ferrous  con- 
dition by  the  current  action.  After  weighing  the  iron  it  should  be  re- 
moved from  the  cathode  by  dissolving  in  warm  dilute  sulphuric  acid. 

LEAD 

When  salts  of  lead  in  solution  are  subjected  to  the  action  of  a 
current  the  lead  may  deposit  upon  both  cathode  and  anode— 
upon  the  former  as  elementary  lead  and  upon  the  latter  as 
lead  peroxide.  Upon  the  cathode  the  lead  is  so  spongy  that  it 
becomes  -impossible  to  wash  and  dry  it  properly.  This  is  the 
familiar  action  of  the  lead  storage  cell  during  charging,  where 
lead  sulphate  is  electrolyzed,  producing  spongy  lead  at  the 
" negative"  and  lead  peroxide  at  the  " positive."  If  the  lead 
salt  solution  contains  a  considerable  excess  of  nitric  acid  the 
entire  quantity  of  lead  deposits  upon  the  anode  as  peroxide  and 
this  is  the  only  practicable  quantitative  method  for  the  electroly- 
sis of  lead  salts  except  where  the  mercury  cathode  is  used. 
Lead  dioxide  cannot  be  completely  dehydrated  unless  heated  to 
a  temperature  above  200°. 


ELECTRO-ANAL  YSIS  1 63 

The  deposition  of  lead  peroxide  upon  the  anode  when  nitric 
acid  is  present  is  to  be  explained  exactly  as  in  the  case  of  silver. 
The  solution  contains  a  small  concentration  of  the  amphoteric 
lead  perhydroxide,  Pb(OH)4,  which  furnishes  anions  of  an  oxy- 
acid  of  lead,  H4Pb04.  The  electrolysis  of  this  acid  and  dis- 
charge of  its  anion  produces  lead  peroxide  and  oxygen: 

Pb=04-+Pb02+02. 

As  in  the  case  of  silver  the  formation  of  the  peroxide  may  be 
prevented  by  the  addition  of  some  substance  which  will  diminish 
the  concentration  of  lead  cations,  such  as  ammonium  oxalate. 
Since  the  cathodic  deposit  of  lead  cannot  well  be  washed  and 
dried  without  loss  or  oxidation  the  anodic  deposition  of  peroxide 
is  assisted  by  addition  of  nitric  acid. 

Determination. — Weigh  enough  lead  salt  (preferably  nitrate)  to 
produce  about  0.3  gm  of  lead,  dissolve  in  the  proper  quantity  of  water 
to  cover  the  electrodes  and  add  20  cc  of  concentrated  nitric  acid  for 
each  100  cc  of  solution.  Connect  the  electrodes  so  that  the  one  with 
the  largest  surface  will  be  he  anode,  instead  of  the  cathode  as  is  the 
case  in  the  electrolysis  of  most  other  metals.  The  lead  peroxide  will  not 
adhere  well  unless  the  anode  surface  has  been  roughened,  as  by  sand 
blasting.  Warm  to  about  50°  and  keep  at  this  temperature  until  the 
electrolysis  is  finished.  Use  2.4  volts.  NDioo  =  1.5  amp.  At  the  end 
of  the  operation  test  the  solution  for  lead  by  adding  a  few  drops  of 
hydrogen  sulphide  solution  to  a  small  amount  of  the  electrolyte.  Care- 
fully remove  the  anode,  having  previously  washed  it  in  the  usual  way. 
Dry  at  200°  to  230°  until  the  weight  is  constant. 

NICKEL 

The  best  solution  from  which  to  deposit  nickel  is  that  of  the 
sulphate,  containing  ammonium  sulphate  and  ammonium  hy- 
droxide. If  nitric  acid  is  present  there  is  usually  some  trouble, 
due  to  the  oxidation  of  the  deposited  nickel.  Nickel  may  also  be 
precipitated  from  solutions  containing  ammonium  oxalate, 
tartrate  or  citrate  or  from  solutions  containing  an  excess  of  potas- 
sium cyanide. 

Determination.  Separation  of  Copper,  Nickel  and  Iron. — Thoroughly 
clean  and  dry  a  nickel  coin,  weigh  it  accurately,  place  in  a  casserole  and 
dissolve  in  nitric  acid  (sp.  gr.  1.2)  the  casserole  being  covered  while  the 
coin  is  dissolving.  Carefully  add  10  cc  of  concentrated  sulhpuric  acid 


164  QUANTITATIVE  ANALYSIS 

and  evaporate  over  a  flame  until  the  characteristic  dense,  white  fumes 
of  sulphuric  acid  appear.  The  evaporation  should  be  accomplished 
while  holding  the  casserole  in  the  hand,  giving  it  a  continuous  rotary 
motion  to  hasten  evaporation  and  prevent  spattering.  Allow  the  mate- 
rial to  cool  then  wash  into  a  250  cc  graduated  flask  and  dilute  to  the 
mark. 

Measure  50  cc  of  the  solution  into  the  vessel  in  which  electrolysis 
is  to  be  accomplished  and  deposit  the  copper  in  the  manner  already 
described.  The  voltage  should  not  exceed  2;  7,  which  is  nearly  the 
decomposition  voltage  of  nickel.  Unusual  care  should  be  exercised 
in  washing  and  saving  the  washings  because  other  metals  are  to  be 
determined. 

Evaporate  the  solution  from  which  the  copper  has  been  removed, 
until  the  volume  is  about  100  cc.  Neutralize  with  ammonium  hydrox- 
ide, boil  to  flocculate  the  colloidal  ferric  hydroxide  which  is  always 
present  and  filter  off  the  precipitate,  washing  the  paper  'and  precipitate 
with  hot  water.  Add  to  the  nitrate  7  gm  of  ammonium  sulphate 
and  20  cc  of  ammonium  hydroxide  (sp.  gr.'  0.90),  and  electrolyze. 
Decomposition  voltage  is  about  2.8  and  any  voltage  above  this  value 
may  be  used,  the  upper  limit  being  fixed  by  the  nature  of  the  deposit 
obtained.  Record  the  current  density. 

Dissolve  the  ferric  hydroxide  in  the  filter  paper  with  1  cc  of  oxalic 
acid  solution,  saturated  at  about  20°.  Wash  the  solution  out  of  the 
paper  with  hot  water  and  into  a  solution  of  5  gm  of  ammonium  oxalate 
in  100  cc  of  water.  Electrolyze  as  previously  directed. 

Moving  Electrodes. — In  the  discussion  of  decomposition 
voltage  it  was  noted  that  if  the  voltage  is  unduly  increased  in 
or.der  to  hasten  the  decomposition,  gas  evolution  prevents  the 
formation  of  a  dense  deposit  of  metal.  Migration  of  the  ions 
is  comparatively  slow  and  current  is  transferred,  not  only  across 
the  solution,  but  also  from  the  solution  to  the  cathode,  by  hydro- 
gen ions.  If  the  migration  of  the  metal  ions  is  aided  by  stirring 
the  solution  large  currents  may  sometimes  be  carried  without 
the  deposition  of  enough  hydrogen  on  the  cathode  to  injure  the 
metal  deposit.  Stirring  may  be  accomplished  by  any  one  of 
five  different  methods:  (1)  Heating  to  produce  convection 
currents,  (2)  use  of  stirring  apparatus  not  connected  with  the 
electrodes,  (3)  rotation  of  the  anode,  (4)  rotation  of  the  cathode, 
(5)  electromagnetic  action.  Convection  currents  are  of  limited 
usefulness  because  they  are  not  sufficiently  rapid.  They  will, 


ELECTRO-ANAL  YSIS  165 

however,  materially  shorten  the  time  of  electrolysis.  Mechanical 
stirring,  whether  by.  the  second,  third  or  fourth  method,  has 
practically  the  same  use.  Which  method  of  these  three  is 
to  be  chosen  will  be  decided  chiefly  by  matters  of  convenience. 
If  a  stirrer  of  glass  or  other  non-conducting  material  is  to  be 
used  it  will  require  room  for  its  movement  and,  since  it  is  as 
easy  to  rotate  one  of  the  electrodes,  the  stirrer  is  generally  made 
one  of  these.  Either  electrode  may  be  rotated 
with  success.  The  anode  is  generally  the  one 
chosen  for  this  purpose  because  it  has  not  the 
large  surface  of  the  cathode  and  is  therefore  more 
easily  manipulated.  Fig.  50  shows  one  of  the 
forms  of  anodes  that  may  be  rotated  rapidly 
without  becoming  bent  or  distorted.  The 
cathode  is  most  frequently  a  dish.  In  order  to 
provide  a  dish  of  large  surface  and  capacity 
Classen  devised  a  very  thin  platinum  dish.  The 
speed  of  rotation  of  the  anode  should  be  as  high 
as  may  be  attained  without  danger  of  throwing 
the  solution  out  of  the  vessel.  500  to  1000  r.p.m. 
may  be  used.  Recently  a  comparatively  slow 
rotation  (150  r.p.m.)  of  the  ordinary  spiral  anode 
inside  a  gauze  cathode  has  been  used,  with  a  con- 
siderable degree  of  success.  Indeed,  it  is  ques- 
tionable whether  this  is  not  more  practicable  than 
the  rapid  rotation  because  the  gauze  electrodes 
may  be  used  without  danger  and  the  apparatus 
needs  no  watching  after  starting.  The  time 
necessary  for  complete  deposition  of  the  metal 
may  be  made  about  one-fifth  of  that  required 
when  stationary  electrodes  are  used. 

In  the  electromagnetic  stirring  apparatus  de-        FlG    50_ 
( vised  by  Frary1  the  solution  is  placed  within  an     Anode  suitable 
electromagnetic  field,  generated  by    a    current 
passing  through  a  solenoid  surrounding  the  electrolyte.     The 
moving  ions  constitute  a  conductor  in  which  the  current  moves 
radially  while  the  electromagnetic  field  is  vertical.      In  another 
form  the  apparatus  is  changed  using  a  vertical  field  and  vertical 

1  J.  Am.  Chem.  Soc.,  29,  1592  (1907). 


166 


QUANTITATIVE  ANALYSIS 


current  lines.      In  either  case  the  mutual  action  of  the  fields 
causes  rotation  of  the  solution  within. 

The  Mercury  Cathode.— By  making  the  cathode  of  mercury 
instead  of  platinum  two  important  gains  are  made.  The  large 
expenditure  for  electrodes  is  largely  eliminated  because  of  the 
relative  cheapness  of  mercury,  also  there  is  no  longer  any  ques- 
tion as  to  the  satisfactory  nature  of  the  deposit  of  metal,  since 
the  latter  amalgamates  with  the  mercury  instead  of  forming  a 
surface  deposit.  The  one  obstacle  to  a  nearly  universal  use  of 
this  form  of  electrode  lies  in  the  small  surface  that  may  be  used, 


FIG.  51. — (a)  Mercury  cathode  cell  with  (b)  drying  apparatus. 

the  large  specific  gravity  of  mercury  prohibiting  the  use  of  more 
than  a  few  cubic  centimeters.  The  most  satisfactory  form  of 
apparatus  for  this  purpose  is  a  glass  cup  having  a  small  platinum 
wire  fused  into  the  bottom  for  the  cathode  connection  (Fig.  51). 
The  upper  limit  to  the  normal  density  is  approximately  fixed 
by  the  undesirable  heating  effect  of  large  current  densities.  The 
anode  should  be  rotated. 

In  the  following  exercises  are  given  the  changes  necessary  to 
adapt  the  foregoing  exercises  to  the  use  of  rotating  electrodes  and 
the  mercury  cathode. 

Determination  of  Copper  by  Use  of  the  Rotating  Anode. — Set  up 
the  apparatus  and  add  to  the  solution  1  cc  of  dilute  sulphuric  acid 


ELECTRO-ANALYSIS  167 

instead  of  nitric  acid.  Use  rapid  rotation  and  note  the  current  density 
that  can  be  employed. 

For  slow  rotation  use  the  gauze  cathode  and  spiral  anode.  Use  the 
same  solution  as  with  stationary  electrodes  but  increase  the  current 
density  as  much  as  possible,  noting  the  character  of  the  deposit  in  deter- 
mining the  practicable  maximum. 

Determination  of  Copper  by  Use  of  the  Mercury  Cathode  and  Rotating 
Anode. — Use  the  small  amount  of  solution  made  necessary  by  the  size 
of  the  cup.  Thoroughly  clean  the  cup  and  place  in  it  pure  mercury 
until  the  whole  apparatus  weighs  not  more  than  60  gm.  Wash  with 
distilled  water,  then  with  redistilled  alcohol  and  finally  dry  by  passing 
dried  air  through  the  cup.  The  drying  tube  on  the  inlet  of  the  drying 
apparatus  must  be  so  arranged  that  the  air  is  well  filtered  by  cotton 
and  no  calcium  chloride  can  enter  the  cell.  The  drying  will  cool  the 
mercury  and  this  must  be  allowed  to  warm  to  the  temperature  of  the 
laboratory  before  weighing.  Connect  the  platinum  wire  with  the  nega- 
tive binding  post  and  place  the  anode  in  position.  Pour  in  the  solution 
but  do  not  add  acid  of  any  kind.  Rotate  the  anode  as  rapidly  as  may 
be  done  without  loss  of  solution  by  spattering.  Limit  the  current 
density  only  by  the  tendency  of  the  solution  to  boil.  After  the  electroly- 
sis is  completed  stop  the  rotation  of  the  anode,  reduce  the  voltage 
(but  not  below  2  volts),  lower  the  anode  until  it  almost  touches  the 
mercury  and  siphon  and  wash  until  no  acid  remains.  Finally  rinse  the 
cell  with  redistilled  alcohol  and  dry  at  the  temperature  of  the  laboratory. 

The  copper  amalgam  may  be  used  as  a  cathode  in  more  experiments 
but  it  should  be  replaced  by  pure  mercury  as  soon  as  it  shows  any  tend- 
ency to  form  a  scale  of  undissolved  copper. 

Determination  of  Silver  by  Use  of  the  Rotating  Anode. — Use  the 
same  solution  as  with  stationary  electrodes.  The  apparatus  and  general 
manipulation  are  the  same  as  with  copper.  Either  rapid  or  slow  rotation 
of  the  anode  will  prove  to  be  satisfactory. 

Determination  of  Silver  by  Use  of  the  Mercury  Cathode  and  Rotating 
Anode. — Dissolve  the  silver  salt  in  the  proper  amount  of  water  and  do 
not  add  potassium  cyanide.  Proceed  as  with  copper.  The  deposit 
of  silver  peroxide  that  usually  appears  upon  the  anode  at  first  should 
later  redissolve  so  that  all  of  the  silver  will  finally  amalgamate  at  the 
cathode. 

Determination  of  Iron  by  Use  of  the  Rotating  Anode. — The  iron  should 
be  in  the  form  of  sulphate.  Note  the  current  conditions  required  to 
produce  a  good  deposit. 

Determination  of  Iron  by  Use  of  the  Mercury  Cathode  and  Rotating 
Anode. — Use  the  same  solution  as  in  the  preceding  exercise  and  similar 
current  conditions. 


CHAPTER  V 
VOLUMETRIC  ANALYSIS 

The  gravimetric  process  involves  the  conversion  of  a  given 
constituent  of  a  substance  into  a  compound  of  known  composi- 
tion by  the  addition  of  an  excess  of  a  precipitating  reagent. 
The  volumetric  process  consists  in  the  addition  of  a  reagent  of 
accurately  known  concentration  (a  ''standard  solution")  until 
a  definite  reaction  with  the  substance  is  exactly  completed.  In 
the  first  case  the  compound  produced  is  weighed  while  in  the  second 
case  a  solution  of  one  of  the  substances  reacting  is  measured  by 
volume,  the  weight  of  the  reacting  substance  being  thus  obtained 
indirectly.  In  the  gravimetric  process  the  constituent  to  be 
determined  is  actually  weighed  in  a  new  compound  and  its  weight 
calculated  from  the  known  composition  of  that  compound.  In 
the  volumetric  process  the  constituent  to  be  determined  is  cal- 
culated from  its  known  reacting  ratio  to  the  indirectly  observed 
weight  of  reagent.  Since  a  measurement  of  volume  is  much 
more  quickly  and  easily  made  than  is  the  case  with  the  necessary 
filtration,  washing,  drying,  ignition,  cooling  and  weighing  of  a 
gravimetric  determination,  it  follows  that  the  volumetric  method 
frequently  results  in  a  great  saving  of  time.  This  is,  however, 
not  always  the  case  and  the  method  to  be  chosen  will  be  that 
which,  all  circumstances  considered,  can  be  carried  out  most 
easily,  quickly  and  accurately.  No  generalization  can,  at  pres- 
ent, be  made  regarding  this  choice.  The  choice  itself  will  not 
be  difficult  or  uncertain  after  some  experience  is  gained  in  general 
quantitative  analysis. 

Apparatus. — It  may  be  observed  at  the  beginning  that  the 
balance  is  practically  always  concerned  even  in  the  volumetric 
process.  The  concentration  of  the  standard  solution  must  be 
determined  and  this  determination  is  generally  gravimetric. 
This  fact  might  seem  to  remove  the  time-saving  element  from 
the  new  class  of  methods.  This  is  not  so  because  one  gravi- 

168 


VOLUMETRIC  ANALYSIS 


169 


metric  determination  suffices  for  a  large  number  of  volumetric 
determinations  if  a  sufficient  quantity  of  standard  solution  is 
made.  The  volumetric  process  will  involve  many  very  accu- 
rate measurements  of  volume  and  consequently  several  forms 
of  graduated  apparatus.  These  will  be  described  with  some 
detail. 

Flasks. — For  measuring  relatively  large  quantities  (50  cc  to 
2000  cc)  of  liquids  in  one  portion  the  graduated  flask  may  be 
employed.  This  is  the  form  of  apparatus  that 
possesses  the  least  relative  surface  and  conse- 
quently causes  the  least  trouble  in  washing, 
draining,  etc.  The  one  reading  is  made  at  a 
mark  on  the  neck.  The  neck  must  be  suffi- 
ciently small  to  permit  a  reading  with  a  slight 
percentage  error  but  must  be  large  enough  to 
permit  filling  and  emptying  without  trouble. 
These  are  practical  limits,  fixed  as  the  result 
of  experience.  The  requirements  of  practice 
also  demand  a  neck  of  uniform  bore  and  of 
some  margin  above  and  below  the  mark. 

Volumetric  flasks  may  be  graduated  to  con- 
tain a  stated  amount  or  to  deliver  this  amount 
when  emptied.  No  container  can  be  made 
to  deliver  all  of  the  liquid  contained  in  it  if 
the  liquid  is  one  that  wets  glass,  a  condition 
that  obtains  with  water  and  solutions  in 
water.  If  the  flask  is  to  be  graduated  to 
deliver  a  stated  amount  the  mark  must  be 
placed  higher  than  if  it  indicates  the  same  amount  contained  in 
the  flask.  If  a  flask,  upon  emptying,  could  be  made  to  drain 
uniformly  it  could  be  accurately  calibrated  for  deliverance.  On 
account  of  the  necessary  form  of  the  flask  -this  is  impossible  and 
for  accurate  work  the  flask  is  always  calibrated  for  containing 
the  amount  indicated  by  the  inscription  upon  it. 

Pipettes. — Pipettes  are  filled  by  suction  and  allowed  to  de- 
liver the  liquid  by  the  action  of  gravity.  Common  forms  of  pi- 
pettes are  shown  in  the  illustrations.  The  pipette  which  is  gradu- 
ated to  deliver  one  fixed  and  stated  amount  is  known  as  a 
"  transfer  pipette.''  On  account  of  the  smallness  of  the  bore  at 


No.16 
contains 
200  C.C.          II, 
20°C. 


FIG.  52. — Volumetric 
flask. 


170 


QUANTITATIVE  ANALYSIS 


the  point  where  the  mark  is  placed  the  pipette  may  be  made  to 
liquids  with  a  relatively  high   degree  of    accuracy, 
in  their  use  often  render  the  readings  highly 
These  are  chiefly  due  to  inconstancy  in  the  length 
of  the  period  of  draining.    The  pipette  must  be  quite 
A»am^  especially  with  regard  to  the  slight  film  of  oily 

period  of  time  must  be  allowed  for  draining  in  afl  cases 
where  a  given  instrument  is  used  and,  in  the  case  of 
pipettes,  a  uniform  procedure  must  be  followed  re- 
garding the  removal  of  the  last  drop  which  is  retained 
in  the  point  of  the  pipette.  The  approved  practice 
is  to  touch  the  point  of  the  instrument  against  the 


into  which  the  liquid  flows  but  not*  to  blow  out 
the  drop  that  is  still  retained. 

Burettes. — The  burette  finds  a  more  taJUaBu w  use 
than  any  other  form  of  volumetric  apparatus.  The 
flask  is  used  in  ™«lr™ihj»  standard  solution  often  in 


making  solutions  of  the  substance  being  analyzed 

flQmritimflgs in  subdividing thggg SPJntinmR.     Theburette 

is  used  in  each  determination  MM?  it  must  be  so 
graduated  as  to  measure  any  quantity  of  liquid  be- 
tween the  extreme  limits  of  its  graduations.  Its  con- 
struction, calibration  and  use  therefore  require  excep- 
tional care.  Its  bore  must  be  uniform,  its  graduations 
sharp,  distinct  and  correctly  placed.  It 


freely  and  uniformly  and  it  should  be  provided  with 
a  cock  of  proper  construction  to  permit  easy  control 
of  the  outflow.  In  reading  burettes  considerable 
jmiiM*tjiiiPs  result  ifUMii  TMj*?>Ji?t-T'  "jTiiy  is  be— 
the  part  of  the  surface  of  the  liquid  which  is 
observed  is  the  lowest  point  of  the  meniscus 
this  point  is  in  the  center  of  the  bore.  If,  there- 
fore, the  eye  knot  in  the  same  horizontal  plane  with  this  pointer^ 
the  burette  is  not  in  a  vertical  position  the  fine  upon  the  exterior 
of  the  burette,  apparently  marking  the  position  of  the  meniscus, 
does  not  ityuacnt  the  correct  reading.  This  is  made  evident 
from  Fig.  34  in  which  the  errors  are  purposely  exaggerated.  In 
otder  to  increase  the  accuracy  of  reading  and  to  prevent  parallax, 


VOLUMETRIC  ANALYSIS 


171 


various  devices  are  used.  In  the  Schellbach  burette,  Fig.  55,  a 
background  of  white  glass  bears  a  stripe  of  blue.  The  meniscus 
appears  against  this  as  a  point.  This  improvement  is  of  doubtful 
value  except  in  rooms  where  the  light  is  not  good,  because  it 
does  not  prevent  parallax.  The  use  of  floats,  Fig.  56,  sometimes 
renders  readings  more  easily  made,  especially  when  the  liquid  is 
so  dark  in  color  as  to  be  nearly  opaque.  The  mark  on  the  float 
is  brought  so  near  to  the  side  of  the  burette  that  parallax  is  also 
largely  prevented.  Trouble  due  to  sticking  of  the  float  is  suf- 
ficient cause  for  dispensing  with  its  use  whenever  possible.  The 


FIG.  54.— Effect  of  Parallax. 


Fro.  55. — Meniscus  as  seen  in 
SdMflbaea  boratt*. 


best  construction  of  burettes  yet  devised  is  that  specified  by  the 
U.  S.  Bureau  of  Standards.1  Upon  burettes  made  according  to 
these  specifications  the  marks  for  whole  cubic  centimeters  extend 
entirely  around  the  burette  while  those  for  subdivisions  extend 
half  way  around.  This  arrangement  absolutely  obviates  the 
troubles  of  parallax  and  makes  quite  sharp  readings  possible. 
Certain  devices  are  sometimes  used  for  promoting  rapid  fin- 
ing of  the  burette.  Fig  58  illustrates  some  of  these.  To 
set  up  such  a  burette  and  keep  it  in  working  order  requires 
a  certain  amount  of  attention  and  such  devices  are  of  value 
chiefly  in  works  laboratories  where  large  numbers  of  routine 
determinations  are  to  be  made  by  means  of  the  same  standard 

*  Bur.  Stand.,  Che.  No.  9. 


172 


QUANTITATIVE  ANALYSIS 


solution.  The  burette  with  a  plain  glass  cock  is  most  serviceable 
for  ordinary  use.  All  burettes  should  be  covered  by  a  cap  when 
in  use.  This  excludes  dust  and  lessens  evaporation  of  the 
solution. 

Units  of  Volume. — For  the  requirements  of  volumetric  analysis 
the  same  accuracy  may  be  obtained  without  regard  to  the  par- 
ticular unit  of  volume  adopted,  provided  that  all  of  the  different 
pieces  are  calibrated  upon  the  basis  of  the  same  unit.     The  liter  is 
defined  by  the  International  Bureau  of  Weights 
(°)         /*^\    and  Measures  to  be  the  volume  occupied  at 
)  ^        I — W    4°  C.  by  water   having  a  mass  of  1  kg.     This 
is  almost  exactly  1000  cc  and  for  all  practical 
purposes    may    be    regarded    as    such.      (The 
milliliter  is  1.000029  cc.)     It  is  now  customary 
to  use  this  true  liter  as  the  standard,  calibrat- 
ing apparatus  upon  this  basis,  the  apparatus 
to  be  used  at  the  average  room  temperature. 
In  America  the  working  temperature  is  usually 
taken  to  be  20°.     Other  temperatures  are  used 
as    standard    working  temperatures,    particu- 
larly abroad  where  17.5°,  15.5°  or  15°  is  used 
as  a  calibration  and  working  temperature. 

When  the  true  liter  is  made  the  basis  of  cali- 
bration and  higher  temperatures  than  4°  are  used  for  the  experi- 
mental part  of  the  calibration  corrections  must  be  made  for  the 
difference  in  density  of  water  used  for  calibrating  and,  if  the 
water  is  weighed,  also  for  the  buoyant  effect  of  air.  In  order 
to  avoid  making  such  corrections  Mohr1  suggested  a  different 
unit,  the  "Mohr  liter,"  which  is  defined  to  be  the  volume  of 
1000  gm  of  water  weighed  in  air  at  a  standard  pressure  of  760 
mm  of  mercury  and  at  a  temperature  of  17.5°. 

Absolute  or  Relative  Capacities. — In  the  discussion  of  the 
calibration  of  weights  (page  66)  it  was  stated  that  true  gram 
values  were  not  required,  since  accurately  known  relative  values 
of  the  various  pieces  are  sufficient  for  analytical  values,  analytical 
results  being  always  stated  in  some  sort  of  ratio  of  the  con- 
stituent determined  to  the  material  analyzed.  For  a  similar 

1Lehrbuch,  Chemisch-analytischen  Titriermethoden,  6th  ed.  (Rev.  by 
A.  Classen),  42. 


FIG.  56. — Burette 
floats. 


VOLUMETRIC  ANALYSIS 


173 


reason  the  various  pieces  of  volumetric  apparatus  may  be 
graduated  upon  any  desired  basis,  true  liter  or  otherwise,  pro- 
vided only  that  the  different  pieces  shall  have  correctly  indicated 
relative  capacities.  However,  it  is  very  desirable  that  all  pieces 
that  are  to  be  used  in  a  given  laboratory  shall  be  interchangeable 


FIG.  57. — Form  of  burette  approved 
by  the  Bureau  of  Standards. 


FIG.  58. — Burette  with  automatic  fill- 
ing and  overflow  devices. 


and  the  adoption  of  a  common  standard  for  all  workers  in  a 
laboratory  is  a  practical  necessity.  In  such  a  case  calibration 
to  a  basis  of  the  true  liter,  using  weights  that  are  calibrated  in 
true  gram  values,  is  the  logical  procedure. 

Tolerance. — Certain  experimental  errors  in  the  graduation  of 
volumetric  apparatus  may  be  regarded  as  reasonable  errors,  on 
account  of  which  the  apparatus  should  not  be  rejected.  This 


174  QUANTITATIVE  ANALYSIS 

does  not  mean  that  corrections  should  not  be  made  after  the 
results  of  calibration  are  known.  Maximum  permissible  errors 
in  graduation  are  known  as  " tolerances"  and  if  the  tolerance 
is  exceeded  in  a  given  case  the  piece  should  not  be  used  without 
regraduation. 

The  following  quotation  from  the  bulletin  of  the  U.  S.  Bureau 
of  Standards  already  referred  to  gives  the  requirements  of  that 
bureau  for  volumetric  flasks,  burettes  and  pipettes  to  be  accepted 
for  testing.  These  requirements  are  recognized  as,  at  once, 
rigid  and  scientific.  Apparatus  used,  even  by  the  student 
beginning  the  study  of  volumetric  analysis,  should,  whenever 
possible,  conform  to  these  requirements. 

GENERAL  SPECIFICATIONS 

"  (a)  Units  of  Capacity. — The  liter,  defined  as  the  volume  occupied  by 
a  quantity  of  pure  water  at  4°  C.  having  a  mass  of  1  kg,  and  the  one- 
thousandth  part  of  the  liter,  called  the  milliliter  or  cubic  centimeter,  are 
employed  as  units  of  capacity. 

"  (b)  Standard  Temperature. — 20°  C.  is  regarded  by  the  bureau  as  the 
standard  temperature  for  glass  volumetric  apparatus. 

"  (c)  Material  and  Annealing. — The  material  should  be  of  best  quality 
of  glass,  transparent  and  free  from  striae,  which  adequately  resists  chem- 
ical action,  and  has  small  thermal  hysteresis.  All  apparatus  should  be 
thoroughly  annealed  at  400°  C.  for  24  hours  and  allowed  to  cool  slowly 
before  being  graduated. 

"(d)  Design  and  Workmanship. — The  cross  section  must  be  circular 
and  the  shape  must  permit  of  complete  emptying  and  drainage. 

"Instruments  having  a  base  or  foot  must  stand  solidly  on  a  level  sur- 
face, and  the  base  must  be  of  such  size  that  the  instruments  will  stand 
on  a  plane  inclined  at  15°.  Stoppers  and  stopcocks  must  be  so  ground 
as  to  work  easily  and  prevent  leakage. 

"The  parts  on  which  graduations  are  placed  must  be  cylindrical  for  at 
least  1  cm  on  each  side  of  every  mark,  but  elsewhere  may  be  enlarged  to 
secure  the  desired  capacities  in  convenient  lengths. 

"The  graduations  should  be  of  uniform  width,  continuous  and  finely 
but  distinctly  etched,  and  must  be  perpendicular  to  the  axis  of  the  appa- 
ratus. All  graduations  must  extend  at  least  halfway  around,  and  on 
subdivided  apparatus  every  tenth  mark,  and  on  undivided  apparatus 
all  marks  must  extend  completely  around  the  circumference. 

"The  space  between  two  adjacent  marks  must  not  be  less  than  1  mm. 
The  spacing  of  marks  on  completely  subdivided  apparatus  must  show 


VOLUMETRIC  ANALYSIS 


175 


no  evident  irregularities,  and  sufficient  divisions  must  be  numbered  to 
readily  indicate  the  intended  capacity  of  any  interval.  Apparatus 
which  is  manifestly  fragile  or  otherwise  defective  in  construction  will 
not  be  accepted. 

"  (e)  Inscriptions. — Every  instrument  must  bear  in  permanent  legible 
characters  the  capacity  in  liters  or  cubic  centimeters,  the  temperature 
in  Centigrade  degrees  at  which  it  is  to  be  used,  the  method  of  use,  i.e., 
whether  to  contain  or  to  deliver,  and  on  instruments  which  deliver 
through  an  outflow  nozzle  the  time  required  to  empty  the  total  nominal 
capacity  with  unrestricted  outflow  must  be  likewise  indicated. 

"Every  instrument  should  bear  the  name  or  trade-mark  of  the  maker. 
Every  instrument  must  bear  a  permanent  identification  number,  and 
detachable  parts,  such  as  stoppers,  stopcocks,  etc.,  belonging  thereto, 
must  bear  the  same  number. 

SPECIAL  REQUIREMENTS 

"(a)  Flasks. — At  the  capacity  mark  or  marks  on  a  flask  the  inside 
diameter  should  be  within  the  following  limits : 


Capacity  of  flask  (in  cc)  up  to  and 
including 

2000 

1  000 

500 

250 

200 

100 

50 

?5 

Maximum  diameter  (in  mm)  
Minimum  diameter  (in  mm)  

25 

18 

20 

14 

18 
12 

15 
10 

13 
9 

12 

8 

10 
6 

8 
6 

"The  neck  of  a  flask  must  not  be  contracted  above  the  graduation 
mark. 

"The  capacity  mark  on  any  flask  must  not  be  nearer  the  end  of  the 
cylindrical  portion  of  the  neck  than  specified  below: 


Capacity 

Distance  from  upper 
end,  cm 

Distance  from  lower 
end,  cm 

100  cc  or  less. 

3 

1 

More  than  100  cc  

6 

2 

"Flasks  of  1  liter  or  more  but  not  less  may  be  graduated  both  to 
contain  and  to  deliver,  provided  the  intention  of  the  different  marks 
is  clearly  indicated. 

"(6)  Transfer  Pipettes. — Pipettes  for  delivering  a  single  volume  are 
designated  "transfer"  pipettes. 

"The  suction  tube  of  each  transfer  pipette  must  be  at  least  16  cm 
long,  and  the  delivery  tube  must  not  be  less  than  3  cm  nor  more  than 
25  cm  long. 

"The  inside  diameter  of  any  transfer  pipette  at  the  capacity  mark 
must  not  be  less  than  2  mm  and  must  not  exceed  the  following  limits : 


176 


QUANTITATIVE  ANALYSIS 


Capacity  of  pipettes  (in  cc)  up  to  and  including .... 
Diameter  (in  mm)..  . 


25 

4 


50 
5 


200 

6 


"The  outside  diameter  of  the  suction  and  delivery  tubes  of  transfer 
pipettes  exclusive  of  the  tip  must  not  be  less  than  5  mm. 

"The  capacity  mark  on  transfer  pipettes  must  not  be  more  than  6 
cm  from  the  bulb. 

"The  outlet  of  any  transfer  pipette  must  be  of  such  size  that  the 
free  outflow  shall  last  not  more  than  one  minute  and  not  less  than  the 
following  for  the  respective  sizes : 


Capacity  (hi  cc)  up  to  and  including  
Outflow  time  (in  seconds)  

5 
15 

10 
20 

50 
30 

100 
40 

200 
50 

"  (c)  Burettes  and  Measuring  Pipettes. — Only  those  emptying  through 
a  nozzle  permanently  attached  at  the  bottom  are  accepted  for  test. 

"So-called  "Shellbach"  burettes — that  is,  those  having  a  milk-glass 
background  with  a  colored  center  line — will  not  be  accepted  for  test. 

"The  distance  between  the  extreme  graduations  must  not  exceed 
65  cm  on  burettes  nor  35  cm  on  measuring  pipettes. 

"The  rate  of  outflow  of  burettes  and  measuring  pipettes  must  be 
restricted  by  the  size  of  the  tip  and  for  any  graduated  interval  the 
time  of  free  outflow  must  not  be  more  than  three  minutes  nor  less  than 
the  following  for  the  respective  lengths: 


Length 
graduated,  cm 

Time    of 
outflow,  sec 

Length 
graduated,  cm 

Time   of 
outflow,  sec 

70 

160 

35 

60 

65 

140 

30 

50 

60 

120 

25 

40 

55 

105 

20 

35 

50 

90 

15 

30 

45 

80 

40 

70 

"The  upper  end  of  any  measuring  pipette  must  be  not  less  than  10  cm 
from  the  uppermost  mark  and  the  lower  end  not  less  than  4  cm  from 
the  lowest  mark. 

"On  a  burette  the  highest  graduation  mark  should  not  be  less  than 
5  cm  nor  more  than  15  cm  from  the  upper  end  of  the  burette. 

"(d)  Burette  and  Pipette  Tips.— Burette  and  pipette  tips  should  be 
made  with  a  gradual  taper  of  from  2  cm  to  3  cm,  the  taper  at  the  extreme 
end  being  slight. 

"A  sudden  contraction  at  the  orifice  is  not  permitted  and  the  tip 
must  be  well  finished. 


VOLUMETRIC  ANALYSIS 


177 


"In  order  to  facilitate  the  removal  of  drops  and  to  avoid  splashing 
when  the  instrument  is  vertical,  the  tip  should  be  bent  slightly. 

"The  approved  form  of  tips  for  burettes,  measuring  pipettes,  and 
transfer  pipettes  is  shown  in  Fig.  59. 

"Special  Rules  for  Manipulation. — These  rules  indicate  the  essential 
points  in  the  manipulation  of  volumetric  apparatus  which  must  be 
observed  in  order  that  the  conditions  necessary  to  obtain  accurate 
measurements  may  be  reproduced. 

"(a)  Test  Liquid. — Apparatus  will  be  tested  with  water  and  the 
capacity  determined  will,  therefore,  be  the  volume  of  water  contained 
or  delivered  by  an  instrument  at  its  standard  temperature. 


FIG.  59. — Tips  of  burette  and  pipettes,  approved  by  the  Bureau  of  Standards. 


"(6)  Method  of  Reading. — In  all  apparatus  where  the  volume  is 
limited  by  a  meniscus  the  reading  or  setting  is  made  on  the  lowest  point 
of  the  meniscus.  In  order  that  the  lowest  point  may  be  observed 
it  is  necessary  to  place  a  shade  of  some  dark  material  immediately 
below  the  meniscus,  which  renders  the  profile  of  the  meniscus  dark 
and  clearly  visible  against  a  light  background.  A  convenient  device 
for  this  purpose  is  a  collar-shaped  section  of  thick  black  rubber  tubing, 
cut  open  at  one  side  and  of  such  size  as  to  clasp  the  tube  firmly. 

"  (c)  Cleanliness  of  Apparatus. — Apparatus  must  be  sufficiently  clean 
to  permit  uniform  wetting  of  the  surface. 

"(d)  Flasks  and  Cylinders. — In  filling  flasks  and  cylinders  the  entire 
interior  of  the  vessel  will  be  wetted,  but  allowed  a  sufficient  time  to 
12 


178 


QUANTITATIVE  ANALYSIS 


drain  before  reading.     Before  completely  filling  to  the  capacity  mark 
flasks  should  be  well  shaken  to  completely  mix  the  contents. 

"Flasks  and  cylinders  when  used  to  deliver  should  be  emptied  by 
gradually  inclining  them  until  when  the  continuous  stream  has  ceased 
they  are  nearly  vertical.  After  half  a  minute  in  this  position  the  mouth 
is  brought  in  contact  with  the  wet  surface  of  the  receiving  vessel  to 
remove  the  adhering  drop. 

"(e)  Pipettes  and  Burettes. — In  filling  pipettes  and  burettes  excess 
liquid  adhering  to  the  tip  should  be  removed  when  completing  the 
filling. 

"In  emptying  pipettes  and  burettes  they  should  be  held  in  a  vertical 
position,  and  after  the  continuous  unrestricted  outflow  ceases  the  tip 
should  be  touched  with  the  wet  surface  of  the  receiving  vessel  to  com- 
plete the  emptying. 

"Stopcocks,  when  used,  should  be  completely  open  during  emptying. 

"Burettes  should  be  filled  nearly  to  the  top,  and  the  setting  to  the  zero 
mark  made  by  slowly  emptying. 

"  While  under  normal  usage  the  measurements  ordinarily  are  from  the 
zero  mark,  other  initial  points  may  be  used  on  burettes  of  standard 
form  without  serious  error. 

"Tolerances. 

(a)    Flasks 


Capacity    (in   cc)   less   than 
and  including 

Limit  of  error,  cc 

If  to  contain         |         If   to   deliver 

25 

0.03 

0.05 

50 

0.05 

0..10 

100 

0.08 

0.15 

200 

0.10 

0.20 

300 

0.12 

0.25 

500 

0.15 

0.30 

1,000 

0.30 

0.50 

2,000 

0.50 

1.00 

(6)   Transfer  pipettes 


Capacity 

(in  cc)  less  than  and  including                 Limit  of  error,  cc 

2 
5 
10 
30 
50 
100 
200 

0.006 
0.01 
0.02 
0.03 
0.05 
0.08 
0.10 

VOLUMETRIC  ANALYSIS 


179 


(c)  Burettes  and  measuring  pipettes 


Capacity  (in  cc)  of  total 
graduated  portion  less 
than  and  including 

Limit  of  error   (in  cc)   of  total  or    partial 
capacity 

Burettes             |    Measuring  pipettes 

2 

• 

0.01 

5 

0.01 

0.02 

10 

0.02 

0.03 

30 

0.03 

0.05 

50 

0.05 

0.08 

100 

0.10 

0.15 

"  Further,  the  error  of  the  indicated  capacity  of  any  ten  consecutive 
subdivisions  must  not  exceed  one-fourth  the  capacity  of  the  smallest 
subdivision." 

Calibration. — For  accurate  work  the  apparatus  as  supplied 
by  the  makers  should  never  be  regarded  as  correctly  graduated 
until  it  has  been  tested  (calibrated)  by  the  user.  Some  manu- 
facturers use  great  care  in  graduating,  especially  with  such  ap- 
paratus as  must  pass  inspection  by  one  of  the  national  standardiz- 
ing bureaus.  Others  are  less  careful  and  pieces  are  often  found  to 
have  large  errors  in  their  graduation.  Two  general  methods  are 
in  use  for  calibrating  instruments  for  capacity.  In  the  first 
the  quantity  of  pure  water  or  other  liquid  of  known  density 
which  would  exactly  occupy  the  desired  volume  at  the  stated 
temperature  is  measured  by  weighing.  The  position  of  the 
meniscus  is  then  compared  with  the  mark  upon  the  apparatus. 
If  the  latter  was  previously  unmarked  the  position  of  the  menis- 
cus is  then  marked.  In  the  second  method  the  capacity  of  the 
instrument  is  determined  by  allowing  water  or  another  liquid 
to  flow  into  it  from  a  previously  standardized  piece  and  the 
capacities  are  compared.  In  the  first  method  temperatures 
must  be  accurately  noted  and  corrections  made  for  any  departure 
from  the  standard  temperature,  also  for  air  displacement.  In 
the  second  method  such  corrections  have  already  been  made 
when  the  standard  piece  was  calibrated  and  we  have  merely  a 
comparison  to  make  of  two  instruments  of  capacity.  The  second 
method  is,  therefore,  shorter  in  point  of  time.  Errors  may  occur 
if  proper  attention  is  not  given  to  certain  details  of  manipulation. 

Calibration  by  Weighing. — Water  is  the  most  conveniently 
used  liquid  for  this  purpose.  Since  water  solutions  are  generally 


180  QUANTITATIVE  ANALYSIS 

to  be  used  in  volumetric  analysis  water  possesses  a  second 
advantage  in  that  the  form  of  meniscus  is  most  nearly  that  of  the 
solutions  later  to  be  measured.  The  problem  in  calibrating, 
in  the  case  of  flasks  or  other  pieces  having  but  one  or  two  marks 
and  therefore  easily  remarked,  is  to  determine  the  correct  posi- 
tion of  the  mark  when  the  piece  contains  the  rated  quantity  of 
liquid.  For  this  reason,  in  laboratories  where  all  of  the  apparatus 
purchased  is  regularly  calibrated  it  is  best  and  cheapest  to 
purchase  flasks  unmarked,  though  conforming  to  certain  specifi- 
cations as  to  dimensions  and  shape. 

If  the  true  liter  is  to  be  taken  as  a  basis  it  will  first  be  necessary 
to  determine  the  apparent  weight  of  this  volume  of  water  in  air, 
correcting  also  for  the  expansion  due  to  the  difference  in  tempera- 
ture between  that  at  which  the  apparatus  is  calibrated  and  used 
and  the  temperature  upon  the  basis  of  which  the  liter  is  defined 
(4°).  From  the  table  on  page  181  it  is  seen  that  the  density  of 
water  at  20°  working  temperature  is  0.99823. 

One  liter  of  water  would  therefore  weigh  998.23  gm  in  a 
vacuum.  In  air  both  water  and  weights  are  apparently  lighter 
than  in  a  vacuum.  If  the  density  of  these  were  equal  the  effect  of 
air  upon  them  would  be  equal  and  no  error  would  be  introduced. 
Since  the  density  of  the  weights  is  greater  and  their  volume  less, 
the  buoyant  effect  is  greater  upon  the  water  and  the  apparent 
weight  of  the  water  in  air  is  less  than  the  true  weight.  One  liter 
of  air,  at  760  mm  pressure  and  at  20°  and  having  a  humidity 
of  50  percent,  weighs  1.19  gm.  This  is  the  buoyant  effect  upon 
the  water.  Analytical  weights,  whether  plated  or  not,  are 
usually  constructed  of  brass.  The  density  of  brass  may  be  taken 
as  8.4.  The  weight  of  air  displaced  by  998.23  gm  of  brass  weights 
is  then 

0.99823  X  1.19 

-£4-     "  =  0.14gm. 

This  is  the  buoyant  effect  upon  the  weights.  The  difference 
1.19  —  0.14  =  1.05  gm  is  the  apparent  loss  in  weight  of  the  liter 
of  water  when  weighed  in  air.  998.23-1.05  =  997.18.  There- 
fore one  liter  of  water  at  20°  and  in  air  apparently  weighs  997.18 
gm.  The  weight  of  fractions  of  a  liter  will  be  calculated  from 
this. 


VOLUMETRIC  ANALYSIS 


181 


EXPANSION  OF  WATER  ACCORDING  TO  P.   CHAPPUIS' 

Density  of  pure  water  free  from  air,  by  tenths  of  degrees  from  0°  to  40°  and  under  standard 

pressure 


Standard 
degrees 

Tenths  of  degrees 

Mean 
differ- 
ences 

0 

1 

2     3 

4 

5  |   6     7  |  8  |   9 

0 

0 

0.999  8681 

8747 

8812 

8875 

8936 

8996 

9053 

9109 

9163 

9216 

+  59 

1 

9267 

9315 

9363 

9408 

9452 

9494 

9534 

9573 

9610 

9645 

+  41 

2 

9679 

9711 

9741 

9769 

9796 

9821 

9844 

9866 

9887 

9905 

+  24 

3 

9922 

9937 

9951 

9962 

9973 

9981 

9988 

9994 

9998 

•0000 

+  8 

4 

1.000  0000 

*9999 

•9996 

*9992 

*9986 

*9979 

•9970 

*9960 

*9947 

*9934 

-   8 

5 

0.999  9919 

9902 

9884 

9864 

9842 

9819 

9795 

9769 

9742 

9713 

-  24 

6 

9682 

9650 

9617 

9582 

9545 

9507 

9468 

9427 

9385 

9341 

-  39 

7 

9296 

9249 

9201 

9151 

9100 

9048 

8994 

8938 

8881 

8823 

-  53 

8 

8764 

8703 

8641 

8577 

8512 

8445 

8377 

8308 

8237 

8165 

-  67 

9 

8091 

8017 

7940 

7863 

7784 

7704 

7622 

7539 

7455 

7369 

-  81 

10 

7282 

7194 

7105 

7014 

6921 

6826 

6729 

6632 

6533 

6432 

-  95 

11 

6331 

6228 

6124 

6020 

5913 

5805 

5696 

5586 

5474 

5362 

-108 

12 

5248 

5132 

5016 

4898 

4780 

4660 

4538 

4415 

4291 

4166 

-121 

13 

4040 

3912 

3784 

3654 

3523 

3391 

3257 

3122 

2986 

2850 

-133 

11 

2712 

2572 

2431 

2289 

2147 

2003 

1858 

1711 

1564 

1416 

-145 

13 

1266 

1114 

0962 

0809 

0655 

0499 

0343 

0185 

0026 

*9865 

-156 

16 

0.998  9705 

9542 

9378 

9214 

9048 

8881 

8713 

8544 

8373 

8202 

-168 

17 

8029 

7856 

7681 

7505 

7328 

7150 

6971 

6791 

6610 

6427 

-178 

13 

6244 

6058 

5873 

5686 

5498 

5309 

5119 

4927 

4735 

4541 

-190 

19 

4347 

4152 

3955 

3757 

3558 

3358 

3158 

2955 

2752 

2549 

-200 

20 

2343 

2137 

1930 

1722 

1511 

1301 

1090 

0878 

0663 

0449 

-211 

21 

0233 

0016 

*9799 

*9580 

*9359 

*9139 

*8917 

*8694 

*8470 

•8245 

-221 

22 

0.997  8019 

7792 

7564 

7335 

7104 

6873 

6641 

64C8 

6173 

5938 

-232 

23 

5702 

5466 

5227 

4988 

4747 

4506 

4264 

4021 

3777 

3531 

-242 

24 

3286 

3039 

2790 

2541 

2291 

2040 

1788 

1535 

1280 

1026 

-252 

25 

0770 

0513 

0255 

*9997 

*9736 

*9476 

*9214 

*8951 

*8688 

*8423 

-261 

26 

0.996  8158 

7892 

7624 

7356 

7087 

6817 

6545 

6273 

6000 

5726 

-271 

27 

5451 

5176 

4898 

4620 

4342 

4062 

3782 

3500 

3218 

2935 

-280 

28 

2652 

2366 

2080 

1793 

1505 

1217 

0928 

0637 

0346 

0053 

-289 

29 

0.995  9761 

9466 

9171 

8876 

8579 

8282 

7983 

7684 

7383 

7083 

-298 

30 

6780 

6478 

6174 

5869 

5564 

5258 

4950 

4642 

4334 

4024 

-307 

31 

3714 

3401 

3089 

2776 

2462 

2147 

1832 

1515 

1198 

0880 

-315 

32 

0561 

0241 

*9920 

*9599 

*9276 

*8954 

•8630 

*8304 

*7979 

*7653 

-324 

33 

0.994  7325 

6997 

6668 

6338 

6007 

5676 

5345 

5011 

4678 

4343 

-332 

34 

4007 

3671 

3335 

2997 

2659 

2318 

1978 

1638 

1296 

0953 

-340 

35 

0610 

0267 

*9922 

*9576 

*9230 

*8883 

*8534 

*8186 

•7837 

*7486 

-347 

36 

0.993  7136 

6784 

6432 

6078 

5725 

5369 

5014 

4658 

4301 

3943 

-355 

37 

3585 

3226 

2866 

2505 

2144 

1782 

1419 

1055 

0691 

0326 

-362 

38 

0.992  9960 

9593 

9227 

8859 

8490 

8120 

7751 

7380 

7008 

6636 

-370 

39 

6263 

5890 

5516 

5140 

4765 

4389 

4011 

3634 

3255 

2876 

-377 

40 

2497 

2116 

1734 

1352 

0971 

0587 

0203 

•98X8 

•9433 

*9t)47 

-384 

41 

0.991  8661 



1  Travaux  et  Memoires,  Bur.  Intern.  Poids  Mesures,  13  (1907.) 


182 


QUANTITATIVE  ANALYSIS 


Calibration   at   Other   Than   the    Standard   Temperature.— 

Carelessness  with  regard  to  the  temperature  of  the  water  used 
in  calibrating  may  lead  to  serious  errors.  The  apparent  weight 
of  one  liter  of  water  at  25°  is  996.04  gm  and  at  15°  it  is  998.05 
gm,  instead  of  997.18  gm.  This  gives  a  mean  variation  of 
±0.20  gm  for  each  degree  of  variation  from  the  standard  tem- 
perature of  20°,  within  the  limits  of  15°  and  25°.  This  error 
is  partly  compensated  by  the  change  in  the  actual  capacity 
of  the  glass  apparatus,  due  to  contraction  or  expansion  with 
change  in  temperature.  The  average  coefficient  of  cubical 
expansion  of  the  glass  used  for  volumetric  apparatus  is  0.000025 
for  each  degree  of  change  in  temperature.  The  correction  is, 
of  course,  similar  in  sign  to  the  temperature  variation  from 
20°,  while  the  correction  to  be  applied  to  the  required  weight  of 
water  is  opposite  in  sign  to  the  temperature  variation.  That  is, 
if  the  temperature  is  above  20°  a  smaller  weight  of  water  should 
be  taken  for  a  true  liter;  but  a  glass  vessel  is  actually  larger 
and  should  have,  from  this  standpoint  alone,  a  greater  weight 
of  water  to  indicate  a  mark  which  can  be  used  at  20°.  For 
this  reason  the  net  correction  represents  the  difference  between 
the  two. 

The  mean  correction  of  0.20  gm  in  the  apparent  weight  of  a 
liter  of  water  is  not  sufficiently  accurate  for  a  range  of  more  than 
a  degree  or  two  variation.  The  following  table  gives  the  apparent 
weight  of  a  liter  of  water  for  various  temperatures  between  15° 


Temperature,  degrees 

Apparent  weight  of  1  1 
of  water  in  air,  grams 

Weight  to  be  taken 
for  calibration 

15 

998.05 

997.93 

16 

997.90 

997.80 

17 

997.74 

997.66 

18 

997.56 

997.51 

19 

997.38 

997.36 

20 

997.18 

997.18 

•  21 

996.97 

996.99 

22 

996.76 

996.81 

23 

996.53 

996.61 

24 

996.29 

996.39 

25 

996.04 

996.16 

VOLUMETRIC  ANALYSIS 


183 


FIQ.  60. — Calibrating  device  of  the  Bureau  of  Standards. 


184 


QUANTITATIVE  ANALYSIS 


and  25°,  also  the  corrected  weight  of  water  to  be  taken  for  cali- 
brating flasks  that  are  to  be  used  at  20°.  The  latter  weight 
includes  the  correction  for  expansion  and  contraction  of  glass. 
This  table  is  to  be  used  only  in  case  it  becomes  difficult  to  main- 
tain the  laboratory  temperature  at  20°. 

Calibration  by  Standardized  Bulbs. — Any  bulb  or  tube  that  is 
to  be  used  as  a  comparison  standard  for  calibrating  by  the  second 


50 


FIG.  61. — Morse-Blalock  calibrating  bulbs. 

method  must  be  of  such  a  form  that  it  will  drain  well  upon 
emptying.  It  must  also  have  the  graduated  portion  small 
enough  to  make  possible  accurate  readings  at  this  part.  The 
apparatus  must  be  quite  rigid  so  that  varying  pressure  may  have 
only  an  inappreciable  effect  upon  the  volume.  The  apparatus 
shown  in  Fig.  60  is  that  used  by  the  Bureau  of  Standards. 


VOLUMETRIC  ANALYSIS  185 

In  Fig.  61  are  shown  the  standard  bulbs  devised  by  Morse 
and  Blalock.1  The  three  pieces  shown  provide  a  means  for 
calibrating  vessels  of  all  of  the  various  capacities  common  to 
the  analytical  laboratory,  if  proper  combinations  are  made. 
In  calibrating  these  bulbs  it  is  necessary  to  determine  the 
capacity  from  the  single  mark  to  the  first  stem  division,  also 
the  capacity  of  the  stem  for  the  smallest  subdivision.  If  the 


FIG.  62. — Morse-Blalock  bulb  arranged  for  calibrating  flasks. 

water  used  is  kept  at  the  standard  working  temperature  no 
correction  for  this  factor  need  be  introduced  and  the  value 
997.18  gm,  previously  deduced  as  the  apparent  weight  of  a 
liter  of  water,  may  be  used  without  change.  The  bulbs  are 
supported  in  such  a  manner  that  they  may  be  readily  filled  from 
a  reservoir  of  distilled  water  at  20°.  The  water  from  the  bulb 
is  carefully  weighed  and  its  volume  calculated.  That  from 
the  entire  graduated  portion  of  the  stem  is  then  weighed  in  a 
smaller  vessel  and  the  calculated  volume  is  divided  by  the  total 

1  Am.  Chem.  J.,  16,  479  (1894). 


186 


QUANTITATIVE  ANALYSIS 


number  of  stem  divisions  and  the  result  recorded  as  the  value 
of  one  division.  In  order  to  use  the  bulbs  for  the  calibration  of 
vessels  that  are  to  contain  a  specified  volume  of  liquid,  the 
vessel  to  be  calibrated  must  first  be  cleaned  as  hereafter  directed. 
The  proper  bulb  is  then  placed  in  such  a  position  that  it  may 
drain  directly  into  the  instrument  being  calibrated  and  the 
latter  is  marked  at  the  meniscus.  Instruments  to  be  calibrated 
to  deliver,  such  as  burettes,  are  better  calibrated  by  weighing 
the  water  delivered.  If,  however,  it  is  desired  to  use  the  standard 
bulbs  for  this  purpose  the  burette  is  so  connected  that  it  may 
empty  into  the  bulb  from  below.  The  details  of  manipulation 
will  be  made  clear  in  the  exercises  that  follow. 

Cleaning  Solution. — Prepare  a  cleaning  solution  by  dissolving  5  gm 
of  powdered  commercial  sodium  dichromate  in  500  cc  of  commercial 
sulphuric  acid.  The  solution  may  be  kept  in  a  bottle  having  a  wide 
mouth,  such  as  those  in  which  dry  chemicals  are  purchased.  Burettes 
may  be  inverted  and  left  standing  in  the  bottle  the  solution  then  being 
drawn  up  by  suction  and  held  in  the  burette  by  closing  the  cock.  For 
cleaning  flasks  the  solution  may  be  allowed  to  remain  in  the  flask  for  some 
time  or  a  small  amount  may  be  warmed  and  the  flask  rinsed  with  it. 
The  chromic  acid  produced  by  the  interaction  of  sulphuric  acid  and 
sodium  dichromate  oxidizes  all  organic  matter  and  leaves  the  glass 
thoroughly  free  from  it. 

Exercise:  Calibration  of  the  Standard  Bulbs. — Clean  the  bulb  and 
set  it  up  in  a  manner  similar  to  that  shown  in  Fig.  60.  Distilled  water 
must  be  used.  Place  the  bulb  so  that  the  graduated  stem  extends  down- 
ward. A  glass  stopcock  must  be  used  for  controlling  the  outflow,  since 
pinchcocks  with  rubber  connections  would  involve  an  uncertain  change 
in  volume  of  the  apparatus.  The  tip  of  the  outflow  tube  should  be 
contracted  to  restrict  the  outflow,  according  to  the  size  of  the  bulb,  as 
follows : 


Size  of  bulb,  cc 

Time  of  outflow,  sec 

50 
200 
500 

30 
50 

65 

Fill  the  bulb  to  the  upper  mark.  With  the  stopcock  wide  open  allow 
the  water  to  flow  into  a  previously  dried  and  weighed  flask  until  the 
first  division  (zero)  on  the  lower  stem  is  reached.  After  15  seconds 


VOLUMETRIC  ANALYSIS  187 

adjust  the  water  level  to  coincide  with  the  mark,  put  the  stopper  in  the 
flask  (a  glass  stopper  is  desirable)  and  weigh.  Place  a  weighing  bottle 
under  the  delivery  tube  and  allow  the  stem  to  drain  to  the  last  mark, 
stopper  the  bottle  and  weigh.  If  the  water  was  at  20°  divide  its  weight 
in  grams  by  0.99718  and  record  the' result  as  cubic  centimeters  capacity 
of  bulb  and  of  stem.  Record  also  the  value  of  each  stem  division  and  the 
division  to  be  used  as  the  mark  for  the  rated  capacity  of  the  bulb.  If 
the  temperature  was  not  20°  determine  from  the  table  on  page  182  the 
weight  of  water  that  should  be  used  at  the  observed  temperature 
for  calibrating  bulbs  to  be  used  at  20°. 

In  the  preceding  exercise  as  in  those  that  follow  the  bulb 
and  reservoir  may  also  be  set  up  as  in  Fig.  62.  The  chief 
objection  to  this  method  of  assembling  lies  in  the  fact  that 
the  water  entering  the  bulbs  must,  for  reasons  already  explained, 
pass  through  a  glass  stopcock,  which  is  necessarily  lubricated 
with  some  kind  of  grease.  The  result  is  that  no  matter  how 
well  the  bulb  may  have  been  previously  cleaned  it  acquires,  at 
the  first  filling,  a  film  of  oil  that  absolutely  prevents  the  proper 
draining  at  the  next  stage  in  the  experiment.  If  the  first  arrange- 
ment is  used  and  the  bulb  is  filled  from  above  a  rubber  connec- 
tion and  pinchcock  may  be  used  and  this  annoyance  avoided. 

Exercise :  Calibration  of  Flasks  by  the  Standard  Bulbs. — With  the 
proper  bulb  in  the  position  used  in  the  preceding  exercise,  and  with  the 
dried  flask  to  be  calibrated  placed  under  the  delivery  tube,  allow  water 
to  flow  into  the  flask  until  the  proper  mark  on  the  lower  stem  is  reached, 
exactly  following  the  directions  as  to  time  of  outflow  as  given  in  the  pre- 
ceding exercise.  Avoid  handling  either  bulb  or  flask  at  the  parts  con- 
taining the  water  as  the  temperature  is  thereby  raised.  It  has  already 
been  explained  that  after  the  bulbs  have  been  standardized  they  may  be 
used  for  further  calibrations  without  regard  to  the  temperature  of  the 
water  provided  only  that  the  temperature  does  not  change  during  the  progress 
of  the  experiment. 

To  mark  the  flask  cut  a  strip  of  gummed  label,  long  enough  to  reach 
around  the  neck  and  about  1/4  inch  wide.  Carefully  paste  this  with  the 
original  straight  edge  at  the  level  of  the  meniscus,  where  the  mark  is  to 
be  made.  Melt  a  small  quantity  of  paraffin  and  brush  a  thin  layer  over 
the  label  and  over  a  space  of  about  1  inch  on  either  side  of  it.  Using 
the  point  of  a  knife  or  of  a  sharpened  piece  of  wood  trace  the  straight  edge 
of  the  label  around  the  neck  of  the  flask,  making  a  mark  sufficiently  wide 
to  be  easily  visible.  The  label  here  merely  serves  as  a  guide,  making  a 


188  QUANTITATIVE  ANALYSIS 

regular  line  possible.  Using  a  small  feather  as  a  brush  apply  a  few  drops 
of  hydrofluoric  acid  .and  allow  this  to  remain  on  the  flask  for  two  or  three 
minutes,  after  which  the  acid  may  be  washed  off  and  the  paraffin 
removed  by  warming. 

In  case  the  flask  already  has  a  graduation  and  the  calibration  shows 
this  mark  to  be  incorrectly  placed  it  is  desirable  to  indicate  the  new 
mark  by  making  a  small,  well-defined  arrow  with  the  point  resting 
exactly  upon  the  new  mark.  The  operator's  initials  may  be  placed 
beside  the  arrrow  and  if  this  is  done  carefully,  no  interference  will 
result. 

If  the  flask  contains  no  inscription  etch  the  side  like  that  shown  in 
Fig.  52,  page  169. 

Exercise:  Calibration  of  Flasks  by  Weighing. — This  is,  in  many 
respects,  the  most  satisfactory  method  of  calibration  although  more 
time  is  required.  Have  the  flask  clean  and  quite  dry.  Place  on  a 
balance  of  capacity  sufficiently  great  to  carry  the  filled  flask.  Counter- 
poise, then  add  weights  to  the  right  pan  at  the  rate  of  997.18  gm  for  each 
liter.  Remove  the  flask  from  the  balance  and  fill  with  recently  boiled 
distilled  water  at  20°,  nearly  to  the  point  where  it  is  thought  that  the 
mark  will  be  placed.  Remove  drops  from  the  inside  of  the  neck,  above 
the  level  of  the  water,  using  a  roll  of  filter  paper.  Replace  the  flask 
upon  the  balance  pan,  then  carefully  drop  in  water  from  a  pipette  until 
the  balance  is  in  equilibrium.  Mark  as  directed  in  the  preceding 
exercise. 

In  both  of  these  exercises  the  flasks  have  been  calibrated  to 
contain  the  rated  quantity  and  this  is  the  only  way  in  which 
flasks  will  be  used  in  this  work. 

Exercise :  Calibration  of  Burettes  by  Weighing. — The  marking  of  a 
burette  is  too  complex  to  be  easily  changed  and  the  calibration  will 
therefore  consist  of  finding  what,  if  any,  corrections  must  be  applied 
to  the  existing  graduations. 

First  inspect  the  burette  to  determine  whether  it  conforms  to  specifi- 
cations, especially  with  respect  to  outflow  time.  Clean  the  burette  with 
cleaning  solution  and  distilled  water.  Fill  with  distilled  water  at  20°. 
Weigh  accurately  a  25  cc  weighing  bottle  to  the  third  decimal  then 
measure  5  cc  of  water  into  it  from  the  burette,  and  reweigh.  Add 
another  5  cc  and  weigh,  continuing  until  the  bottle  is  full.  Empty  the 
bottle,  reweigh  and  continue  the  process  until  the  water  from  the  entire 
graduated  portion  of  the  burette  has  been  weighed.  Repeat  the  process 
in  order  to  have  a  check  upon  the  work.  Calculate  the  true  capacity 
of  each  of  the  ten  portions,  using  the  weight  0.99718  gm  for  1  cc  of 


VOLUMETRIC  ANALYSIS  189 

water.     Record  as  follows,  the  capacities  in  the  last  two  columns  being 
recorded  only  as  far  as  the  second  decimal  place. 


Mark. 


Weight  of  water, 
each  interval. 


True  capacity,  each 
interval. 


True  total  capacity,  zero  to  e 
of  interval. 


BBtj 


Construct  a  curve  showing  the  true  reading  at  all  points.  In  case 
any  marked  irregularity  is  observed  at  any  part  of  the  burette  so  that 
corrections  taken  from  the  curve  would  be  inaccurate,  recalibrate  this 
portion,  using  1  cc  at  a  time. 

Exercise:  Calibration  of  Burettes  by  the  Standard  Bulbs. — Set  up 
the  apparatus  as  in  Fig.  63.  The  reservoir  must  be  higher  than  the 
top  of  the  burette  and  this,  in  turn,  must  be  placed  so  that  the  lowest 
graduation  is  higher  than  the  bulbs.  The  tubing  leading  from  the  reser- 
voir to  the  burette  may  be  of  well-cleaned  rubber.  That  between  the 
stopcock  a  and  the  burette  and  bulbs  respectively  must  be  of  glass,  the 
necessary  connections  being  of  heavy  rubber  tubing  with  the  glass 
tubes  pushed  together  until  they  touch  inside  the  rubber. 

With  the  three-way  cock  b  closed,  open  the  cock  a  and  fill  the  burette 
with  water.  -Close  a  and  open  b  so  that  the  2  cc  and  3  cc  bulbs  may 
fill,  then  drain  the  burette  to  the  zero  mark  and  the  bulbs  to  that  mark 
on  the  stem  of  the  2  cc  bulb  which  represents  exactly  2  cc.  This 
leaves  the  bulbs  moistened  as  they  will  be  throughout  the  experiment. 
Leave  the  burette  cock  open.  Turn  the  cock  b  and  measure  5  cc  of 
water  from  the  burette  into  the  small  bulbs.  Observe  the  position  of 
the  meniscus  upon  the  stem  of  the  3  cc  bulb  and  calculate  the  true 
capacity  of  the  first  portion  of  the  burette,  using  the  values  for  the  stem 
divisions  as  determined  in  the  calibration  of  the  bulbs.  Repeat  this 
process  for  the  other  nine  portions  of  the  burette  and  record  as  follows: 


Mark. 

Bulb  reading. 

True  capacity,  each 
interval. 

True  total  capacity,  zero  to  end 
of  interval. 

For  a  more  nearly  complete  calibration  the  burette  may  be  calibrated 
2  cc  at  a  time,  using  the  2  cc  bulb  alone  or  1  cc  or  any  fraction  at  a  time 
using  the  standard  tube,  Fig.  64.  Such. calibration  is  necessary  if  the 
bore  of  the  burette  is  found  to  be  very  irregular. 

Calibration  of  Pipettes. — Pipettes  which  are  graduated  in 
small  subdivisions  from  zero  to  full  capacity  ("  measuring  pi- 
pettes") are  calibrated  in  the  same  manner  as  are  burettes. 
Transfer  pipettes  are  best  calibrated  by  the  method  of  weighing. 

Exercise :  Calibration  of  Transfer  Pipettes. — Determine  whether  the 
time  of  outflow  conforms  to  the  requirements  as  set  forth  on  page  176. 


190 


QUANTITATIVE  ANALYSIS 


If  not  alter  the  tip  of  the  pipette  before  calibrating.  Provide  a  weigh- 
ing bottle  having  a  capacity  of  10  cc,  also  a  larger  one  having  a  capacity 
equal  to  that  of  the  pipette.  Cut  a  strip  of  paper  about  2  mm  wide 


FIG.  63. — Morse-Blalock  bulb  set  up  for  calibrating  burettes. 

and  5  cm  long  and  carefully  rule  this  in  divisions  of  centimeters,  mark- 
ing from  0  to  5,  and  subdivisions  of  millimeters,  using  fine  lines.  Deter- 
mine the  approximate  location  of  the  capacity  mark  on  the  pipette 
by  a  rough  experiment,  unless  the  pipette  is  already  marked.  Paste 


VOLUMETRIC  ANALYSIS 


191 


the  paper  strip  on  the  stem  of  the  pipette  with  the  division  2.5  at  the 
supposed  place  for  the  capacity  mark  and  with  the  zero  toward  the  point 
of  the  pipette.  Having  cleaned  the  pipette  with  chromic  acid  solution 
it  is  drawn  full  of  distilled  water  which  is  at  a  temper- 
ature of  20°,  and  the  water  is  allowed  to  flow  out 
until  the  zero  mark  is  exactly  reached.  The  pipette 
must  be  held  in  a  vertical  position  and  the  eye  must 
be  in  the  same  horizontal  plane  as  is  the  meniscus. 
The  pipette  tip  is  now  touched  against  the  side  of 
the  beaker  to  remove  the  last  drop.  The  finger  is 
then  removed  from  the  top  of  the  pipette  and  the 
water  is  allowed  to  flow,  at  full  speed,  into  the  larger 
weighing  bottle,  which  has  already  been  weighed. 
The  tip  is  immediately  touched  to  the  side  of  the 
weighing  bottle  to  remove  the  hanging  drop.  The 
weighing  bottle  is  ther  stoppered  and  weighed.  Cal- 
culate the  volume  of  the  water  from  the  observed 
weight  and  record  this  as  the  capacity  of  the  pipette 
to  the  zero  mark. 

Using  the  small  weighing  bottle  determine  in  a 
similar  manner  the  capacity  of  the  pipette  stem 
between  0  and  5.  Divide  this  capacity  by  50 
in  order  to  obtain  the  value  of  the  smaller  sub- 
divisions. 

From  the  capacities  so  determined  calculate  the 
number  of  stem  divisions  to  be  added  to  the  zero  in 
order  to  obtain  the  rated  capacity  of  the  pipette. 
Mark  the  point  so  determined,  using  the  method 
directed  for  marking  flasks. 

CALCULATION  OF  THE  RESULTS  OF 
VOLUMETRIC  ANALYSIS 

Although  the  first  exercises  in  volumetric 
analysis  will  necessarily  have  to  do  with  the 
making  of  solutions  and  with  their  standardiza- 
tion and  adjustment  to  desired  concentration, 
it  will  be  simpler  to  deal  first  with  the  calcula- 
tion of  the  results  of  the  analysis.  When  making  the  determi- 
nation of  silver  by  the  gravimetric  method,  a  definite  amount 
of  the  silver  compound  was  weighed,  dissolved  in  water  and  a 
slight  but  somewhat  indefinite  excess  of  hydrochloric  acid  was 


FIG.    64. — Morse 
Blalock  tube. 


192  QUANTITATIVE  ANALYSIS 

added,  thus  precipitating  all  of  the  silver  as  silver  chloride,  the 
following  reaction  taking  place: 

AgNOs+HCl-^AgCl+HNOt. 

The  silver  chloride,  representing  the  entire  amount  of  silver 
present  in  the  compound  of  unknown  composition,  was  then 
filtered,  washed,  dried,  and  weighed  and  from  this  observed 
weight  and  the  known  weight  relations  between  silver  chloride 
and  silver  the  percent  of  the  latter  was  calculated.  The  formula 
AgCl  expresses  the  fact  that  for  each  143.34  parts,  by  weight,  of 
silver  chloride,  there  was  involved  35.46  parts  of  chlorine  and 
107.88  parts  of  silver.  In  other  words,  in  any  given  weight  of 

,  .    .  ,      35.46      ,    .:.         .  ,  ^  .      ,  .    .  ,  107.88  . 

silver  chloride,  1/to  OA  of  this  weight  is  chlorine  and  1/)0  0,  is 
'  143.34  143.34 

silver.     If  the  weight  of  silver  chloride  found  in  the  analysis 

107  88 
is  multiplied  by  the  fraction  1  AQ  QA   and  the  result  divided  by 

14O.OT: 

the  weight  of  sample  taken,  the  quotient  will  be,  when  multiplied 
by  100,  the  percent  of  silver  in  the  sample.  If  W  =  the  weight 
of  silver  chloride  found,  and  S  =  the  weight  of  sample  taken, 
this  would  be  expressed  shortly  as  follows: 

107.88  W  X  100 

A  p  --  =  percent  of  silver  in  sample.  (I) 


Instead  of  adding  the  hydrochloric  acid  in  slight  but  indefinite 
excess  to  the  solution  of  the  silver  salt,  one  might  add  exactly 
the  amount  required  to  complete  the  reaction,  but  no  more. 
This  would  involve  the  use  of  some  method  for  determining 
when  the  reaction  is  exactly  completed  (the  "end  point  "),  such 
as  noting  when  another  drop  of  hydrochloric  acid  solution  fails 
to  produce  any  further  precipitation  of  silver  chloride.  Suppose, 
also,  that  the  concentration  of  the  hydrochloric  acid  solution,  in 
grams  per  cubic  centimeter,  were  very  accurately  known.  We 
should  then  have  the  following  data: 

(a)  Weight  of  silver  salt  taken, 

(6)  Volume  of  hydrochloric  acid  required  to  react  with  silver, 

(c)   Concentration  of  hydrochloric  acid. 

Just  as  the  formula  for  silver  chloride  expresses  the  weight 
relations  between  silver,  chlorine  and  silver  chloride,  so  the 


VOLUMETRIC  ANALYSIS  193 

equation  for  the  reaction  between  silver  salt  and  hydrochloric 
acid  expresses  the  weight  relations  between  all  of  the  elements 
and  compounds  involved.  We  are  here  particularly  concerned 
with  the  relations  between  silver  and  hydrochloric  acid,  and  we 
note  that  for  every  107.88  parts,  by  weight,  of  silver,  we  require 
36.468  parts  of  hydrochloric  acid  for  complete  precipitation  of 
the  silver  as  silver  chloride.  Conversely,  if  the  reaction  has  been 
exactly  completed,  for  every  36.468  parts  of  hydrochloric  acid 
used  there  will  have  been  present  107.88  parts  of  silver.  The 
weight  of  pure  hydrochloric  acid  used  is  found  by  multiplying 
the  number  of  cubic  centimeters  by  the  concentration  in  grams 
per  cubic  centimeter,  i.e., 

VC  =  Wt.  HC1  used, 

where  F  =  cc   of  acid   solution   used   and   Cf  =  gm  hydrochloric 

107  88 
acid  in  1  cc.     If  this  weight  is  multiplied  by  the  fraction  QA 

ob .  4o 

the  result  will  be  the  weight  of  silver  in  the  sample.  Expressed 
briefly: 

107.887CX100 


36.46S 


percent  silver.  (II) 


This  is  the  most  general  expression  for  the  calculation  of  the 
results  of  a  volumetric  analysis.  A  comparison  of  expressions 
(I)  and  (II)  will  show  that  the  volumetric  calculation  differs 
from  the  gravimetric  calculation  in  two  respects  only:  (1)  A 
weight  of  a  substance  is  obtained  indirectly  by  measuring  the 
volume  of  a  solution  of  known  concentration,  instead  of  directly 
by  weighing  the  substance.  (2)  The  substance  whose  weight  is 
desired  is  one  which  reacts  with  the  substance  being  determined 
instead  of  one  which  is  produced  by  this  substance.  The  gravi- 
metric factor  for  the  ratio  of  the  weight  of  one  substance  to  the 
weight  of  another  substance  which  contains  the  first  becomes  the 
volumetric  factor  for  the  ratio  of  the  weight  of  one  substance  to 
the  chemically  equivalent  weight  of  another  substance  which 
does  not  contain  the  first. 

The  substance  which  is  a  visible  indication  of  the  end  point  of 
the  reaction  is  called  the  "  indicator."  Indicators  will  be 
discussed  at  length  in  a  later  section.  The  solution  whose  con- 
is 


194  QUANTITATIVE  ANALYSIS 

centration  is  accurately  known  and  of  which  we  measure  the 
volume  required,  is  called  a  " standard  solution"  because  it  is 
actually  a  standard  by  which  the  quantity  of  the  substance  under 
investigation  is  measured.  The  process  of  running  in  the  stand- 
ard solution  until  the  end  point  is  reached  is  called  "titration." 

The  examples  given  below  will  serve  to  illustrate  the  principles 
above  outlined: 

1.  0.5436  gm  of  a  silver  salt  was  dissolved  and  titrated  by  a 
standard  solution  of  hydrochloric  acid,  1  cc  of  which  contained 
0.00304  gm  of  the  pure  acid.     27.2  cc  of  the  standard  was  used. 
Required,  the  percent  of  silver  in  the  salt. 

27.2  X  0.00304  =  gm  of  pure  acid  used; 
107  88 
Sfi  46  ^£m  °^  hydrochloric  acid  =  gm  of  silver  present. 

107.88X27.2X0.00304X100 

Therefore  36.46X0.5436  =  45-°° =  percent  Sl1' 

ver  in  the  original  sample  of  silver  salt. 

Use  of  Aliquot  Parts. — It  often  happens  that,  in  order  to  elimi- 
nate the  error  due  to  the  lack  of  uniformity  of  a  sample  being 
analyzed,  or  for  reasons  of  convenience,  a  larger  quantity  than  is 
necessary  for  the  titration  is  weighed  and  this  dissolved  in  a 
definite  quantity  of  solvent  and  an  aliquot  part  taken  for  the 
titration.  In  such  a  case  the  final  calculation  must  include  an 
expression  of  this  fact.  Thus  in  Example  (1)  instead  of  weighing 
0.5436  gm  of  the  silver  salt,  suppose  that  10.8720  gm  was 
weighed,  dissolved,  and  the  solution  diluted  to  1000  cc,  50  cc 
being  then  titrated.  The  statement  would  then  be 
27.2X0.00304X107.88X20X100 

10.8720X36.46  =  percent  Sllver  m 

2.  In  a  sample  of  undried,  but  otherwise  pure,  sodium  hydrox- 
ide, the  percent  of  the  base  was  to  be  determined.     For  this 
purpose  5.5310  gm  of  the  sample  was  weighed  and  dissolved 
and  diluted  to  250  cc.     Portions  of  25  cc  each  were  measured 
and  titrated  by  a  standard  solution  of  hydrochloric  acid,  the 
average  amount  of  acid  solution  required  for  complete  neutrali- 
zation  being   45.1    cc;    1    cc   of   the  standard  acid  contained 
0.00960  gm  of  the  pure  acid.     Required,  the  percent  of  sodium 
hydroxide.  "  The  solution  of  the  problem  is  as  follows: 


VOLUMETRIC  ANALYSIS  195 

45. 1X0. 0096  =  gm  of  hydrochloric  acid  used; 
'     '   .„  Xgm   of   hydrochloric   acid  =  gm   of   sodium   hydroxide 

in  25  cc  of  solution. 

250 

-7^-Xgm  of  sodium  hydroxide  =  gm  in  5.5310  gm  of  sample. 

AO 

The  condensed  expression  is 

45.1X0.0096X40.008X250X100 

"36.46X25X5. 5310  =  85. 89  =  percent  sodium 

hydroxide. 

The  " equivalent  weight"  of  a  substance  is  the  number  of 
weight-units  chemically  equivalent  to  eight  weight-units  of  oxygen. 
This  definition  is  sufficiently  broad  to  apply  to  any  system  of 
weights,  although  there  are  very  few  cases  in  scientific  work 
where  "grams"  might  not  be  substituted  for  "weight-units," 
since  the  metric  system  is  quite  universally  accepted  and  used 
in  scientific  work.  The  result  of  this  substitution  is  the  "gram- 
equivalent."  In  the  effort  to  determine  what  is  the  equivalent 
weight  of  a  substitute  it  is  always  necessary  to  inspect  the 
equation  for  the  reaction  that  occurs  in  that  particular  case.  In 
the  reaction 

HC1 + NaOH-^NaCl + H20, 

it  is  easily  seen  that  the  equivalent  weights  for  all  of  the  elements, 
radicals,  ions  or  compounds  are  the  atomic,  radical,  ionic  or 
molecular  weights,  respectively.  In  the  reaction: 

H2SO4+BaCl2-»BaS04+2HCl, 

the  equivalent  weights  of  the  compounds  are  seen  to  be  one- 
half  of  their  molecular  weights,  with  the  exception  of  hydro- 
chloric acid,  whose  equivalent  weight  is  its  molecular  weight. 
This  will  be  more  easily  understood  if  we  first  determine  the 
''hydrogen  equivalent"  of  each  substance,  this  being  the  number 
of  atoms  of  hydrogen  chemically  equivalent  to  one  molecule,  atom, 
radical,  etc.,  of  the  substance  under  consideration,  as  denoted  by  the 
equation  for  the  reaction  that  has  taken  place.  The  hydrogen 
equivalent  of  sulphuric  acid  is  2  because  it  reacts  by  substituting 
another  element  for  two  hydrogen  atoms.  That  of  barium 
chloride  is  2,  because  one  atom  of  a  bivalent  element  gives  place 


196  QUANTITATIVE  ANALYSIS 

to  hydrogen  or  because  2  atoms  of  a  univalent  element  give 
place  to  one  radical  which  is  bivalent.  That  of  barium  sulphate 
is  2  and  that  of  hydrochloric  acid  is  1,  for  similar  reasons.  Since 
one  atom  of  oxygen  is  chemically  equivalent  to  two  atoms  of 
hydrogen,  it  follows  that  any  body  that  is  equivalent  to  1.008 
weight  units  of  hydrogen  will  be  equivalent  to  8  weight  units  of 
oxygen  and  therefore  the  equivalent  weight  is  the  molecular  (atomic, 
etc.]  weight  divided  by  the  hydrogen  equivalent. 

Problems 

Find  the  equivalent  weights  of  the  substances  whose  formulas  are  in  bold 
face  in  the  following  equations : 

4.  2HCl+Na2C03— »2NaCl+H2CO3. 
6.  HCl+NaHCOs— >NaCl+H2CO3. 

6.  HCl+Na2CO8— >NaHCO,+NaCl. 

7.  (NH4)2C204  +  CaCl2-»CaC2O4+2NH4CL 

8.  (NH4)2C2O4+HC1-»NH4C1+NH4HC2O4. 

9.  (NH4)2C204+2HC1->2NH4CH-H2C2O4. 

10.  FeCli+2AgNO,->Fe(NO,)»+2AgCl. 

11.  2FeCl2+Cl2— »2FeCl3. 

12.  H2SO4 +Zn— »ZnSO4 +H2. 

13.  H2S04+2H->SO2+2H2O. 

14.  CuCl2+2AgNO3-»Cu(NO3)2+2AgCl. 
16.  2CuCl2+Fe— >2CuCl+FeCl2. 

Calculation  of  the  Weight  of  One  Substance,  Chemically  Equiv- 
alent to  a  Stated  Weight  of  Another. — In  the  solution  of  nearly 
all  problems  of  quantitative  chemistry  there  is  involved  a  calcu- 
lation of  the  weight  of  one  substance  chemically  equivalent  to  a 
given  weight  of  another.  In  the  examples  just  considered  this 
is  a  calculation  of  the  weight  of  silver  or  of  sodium  hydroxide 
equivalent  to  the  weight  of  hydrochloric  acid  that  is  contained 
in  the  quantity  of  standard  solution  used.  In  example  (1)  this 

1 07  88 
was   expressed    as    on  4^  FX  0.00304    and   in    example    (2)    as 

36  46  ^X°-QQ96-  I*  is  easilv  seen  tnat  both  of  these  cal- 
culations involve  the  multiplication  of  the  weight  of  hydro- 
chloric acid  by  a  ratio  of  equivalent  weights.  From  this  fol- 
lows the  rule  that  to  find  the  weight  of  one  substance  chemically 


VOLUMETRIC  ANALYSIS  197 

equivalent  to  a  stated  weight  of  another,  multiply  the  stated  weight  by 
the  fraction: 

equivalent  weight  of  substance  calculated 
equivalent  weight  of  substance  given 

This  is  a  simple  and  very  useful  rule  and  its  application  will 
obviate  the  use  of  the  more  cumbersome  rule  of  proportions. 
Applied  to  gravimetric  analysis  the  fraction  given  above  is  the 
factor. 

Problems 

16.  What  weight  of  oxygen  is  equivalent  to  0.3460  gm  of   hydrogen, 
direct  oxidation  to  water  being  understood? 

17.  What  weight  of  carbon  dioxide  is  equivalent  to  0.5693  gm  of  carbon, 
direct  oxidation  being  understood? 

18.  Calculate  the  weight  of  ferric  chloride  and  of  iron  equivalent  to 
0.5243  gm  of  chlorine,  the  following  reaction  taking  place: 

2Fe+3Cl2-»2FeCl3. 

19.  Calculate  the  weight  of  potassium  hydroxide  equivalent  to  1.7521 
gm  acetic  acid,  assuming  complete  neutralization. 

20.  In  problems  (4)  to  (15)  calculate  the  weights  of  the  substances  whose 
formulas  are  in  bold  face,  equivalent  to  3  gm  of  the  substances  reacting 
with  them. 

21.  A  solution  of  hydrochloric  acid  of  specific  gravity  1.05  contains  10 
percent  by  weight  of  the  pure  acid.     What  volume  of  the  solution  is  required 
to  precipitate  the  silver  from  0.75  gm  of  silver  sulphate? 

22.  0.4321  gm  of  impure  potassium  sulphide  was  oxidized  to  potassium 
sulphate   and   precipitated   by   barium    chloride.     0.8035   gm   of   barium 
sulphate  was  produced.     What  was  the  percent  of  sulphur  in  the  sample? 

23.  What  weight  of  tartaric  acid  is  equivalent  to  3.52  gm  of  sodium 
hydroxide,  the  following  reaction  taking  place? 


Na2C4H4O6  +  2H2O. 

Use  of  a  Standard  Solution  for  the  Titration  of  but  One  Sub- 
stance. —  When  a  standard  solution  is  to  be  used  for  the  titra- 
tion  of  but  one  substance  the  calculations  will  all  involve  the 
constants  representing  (1)  equivalent  weight  of  the  active  sub- 
stance in  the  standard  solution,  (2)  equivalent  weight  of  the 
substance  to  be  determined  and  (3)  concentration  of  the  standard 
solution.  If  the  standard  solution  of  example  (1)  is  to  be  used  for 

107.88X0.00304 
tne   determination    ot    silver,   the    expression  :   -      —  ~a  A&  — 


198  QUANTITATIVE  ANALYSIS 

contains  quantities  that  are  constants  for  all  such  determinations. 
These  constants  should  then  be  combined  in  a  single  constant: 
0.00899.  From  what  was  said  in  the  preceding  paragraph  this 
is  seen  to  be  the  weight  of  silver  equivalent  to  1  cc  of  this  par- 
ticular standard  solution  of  hydrochloric  acid.  All  such  calcu- 
lations would  therefore  be  made  by  the  expression 

7X0.00899X100 

— ; p —       —  =  percent  silver  (III) 

o 

This  is  obviously  a  very  simple  calculation  and  such  simplifica- 
tion is  possible  and  should  be  made  whenever  a  given  standard 
solution  is  to  be  used  for  a  considerable  number  of  determinations 
of  a  single  substance. 

Burette  Reading  a  Direct  Percentage  Reading. — If  some  care 
were  exercised  in  adjusting  the  weight  of  silver  salt  used  for 
analysis  where  statement  (III)  is  to  enter  the  calculation,  so 
that  exactly  0.8990  gm  of  sample  were  used,  the  expression 
would  become 

VX0.00899X100 

0.8990  "        =  percent  Sllver 
whence,  V  =  percent  silver. 

In  this  case  the  volume  of  standard  solution  used  is  the  percent 
of  silver  in  the  sample.  From  this  follows  the  rule:  To  make 
the  burette  reading  a  percentage  reading  first  calculate  the  weight 
of  the  titrated  substance  that  is  equivalent  to  1  cc  of  the  standard 
solution,  then  use  100  times  this  weight  of  sample. 

Problems 

24.  What  weight  of  soda  ash  must  be  used  for  analysis  in  order  that  1  cc 
of  the  hydrochloric  acid  solution  containing  0.0031  gm  shall  be  equivalent 
to  1  percent  of  sodium  carbonate,  assuming  complete  decomposition? 

26.  A  standard  solution  of  sulphuric  acid  contains  40.2  gm  in  1000  cc. 
What  weight  of  potassium  hydroxide  must  be  taken  so  that  each  cubic 
centimeter  of  the  standard  acid  required  shall  indicate  0.1  percent  of 
potassium  hydroxide? 

26.  A  standard  solution  of  barium  hydroxide  contains  20.35  gm  in  1000 
cc.  What  weight  of  vinegar  is  necessary  in  order  that  1  cc  of  barium 
hydroxide  solution  shall  indicate  0.1  percent  of  acetic  acid  in  the  vinegar? 

No  System. — In  the  examples  given  above  there  was  no  definite 
basis  for  the  choice  of  the  concentration  of  the  standard  solution, 


VOLUMETRIC  ANALYSIS  199 

all  that  was  required  being  an  accurate  knowledge  of  the  existing 
concentration.  Thus,  in  the  first  example  the  standard  hydro- 
chloric acid  contained  0.00304  gm  in  1  cc,  while  in  the  second 
it  contained  0.00960  gm  in  1  cc.  There  is  no  connectio  ,  ap- 
parent or  real,  between  these  concentrations;  they  were  chosen, 
at  least  to  a  certain  extent,  at  random,  upon  the  assumption 
that  the  determination  of  the  concentration  (standardization) 
was  carried  out  with  the  greatest  possible  accuracy  but  that 
in  making  the  solution  no  particular  care  was  exercised.  The 
method  of  calculating  analyses  made  by  means  of  such  standard 
solutions  would  in  all  cases  be  analogous  to  these  examples 
and  any  substance  can  be  determined  by  means  of  such  solu- 
tions, provided  that  the  reaction  involved  is  definite,  complete 
and  well  understood,  and  that  the  end  point  can  be  determined 
accurately. 

Normal  System. — The  calculations  of  volumetric  analysis 
may  be  considerably  shortened  by  the  proper  adjustment  of 
the  concentration  of  the  standard  solution. 

The  reaction  between  hydrochloric  acid  and  silver  nitrate  is 
expressed  by  the  equation: 

HC1 + AgNO3-»HNO3 + AgCl, 

and  expression  (II)  was  deduced  for  the  calculation  of  the  per- 
cent of  silver  in  a  salt  that  had  been  titrated  by  a  standard 
solution  of  hydrochloric  acid.  This  expression  was 

107.887CX100 

36.46S      -=  P^cent  silver. 

By  using  the  proper  indicator  in  each  case  we  might  have 
completed  such  reactions  as  the  following  for  the  titration  of 
the  substances  indicated : 

HC1 + NaOH-^NaCl + H  20,  (a) 

HC1 + KOH->KC1 + H  2O,  (b) 

HC1 + NH  4OH-+NH  4C1 + H20,  (c) 

2HCl+Na2CO3^2NaCl+H20+CO2,  (d) 

HCl+NaHCO3-*Naa+H2O+C02,  (e) 

2HCl+Ba(OH)2-*BaCl2+2H20,  (f) 

2HCl+CaC03->CaCl2H-H20+C02.  (g) 


200  QUANTITATIVE  ANALYSIS 

Many  other  substances  may  also  be  titrated  by  this  same 
standard  solution  and  in  each  case  the  expression  for  the  per- 
cent of  the  substance  to  be  calculated  would  be  the  same  as 
(II)  with  the  exception  that,  for  the  equivalent  weight  (combining 
weight)  of  silver  (107.88)  we  should  substitute  the  equivalent 
weight  of  the  substance  to  be  calculated;  we  should  then  have: 

40.0087CX100 

=  percent  NaOH  (a') 


36  46 
56.1087CX100 


percent  KOH  (b') 


35.05  FCX100 

=  percent  NH4OH  (c') 


36 

17.03  7(7  -X  100 

36.46S         =  Percent  NH3 

53  7CX100  ,,„ 

-  36  46^  --  =  percent  Na2CO3  (d') 

84.0087CX100  ,  ,. 

36  46  s  --  =  percent  NaHCO3  (e') 

85.6937CX100 
36  46  s  -- 

50.0457CX100 


36 


,ff. 
=  percent  Ba(OH)2 

f  ,, 
=  percent  CaC03  (gO 


In  all  of  these  expressions  for  the  percent  of  the  various 
substances  as  titrated  by  a  single  standard  solution,  the  only 
difference  lies  in  the  equivalent  weight  of  the  substance.  The 
volume  of  standard  required  will  depend,  among  other  things, 
upon  the  purity  of  the  sample  and,  since  this  is  unknown,  the 
volume  required  cannot  be  predicted.  The  concentration  of 
the  standard  is  under  control  and  may  be  arbitrarily  fixed  at 
any  desired  figure.  The  equivalent  weights  concerned  are 
constants,  in  any  given  case,  and  the  weight  of  sample  may  be 
made  whatever  is  desired. 

If  the  standard  solution  is  made  of  such  strength  that  the 
tumber  of  grams  contained  in  1000  cc  will  be  represented  by 
the  equivalent  weight  (in  the  case  of  hydrochloric  acid  36.46  gm) 
the  concentration  in  grams  per  cubic  centimeter  will  then  be 


VOLUMETRIC  ANALYSIS  201 

Q 
0.03646,  and  the  fraction  „„    .„>  which  is  involved  in  all  of  the 

expressions,  will  become  =0.001,  so  that  we  shall  then 


have 

0.04008FX100 


=  percent  NaOH  (ai) 


0.0561087X100 

^          —  =  percent  KOH  (bi) 

0.035057X100 

„         —=  percent  NH4OH  (ci) 

and  so  on. 

The  standard  solution  of  hydrochloric  acid  thus  made,  con- 
taining 1  gram-equivalent  of  the  active  substance  in  1000  cc 
of  solution,  is  a  solution  of  general  application  and  the  calcula- 
tion of  the  results  of  analyses  of  various  substances  is  simplified 
by  this  choice  of  concentration.  Such  a  solution  is  called  a 
"  normal  solution"  which  will  be  defined  as  a  solution  containing 
1  gram-equivalent  of  the  active  substance  in  1000  cc. 

From  the  foregoing  discussion  the  following  deductions  may  be 
made. 

1.  1  cc  of  any  normal  solution  is  equivalent  to  one-thousandth 
of    one   gram-equivalent    (  =  one   milHgram-equivalent)    of    any 
substance.     This  is  because  1  cc  of  any  normal  solution  contains 
one-thousandth  of  one  gram-equivalent  of  the  active  substance.. 

2.  If  the  milligram-equivalent  of  the  substance  titrated  in  a 
given  determination  is  multiplied  by  the  number  of  cubic  centi- 
meters of  a  normal  solution  used  for  the  titration,  the  result  is 
the  weight  of  the  former  in  the  sample.     This  follows  as  a  result 
of  (1). 

3.  1  cc  of  any  normal  solution  is  equivalent  to  1  cc  of  any 
other  normal  solution.     This  also  follows  as  a  result  of  (1). 

4.  The  relative  volumes  of  various  standard  solutions  equiva- 
lent to  each  other  are  inversely  as  the  respective  normalities  of 
these  solutions.     Thus  6  cc  of  a  fifth-normal  solution  is  equivalent 
to  26  cc  of  a  tenth-normal  solution. 

These  principles  are  very  important  and  their  intelligent  ap- 
plication will  serve  to  shorten  many  of  the  calculations  of 
volumetric  analysis. 


202  QUANTITATIVE  ANALYSIS 

It  frequently  happens  that  the  normal  solution  is  too  concen- 
trated or  too  dilute  for  convenient  use  in  a  given  analysis.  In 
this  case  the  advantage  of  the  normal  solution  may  be  retained 
by  making  the  concentration  of  the  solution  to  be  some  simple 
multiple  of  the  concentration  of  the  normal  solution,  such  as 

2>  3,  V,  y~,  -rpr,  .TT^-T,  etc.     This  factor  must  then  be  introduced 

O     JLU    OU     1UU 

into  the  calculations  involving  the  solution.  Solutions  made  oi 
normal  or  a  simple  multiple  of  normal  strength  are  said  to  be 
made  in  the  "normal  system"  and  are,  for  the  sake  of  brevity, 

N    N     N 
designated  as  N,  2N,      ,      ,        ,  etc. 


Problems 

27.  1  cc  of  normal  acid  is  equivalent  to  what  weight  of  ammonium  car- 
bonate, assuming  complete  decomposition? 

N 

28.  1.1256  gm  of  a  silver  alloy  is  dissolved  and  titrated  by  ^  potassium 

thiocyanate  solution  according  to  the  following  equation : 
KCNS  +  AgNO3->KNO3  +  AgCNS. 

35.2  cc  of  standard  solution  is  required.     What  is  the  percent  of  silver  in 
the  alloy? 

N 

29.  0.5  gm  of  limestone  was  dissolved  in  50  cc  of  -=•  acid.     The  unused 

o 

N 
excess  of  acid  was  titrated  by  16.2  cc  of  JK  base.     What  was  the  percent 

of  calcium  carbonate  in  the  limestone?     What  percent  of  calcium? 

N 

30.  0.4  gm  of  soda  ash  was  titrated  by  20.9  cc  of  y  acid.     What  was 

the  percent  of  sodium  carbonate  in  the  sample? 

31.  0.5' gm  of  an  ammonium  salt  was  decomposed  by  sodium  hydroxide 

N 
and  the  resulting  ammonia  distilled  into  50  cc  of  -^  acid  solution.     The 

N 
unused  excess  of  acid  was  titrated  by  29.3  cc  of  ^Q  base.     What  percent 

of  ammonia  in  the  salt? 


Decimal  System. — Instead  of  using  the  normal  system,  a 
further  simplification  may  be  made  by  adjusting  the  standard 
until  each  cubic  centimeter  shall  be  equivalent,  not,  as  in  the 
normal  system,  to  a  decimal  fraction  of  a  gram-equivalent  of  the 


VOLUMETRIC  ANALYSIS  203 

substance  to  be  titrated  but  to  a  decimal  or  simple  fraction  of 
a  gram  of  the  substance.  For  example,  a  solution  of  hydro- 
chloric acid  would  be  made  with  each  cubic  centimeter  equiva- 
lent to  0.0100  gm,  0.0010  gm,  0.0050  gm,  etc.,  of  silver. 
This  results  in  a  very  much  simplified  calculation  and  still  more 
time  is  saved  if  the  weight  of  sample  used  bears  a  definite  and 
simple  relation  to  the  equivalence  of  the  standard. 

Such  solutions  as  these  are  frequently  made  for  technical 
work  in  industrial  laboratories,  where  large  quantities  of  standard 
solutions  are  often  required  for  the  titration  of  a  single  con- 
stituent of  a  large  number  of  samples.  Mention  may  be  made  of 
the  use  of  potassium  permanganate  or  potassium  dichromate 
solutions  for  the  titration  of  iron  in  ores,  sodium  thiosulphate 
solutions  for  the  determination  of  the  available  chlorine  in  bleach- 
ing powder,  potassium  ferrocyanide  solutions  for  the  determina- 
tion of  zinc  and  hydrochloric  acid  solutions  for  the  determina- 
tion of  hardness  of  water. 

The  method  of  calculation  of  the  necessary  concentration  of 
a  solution  to  be  made  in  the  decimal  system  is  the  reverse  of 
the  method  for  calculating  the  equivalence  of  a  solution  of  given 
concentration. 

Example:  What  must  be  the  concentration  of  a  solution  of 
potassium  hydroxide  in  order  that  each  cubic  centimeter  shall 
be  equivalent  to  0.001  gm  of  sulphuric  acid?  The  reaction 
involved  is: 

2KOH+H2S04-»K2S04+2H20. 

The  equivalent  weight  of  potassium  hydroxide  is  56.108  and  that 
of  sulphuric  acid  is  49.02.  Each  cubic  centimeter  must  con- 

rn    -«  /-kQ 

tain      *  no  X  0.001    gm    of    potassium     hydroxide.      This     is 

*±.\}.\jZi 

0.00114  gm. 

Problems 

32.  Calculate  the  concentration  ( — )  of  standard  solutions  of  hydro- 

\  cc  / 

chloric  acid  such  that  1  cc=c=the  following  weights  of  other  substances: 
0.002  gm  of  silver;  0.005  gm  of  silver  chloride;  0.010  gm  of  potassium 
hydroxide;  0.005  gm  of  sodium  hydroxide;  0.002  gm  of  sodium;  0.002 
gm  of  ammonia. 


204  QUANTITATIVE  ANALYSIS 

33.  Calculate  the  concentration  of  a  nitric  acid  solution  such  that  1  cc=c= 
the  following  weights  of  substances:  0.040  gm  of  potassium  hydroxide; 
0.005  gm  of  calcium  carbonate;  0.001  gm  of  nitrogen  as  ammonia. 

34.  What  is  the  concentration  of  a  potassium  hydroxide  solution  of 
which  1  ceo  0.010  gm  of  potassium  acid  tartrate? 

Choice  of  System. — Summarizing,  it  has  been  shown  that 
volumetric  analysis  may  be  carried  out  by  the  use  of  standard 
solutions  made  in  "no  system,"  in  the  "normal  system"  or  in 
the  "decimal  system,"  and  that  for  any  of  these  systems  a  defi- 
nite, precalculated  weight  of  sample  may  be  taken  so  that  the 
burette  reading  in  cubic  centimeters  will  indicate  directly  the 
percent  or  simple  fractions  of  percent  of  the  constituent  being 
determined.  Which  of  these  systems  shall  be  selected  in  prac- 
tical work  will  be  determined  by  the  circumstances.  If  but  a 
few  titrations  are  to  be  made  with  a  given  standard  solution  the 
time  saved  in  simplified  calculations  will  not  justify  the  expendi- 
ture of  time  required  for  adjusting  the  concentration  to  the  nor- 
mal or  decimal  system.  If  many  titrations  are  to  be  made,  one 
of  the  latter  two  systems  will  be  used.  The  normal  system  is 
most  useful  for  standard  acids  and  bases  because  their  application 
is  more  general  and  a  solution  so  made  will  give  simplified  cal- 
culations for  the  titration  of  many  other  substances.  There  are 
many  standard  solutions  which  are  not  to  be  used  so  generally  but 
which  are  made  for  the  titration  of  but  one  substance.  In 
such  instances  the  decimal  system  will  always  be  used. 

Temperature  Correction  for  Standard  Solutions. — It  is  often 
difficult  to  control  the  temperature  of  the  laboratory  within 
close  limits  and  errors  may  thereby  be  introduced  into  volumetric 
determinations,  due  to  changes  in  the  density  of  standard  solu- 
tions and  in  the  capacity  of  measuring  instruments  when  the 
temperature  varies  from  20°.  The  following  table,  adapted 
from  tables  published  by  the  Bureau  of  Standards,1  indicates 
the  corrections  for  water  and  for  two  concentrations  of  most  of 
the  common  standard  solutions  of  acids  and  bases.  Approxi- 
mately these  corrections  will  apply  also  to  most  other  solutions 
of  similar  concentrations. 

1  Bur.  Stand.,  Circ.  19,  table  33, 


VOLUMETRIC  ANALYSIS 


205 


Correction,  cc  per  observed  liter  to  give 

Temperature, 

volume  at  20° 

degrees 

N 

N 

Water 

77:  Solutions 

-^  Solutions 

10 

2 

15 

+0.8 

+0.8 

+  1.0 

16 

+0.6 

+0.7 

+0.8 

17 

+0.5 

+0.5 

+0.6 

18 

+0.3 

+0.4 

+0.4 

19 

+0.2 

+0.2 

+0.2 

21 

-0.2 

-0.2 

-0.2 

22 

-0.4 

-0.4 

-0.5 

23 

-0.6 

-0.6 

-0.7 

24 

-0.8 

-0.9 

-1.0 

25 

-1.0 

-1.1 

-1.3 

It  will  be  seen  that  a  variation  of  2°,  either  way,  from  20° 
involves  an  error  of  0.04  percent  in  measurements  of  tenth- 
normal  solutions,  or  0.04  to  0.05  percent  for  half-normal  solu- 
tions. This  may  be  ignored  for  much  of  the  routine  work  of  the 
industrial  laboratory  but  for  more  exact  work  the  corrections 
should  be  applied. 

The  use  of  standard  acids  and  bases  provides  a  means  for  the 
quantitative  determination  of  practically  any  acid  or  base  and 
of  many  salts.  This  is  an  extremely  useful  department  of  work, 
in  view  of  the  fact  that  no  gravimetric  method  will  serve  to  de- 
termine the  essential  constituent  of  acids  and  bases,  the  ionizable 
hydrogen  and  hydroxyl.  For  example,  from  potassium  hy- 
droxide potassium  may  be  determined  as  chlorplatinate  or 
perchlorate,  but  this  gives  no  information  concerning  the  per- 
cent of  potassium  hydroxide  since  potassium  from  any  salt 
present  is  also  precipitated  and  weighed.  By  using  the  proper 
indicator  salts  of  strong  bases  with  weak  acids  or  of  strong  acids 
with  weak  bases  may  also  be  titrated.  Thus  sodium  carbonate 
may  be  titrated  by  standard  hydrochloric  or  sulphuric .  acid  if 
methyl  orange  is  used  as  indicator. 

Adjustment  to  Exact  Concentration. — In  most  of  the  exercises 
of  the  following  pages  the  student  is  directed  to  adjust  his 
standard  solutions  to  the  exact  stated  concentrations.  If  a 


206  QUANTITATIVE  ANALYSIS 

solution  is  desired  to  be  tenth-normal,  or  perhaps  of  such  con- 
centration that  1  cc  is  equivalent  to  0.1  mg  of  phosphorus  or  0.5 
mg  of  tin,  etc.,  it  is  first  made  to  this  approximate  concentration. 
It  is  then  standardized  and  finally  adjusted  to  the  required 
strength,  with  or  without  an  additional  standardization  to  con- 
firm the  accuracy  of  the  dilution.  The  only  exceptions  to  this 
rule  are  in  cases  of  solutions  of  unstable  substances  which  change 
on  standing. 

Of  course  this  process  of  adjustment  is,  in  some  cases,  a  some- 
what tedious  procedure  and  there  is  a  too  common  custom  of 
omitting  the  final  adjustment,  using  a  correction  factor  in  the 
calculations  of  analyses  made  by  means  of  this  solution,  this 
factor  having  been  found  by  the  first  standardization.  Thus, 

N 

if  the  first  standardization  showed  a  solution  to  be  1.0359-^, 

o 

all  calculations  of  titrations  would  be  made  as  though  the  solu- 
tion were  fifth-normal,  except  that  the  factor  1.0359  would  enter. 
This  figure  is  known  as  a  "normality  factor." 

To  the  inexperienced  analyst  it  may  seem  that  this  is,  after  all, 
the  best  method  of  dealing  with  the  question — that  the  use  of 
such  a  factor  requires  much  less  work  than  is  involved  in  the 
process  of  exact  dilution  and  restandardization.  But  here 
again  the  question  must  be  resolved  with  respect  to  the  way  in 
which  the  solution  is  to  be  used,  as  was  done  in  the  matter  of 
choice  of  system.  The  work  involved  in  adjusting  the  standard 
solution  is  balanced  against  the  labor  that  is  saved  by  simplified 
calculations.  But  the  latter  quantity  is,  of  course,  to  be  mul- 
tiplied by  the  number  of  determinations  that  will  be  made  before 
the  solution  deteriorates  and  requires  restandardization,  or  before 
the  supply  is  exhausted.  Obviously,  this  means  that  if  any 
considerable  use  is  to  be  made  of  a  given  standard  solution  it 
should  be  adjusted  to  the  exact  desired  concentration.  It 
may  also  be  remarked  that  if  adjustment  is  not  to  be  made  there 
is  no  logic  in  trying  to  work  to  either  the  normal  or  the  decimal 
system.  The  method  described  on  page  197  is  far  simpler  in 
this  case. 


CHAPTER  VI 
COLOR  CHANGE  OF  INDICATORS 

In  a  broad  sense  the  word  "indicator"  applies  to  all  substances 
which,  by  undergoing  any  visible  change,  indicate  the  end  point 
of  reactions.  When  the  indicators  are  inorganic  the  reactions 
are  usually  definite  and  well  understood.  The  indicators  used 
in  acidimetry  and  alkalimetry  are  organic  and  the  direct  cause 
of  color  change  is,  even  now,  not  thoroughly  understood.  Many 
of  the  organic  dyes  show,  in  acids,  a  color  different  from  that  in 
bases.  The  color  change  is  generally  reversible  an  indefinite 
number  of  times.  The  molecular  structure  of  the  dye  is  often 
very  complex  and  it  is  not  easy  to  follow  the  changes  in  structure. 

Simple  lonization  Theory.  —  Most  or  all  of  the  indicators  of  this 
class  are  known  to  possess,  in  certain  conditions  at  least,  the  prop- 
erties of  acids  or  bases.  The  acid  or  basic  nature  is  usually 
weakly  emphasized.  From  this  Ostwald  deduced  a  theory  as  to 
the  cause  of  color  change.1  According  to  this  theory  these  dyes 
are,  when  uncombined  with  other  acids  or  bases,  weak  electrolytes 
and  largely  in  the  molecular  state.  If  a  base  is  added  to  a  weakly 
acid  indicator,  the  salt  is  formed  and  this  is  highly  ionized,  ac- 
cording to  the  general  rule.  The  molecule  possesses  one  color  (or 
is  colorless),  while  the  anion  shows  a  different  color.  The  result 
of  the  addition  of  a  base  is  therefore  a  color  change.  If  another 
acid  is  now  added  to  the  ionized  salt  the  weak  acid  is  reformed, 
the  molecule  reappears  and  the  color  change  is  reversed.  The 
added  acid  has  a  further  effect  upon  the  indicator  acid  in  suppress- 
ing the  already  small  ionization.  Similar  reasoning  would  apply 
to  basic  indicators.  Phenolphthalein  is  in  the  presence  of  acids, 
a  derivative  of  phthalic  anhydride  and  phenol  having  the 
following  constitution  : 

CO 


=  (C6H4OH)2. 

1  Scientific  Foundations  of  Analytical  Chemistry,  118. 

207 


208  QUANTITATIVE  ANALYSIS 

According  to  the  theory  of  Ostwald  this  is  a  very  weak  acid, 
giving  a  small  concentration  of  ions  thus: 

HPh  ±5  H  +  Ph, 

the  symbol  Ph  representing  the  negative  radical  of  the  compound. 
The  ionization  constant  is  very  small  and  equilibrium  occurs 
with  an  inappreciable  concentration  of  the  anion.  Upon  the 
addition  of  a  base  the  ionized  acid  is  neutralized,  equilibrium  is 
disturbed  and  the  ionized  salt  is  produced,  hence  the  color  of  the 
anion  (red)  appears.  Methyl  orange  is  known  to  have,  under 
certain  conditions,  the  structure 


This  is  an  acid,  the  red  molecular  form  predominating  in  acid 
solutions,  and  the  yellow  anion  appearing  in  basic  solutions. 


Theory  of  Chromophors.  —  This  explanation  is  not  sufficient 
in  itself  for  several  reasons.  The  silver  salt  of  phenolphthalein 
is  intensely  purple,  even  when  dry,  and  the  dry  salt  cannot  be 
highly  ionized.  Ethers  of  tetrabromphenolphthalein  have  been 
prepared;1  these  are  non-ionizable  but  colored.  The  monoethyl 
ether  is 

xCO2C2H5 

r-C  6H2Br2OH 


Litmus  is  known  as  both  blue  and  red  in  the  dry  state,  when  it 
must  be  chiefly  molecular,  no  matter  what  the  color  may  be. 
Also  the  studies  of  recent  years  upon  the  constitution  of  organic 
dyes  have  shown  that  in  many  cases  a  change  of  molecular 
structure  takes  place  upon  the  addition  of  an  acid  or  a  base. 

Phenolphthalein  is  known  to  have  the  structure  shown  above 
but  in  basic  solutions  there  is  a  salt  of  a  carboxyl  acid  which  is  a 
quinone  derivative.  The  phenol  derivative  is  then  in  equilibrium 

1  Nietzki  and  Burckhardt:  Ber.,  30,  175  (1897). 


COLOR  CHANGE  OF  INDICATORS  209 

with  the  quinone  derivative  and  this  equilibrium  is  disturbed 
in  one  direction  or  the  other  by  the  addition  of  an  acid  or  a  base. 

CO  COO 


C6H4/ 


+H 


(C6H4OH)2  C  =  C6H4  =  0 

\C6H4OH 


If  an  acid  is  added  the  first  (colorless)  molecular  form  is  produced 
because  suppression  of  ionization  results  from  the  increase  in 
concentration  of  hydrogen  ions.  If  a  base  is  added  the  ionized 
form  is  neutralized,  forming  ionized  salt  and  water,  and  thus 
the  new  structure  predominates.  When  methyl  orange  changes 
from  the  sulphonic  acid  to  one  of  its  salts  a  change  of  structure 
also  takes  place.  The  structure  peculiar  to  the  non-ionized  body 
(present  when  an  acid  is  added)  is  not  that  of  an  azo  compound 
but  one  containing  the  quinone  ring.  There  is  then  equilibrium 
between  the  two  forms: 
(CH3)2N-C6H4-N  =  N-CeH4S03H^± 

Yellow,  predominates  in  basic  solution. 

(CH3)2N  =  C6H4  =  N-NH-C6H4SOa 

!  ___  .  _  j 
Red,  predominates  in  acid  solution. 

The  acid,  by  suppressing  the  ionization  of  the  first  form,  causes 
the  second,  a  lactonic  form,  to  predominate.  A  base,  by  forming 
salt  and  water  from  the  sulphonic  acid,  causes  the  first  form  to 
predominate.  With  the  acid  the  quinone  ring  gives  a  red  color. 
With  the  base  the  azo  group  gives  a  yellow  color.  The  quinone 
ring,  =C6H4  =  ,  is  one  of  a  class  of  groups  known  as  "chromo- 
phors"  because,  wherever  they  appear  in  any  compound  they  give 
rise  to  color.  Other  well  characterized  chromophors  are  the  azo 
group  —  N  =  N  —  ,  the  nitro  group,  —  N02,  and  the  dicarbonyl 
group,—  CO—  CO—  -1 

Classification  of  Organic  Indicators.  —  Ostwald  classified  the 
indicators  according  to  their  supposed  dissociation  constants  into 
three  groups: 

(a)  Very  weak  bases  and  relatively  strong  acids. 

(b)  Moderately  strong  acids  and  bases. 

(c)  Very  weak  acids  and  relatively  strong  bases. 

1  Vide  Hantzsch:  Ber.,  32,  575  (1899),  and  Stieglitz:  J.  Am.  Chem.  Soc.f 
26,  1112  (1903). 

14 


210  QUANTITATIVE  ANALYSIS 

Since  he  explained  the  color  change  upon  the  basis  of  salt 
formation  it  would  necessarily  follow  that  the  relative  sensitive- 
ness would  vary  in  the  three  classes.  In  class  (a)  the  indicators 
would  be  highly  sensitive  to  bases  but  not  easily  affected  by 
acids,  except  by  very  strong  ones.  The  indicators  of  class  (b) 
would  be  moderately  sensitive  to  both  acids  and  bases,  while  in 
class  (c)  they  would  be  highly  sensitive  to  acids  and  to  none  but 
strong  bases.  While  some  of  these  indicators  are  here  called 
" relatively  strong"  acids  or  bases,  it  must  be  remembered  that, 
compared  with  the  strongest  electrolytes,  all  are  weakly  ionized 
and  all  lie  in  the  class  of  weak  electrolytes. 

While  the  theory  of  color  change  by  ionization  must  be 
regarded  as  based  upon  incorrect  assumptions,  the  above  classi- 
fication is  still  a  convenient  one  since  the  same  relative  sensi- 
tiveness would  follow  from  the  application  of  any  of  our  theories. 
Phenolphthalein  may  be  taken  as  an  illustration.  According 
to  the  simple  ionization  theory  there  is  to  be  considered  merely 
the  following  system  in  equilibrium : 

HPh^H+Ph, 

where  Ph  is  understood  to  mean  the  negative  radical 
C6H4(CO)2C6H4OH.C6H40. 

Phenolphthalein  falls  in  the  class  of  very  weak  acids  and  it  is 
consequently  chiefly  molecular  unless  a  strong  base  be  present, 
a  weak  base  forming  an  easily  hydrolyzed  salt.  Even  weak 
acids  can  decompose  the  salt,  therefore  phenolphthalein  will  be 
easily  affected  by  acids  but  will  not  be  highly  sensitive  to  bases. 
According  to  the  view  that  color  is  due  to  the  existence  of 
chromophors  the  equation 

CO  COO 

No  <=*    CeH4/  +H 


=  (C6H4OH)2  C  — CeH4  — O 

\C6H4OH 

is  an  expression  of  equilibrium  between  the  non-ionized  colorless 
form  and  the  ionized  form  containing  a  chromophor.  Here 
again  a  strong  base  will  be  required  to  produce  the  ion  containing 


COLOR  CHANGE  OF  INDICATORS 


211 


the  chromophor,  while  a  weak  acid  will  reform  the  colorless 
molecule  and  for  the  same  reasons  that  are  given  above. 

The  classification  according  to  ionization  loses  much  of  its 
significance  when  it  is  remembered  that  each  indicator  exists  in 
at  least  two  forms.  On  this  account  we  shall  rather  lay  stress 
upon  the  sensitiveness  of  the  indicator  toward  acids  and  bases 
and  shall  so  classify  a  few  of  the  more  commonly  used  indicators. 


Class  I 
Highly  sensitive  to 
acids,  less  sensitive  to 
bases 

Class   II 

Moderately  sensitive  to 
acids  and  bases 

Class  III 
Highly  sensitive  to 
bases,  less  sensitive  to 
acids 

Phenolphthalein 
Rosolic  acid 

Litmus 
p-nitrophenol 
Lacmoid 

Methyl  orange. 
Ethyl  orange. 
Cochineal. 
Erythrosine. 
Methyl  red. 

This  classification  shows  that  an  indicator  which  is  highly 
sensitive  to  acids  is,  in  a  corresponding  degree,  weakly  sensitive 
to  bases.  For  this  reason  the  only  generally  serviceable  standard 
acid  or  base  is  a  highly  ionized  one.  Such  a  standard  may  be 
used  for  the  titration  of  either  weak  or  strong  electrolytes,  the 
selection  of  indicator  being  made  with  reference  to  the  substance 
titrated  rather  than  to  the  standard. 

Many  of  the  indicators  undergo  a  color  change  upon  the  addi- 
tion of  certain  salts,  as  well  as  of  acids  or  bases.  In  all  such  cases 
the  salt  is  one  derived  from  an  acid  and  a  base  of  unequal  degree 
of  ionization.  The  result  of  the  partial  hydrolysis  of  such  a  salt 
is  the  production  of  an  excess  of  ions  of  either  hydrogen  or 
hydroxyl,  according  to  whether  the  acid  or  the  base  is  the  more 
strongly  ionized.  An  indicator  of  the  proper  sensibility  will  be 
affected  exactly  as  though  the  solution  were  that  of  an  acid  or  a 
base.  Thus  sodium  carbonate  in  solution  yields,  by  partial 
hydrolysis,  sodium  hydroxide  and  bicarbonate.  The  difference 
in  degree  of  ionization  of  these  two  electrolytes  is  so  great  that  all 
indicators  show  the  color  that  is  ordinarily  exhibited  in  basic 
solutions.  On  the  other  hand  the  hydrolysis  of  ferric  chloride 
yields  a  strong  acid  and  a  weak  base  and  here  again  the  difference 
in  ionization  is  sufficiently  large  to  give  an  "acid  reaction''  with 


212  QUANTITATIVE  ANALYSIS 

most  indicators.  The  exact  point  at  which  the  indicator  will 
change  color  depends  upon  the  relative  strength  of  acid  and  base 
and  also  upon  the  sensibility  of  the  indicator  itself.  If  a  solution 
of  sodium  carbonate  containing  methyl  orange  is  titrated  by 
hydrochloric  acid  the  color  change  does  not  occur  until  the 
sodium  carbonate  is  completely  decomposed,  according  to  the 
following  equation : 

Na2C03+2HC1^2NaCl+H2C03. 

This  is  because  methyl  orange  is  practically  insensible  to  such  a 
weakly  ionized  acid  as  carbonic  acid  and  a  slight  excess  of  hy- 
drochloric acid  is  necessary  in  order  to  affect  the  indicator.  If 
phenolphthalein,  an  indicator  of  high  sensibility  to  acids,  is  used 
instead  of  methyl  orange  the  color  change  occurs  when  one-half 
of  the  reaction  represented  above  is  completed: 

Na2C03+HCl^NaHC03+NaCl. 

In  other  words  sodium  bicarbonate,  yielding  upon  hydrolysis 
two  equivalents  of  carbonic  acid  for  one  of  sodium  hydroxide, 
is  a  neutral  body  to  an  indicator  that  is  easily  affected  by  even 
weak  acids  and  only  with  difficulty  by  bases.  Orthophosphoric 
acid  may  be  taken  as  a  final  example.  The  molecule  of  this  sub- 
stance contains  three  atoms  of  ionizable  hydrogen.  Two  of  these 
atoms  are  ionized  to  but  a  small  extent.  If  the  acid  is  titrated 
by  a  solution  of  a  strong  base  the  point  at  which  the  color  change 
occurs  ("  end-point ")  will  depend  upon  the  indicator  used.  If 
the  indicator  is  litmus  the  color  changes  from  red  to  blue  gradually 
instead  of  suddenly  and  this  change  comes  after  one-third  and 
before  two-thirds  of  the  hydrogen  is  neutralized.  In  the  pres- 
ence of  methyl  orange  the  color  changes  at  the  completion  of 
the  reaction: 

NaOH+H3P04->NaH2PO4+H2O. 

In  the  presence  of  phenolphthalein  the  end  point  is  somewhat  in- 
definite but  occurs  at  the  neutralization  of  two-thirds  of  the  acid : 

2NaOH + H3P04-»Na2HP04 + 2H20. 
DESCRIPTION  OF  INDICATORS 

Following  is  a  brief  discussion  of  the  preparation  and  properties 
of  the  indicators  named  in  the  table  on  page  211. 


COLOR  CHANGE  OF  INDICATORS  213 

Phenolphthalein. — The  chemical  nature  and  changes  of  phenol- 
phthalein  have  already  been  discussed.  The  compound  is  pre- 
pared by  heating  together  5  parts  of  phthalic  anhydride,  10 
parts  of  phenol  and  4  parts  of  concentrated  sulphuric  acid  for 
several  hours  at  a  temperature  between  120°  and  130°.  The 
mass  is  then  boiled  with  water,  filtered  and  the  residue  dissolved 
in  dilute  sodium  hydroxide  solution.  The  solution  is  filtered  and 
neutralized  with  hydrochloric  acid.  Phenolphthalein  precipi- 
tates and  is  purified  by  recrystallization  from  alcohol.  The 
pure  substance  is  a  yellowish-white  crystalline  powder,  practically 
insoluble  in  water  but  soluble  in  alcohol.  For  use  in  volumetric 
analysis  a  solution  of  5  gm  in  1000  cc  of  50  percent  alcohol  is  used. 

Rosolic  Acid  (Corallin). — Commercial  rosolic  acid  is  a  mix- 
ture of  aurine,  CigHuOs,  oxyaurine,  Ci9Hi6O6,  methylaurine, 
CsoHieOs  and  pseudorosolic  acid,  CaoHieO^  Each  of  these  sub- 
stances contains  the  quinone  ring.  It  is  prepared  by  heating 
together  phenol,  sulphuric  acid  and  oxalic  acid.  The  changes 
occurring  as  acids  or  bases  are  added  are  not  thoroughly  under- 
stood. The  solution  as  used  in  the  laboratory  is  a  1  percent 
solution  in  60  percent  alcohol.  The  indicator  is  red  with  bases 
and  yellow  with  acids.  It  is  highly  sensitive  to  acids  and  can  be 
used  for  the  titration  of  weak  acids. 

Litmus. — Litmus  is  obtained  by  the  action  of  ammonium 
hydroxide  and  potassium  hydroxide  upon  certain  species  of 
plants,  followed  by  fermentation.  The  essential  constituent  of 
the  indicator  is  azolitmin,  C7H7N04,  of  unknown  constitution. 
It  is  colored  red  by  acids  and  blue  by  bases.  Its  sensitiveness 
toward  both  acids  and  bases  is  only  moderate  and  this  fact  makes 
it  a  very  valuable  indicator  for  general  qualitative  purposes  but 
of  little  use  for  quantitative  analysis.  A  10  percent  solution 
in  water  is  used  in  the  laboratory. 

p-Nitrophenol. — This  indicator  is  prepared  by  the  action  of 

/N02  (4) 
nitric  acid  upon  phenol.     Its  formula,C6H4<( 

X)H  (1), 

is  indicated  by  its  name.  It  is  yellow  with  bases  and  colorless 
with  acids.  It  is  applicable  to  the  same  uses  as  is  litmus.  The 
indicator  solution  is  a  0.02  percent  solution  in  water.  It  should 
not  be  kept  in  a  closely  stoppered  bottle. 


214  QUANTITATIVE  ANALYSIS 

Methyl  Orange. — The  constitution  and  chemical  properties 
have  already  been  discussed.  The  substance  as  obtained  in 
commerce  is  usually  the  sodium  salt.  This  is  a  yellow  powder, 
soluble  in  water.  A  solution  containing  0.5  gm  in  1000  cc  is 
used.  In  presence  of  ammonium  salts  the  indicator  is  not  very 
sensitive.  Also  the  color  is  destroyed  by  iron  or  aluminium 
salts. 

Ethyl  Orange. — This  substance  is  analogous  in  constitution  to 
methyl  orange,  it  being  the  diethyl  ester  of  amidoazobenzene- 
sulphonic  acid.  Its  properties  and  uses  are  the  same  as  those  of 
methyl  orange. 

Cochineal. — Cochineal  is  the  dried  female  insect  Coccus  cacti 
Linne".  The  essential  coloring  matter  is  carminic  acid,  CiiHi207 
whose  constitution  is  not  definitely  established.  The  solution 
for  use-  as  an  indicator  is  made  by  digesting  3  gm  of  the  dried, 
unpowdered  insects  with  250  cc  of  75  percent  alcohol  until 
the  coloring  matter  is  extracted,  then  decanting.  The  bottles 
containing  the  solution  should  not  be  closely  stoppered.  The 
indicator  is  violet  with  bases  and  red  with  acids.  Its  sensitive- 
ness is  not  diminished  by  ammonium  salts. 

Lacmoid. — Lacmoid  is  prepared  by  heating  together  resorcin, 
sodium  nitrite  and  water.  It  is  a  deep  blue  dye  of  unknown 
constitution;  the  molecular  composition  is  probably  represented 
by  the  formula  Ci2H9N04.  It  is  soluble  in  alcohol  and  less  so 
in  water.  In  solution  it  is  colored  blue  by  bases  and  red  by 
acids.  Its  sensitiveness  toward  both  acids  and  bases  is  moderate 
but  it  finds  an  application  in  the  titration  of  carbonates  in  boiling 
solution,  carbonic  acid  being  decomposed  as  fast  as  it  is  formed. 
Litmus  may  be  used  in  the  same  way  but  is  not  so  sensitive  in  hot 
solutions.  The  indicator  solution  should  contain  2  gm  of  purified 
lacmoid  in  1000  cc  of  50  percent  alcohol. 

Erythrosine  (lodeosine). — This  dye  is  tetraiodofluorescein, 
a  synthetic  derivative  of  fluorescein,  and  it  has  the  constitution 
represented  by  the  formula 

CO 

C6HI2OH 

>0 
6HI2OH. 


COLOR  CHANGE  OF  INDICATORS  215 

The  substance  is  therefore  a  phthalein.  It  is  almost  insoluble 
in  water  but  is  soluble  in  hot  alcohol  and  ether.  A  solution 
of  0.5  gm  of  the  sodium  salt  in  1000  cc  of  recently  boiled  water 
is  used  as  indicator.  It  is  colored  red  by  bases  and  light  yellow 
by  acids.  It  is  highly  sensitive  to  bases  and  is  therefore  appli- 
cable to  the  titration  of  the  alkaloids.  Its  sensitiveness  to  acids 
is  correspondingly  small  so  that  it  may  be  used  for  the  titration 
of  carbonates  of  the  alkali  and  alkaline  earth  metals  without 
boiling  to  expel  carbonic  acid.  On  account  of  its  limited  solu- 
bility titrations  are  made  by  adding  10  to  20  cc  of  the  solution 
in  ether  to  the  titrating  solution,  shaking  after  each  addition 
of  acid. 

Methyl    Red.  —  This    dye    is    p-dimethylaminoazobenzene-o- 
carboxylic  acid: 


The  indicator  solution  is  prepared  by  dissolving  1  gm  of  the  solid 
in  100  cc  of  95  percent  alcohol.  The  solution  is  pale  yellow  in 
basic  solutions  and  violet  red  with  acids.  It  belongs  to  Class  III 
and  is  especially  good  for  the  titration  of  ammonium  hydroxide 
and  the  alkaloids,  all  being  weak  bases.  It  cannot  be  used 
if  much  carbonic  acid  is  present,  hence  is  useless  for  the  titration 
of  carbonates. 

All  indicator  solutions  must  be  adjusted  to  exact  neutrality  before 
using. 


CHAPTER  VII 
STANDARDIZATION 

Thus  far  we  have  dealt  with  only  the  calculation  of  the  results 
of  analyses,  assuming  that  the  standard  solution  was  ready  for 
use  in  the  experiment.  The  determination  of  the  concentration 
of  the  standard  solution  is  called  "  standardization."  The 
details  of  the  experimental  work  will  be  treated  later  and  will  be 
mentioned  here  only  so  far  as  they  may  serve  to  illustrate  the 
methods  of  calculation. 

Standardization  may  be  accomplished  by  one  or  more  of  four 
methods : 

Direct  Weighing. — The  active  substance  of  the  solution 
is  accurately  weighed  and  dissolved  to  make  a  definite  volume 
of  solution.  This  method  is  applicable  to  only  those  substances 
that  can  be  obtained  in  a  pure  state  or  in  a  state  of  uniform  and 
accurately  known  composition.  Most  of  such  substances  are 
crystallized  salts  or  acids,  or  soluble  gases. 

Weighing  a  Substance  Produced  by  a  Measured  Volume 
of  the  Solution. — Sulphuric  acid  solution  may  be  standardized 
by  precipitating  a  measured  volume  by  adding  an  excess  of 
barium  chloride.  From  the  weight  of  barium  sulphate  found 
the  weight  of  sulphuric  acid  may  be  calculated  by  the  method 
given  on  page  196.  Similarly  hydrochloric  acid  solution  may  be 
standardized  by  precipitating  as  silver  chloride. 

Measuring  the  Volume  of  Solution  Required  to  React  with 
a  Known  Weight  of  a  Substance  of  Known  Purity. — An  acid 
may  be  allowed  to  react  with  a  pure  carbonate  and  the  required 
volume  noted.  Sodium  thiosulphate  solution  may  likewise 
be  titrated  against  a  weighed  quantity  of  iodine  (or  indirectly) 
against  a  weighed  quantity  of  arsenic  trioxide. 

Titration  Against  Another  Solution  Which  has  Already  Been 
Standardized. — This  is  a  very  common  expedient. 

216 


STANDARDIZATION  217 

Primary  Standards.  —  It  will  be  noticed  that  in  each  of  these 
cases  there  is  some  substance  of  known  composition  that  is 
measured  or  weighed  and  the  solution  is  somehow  compared 
with  this  for  standardization.  This-  substance  of  known  com- 
position is  called  the  "  primary  standard,"  whether  it  be  the 
substance  dissolved  in  the  solution,  something  produced  by  the 
solution  or  something  reacting  with  the  solution. 

The  following  examples  will  illustrate  the  methods  of  calcula- 
tion in  each  of  the  cases  discussed. 

(1)  The  method  of  calculation  for  the  first  method  of  stand- 
ardization is  self-evident.     The  normality  is  equal  to  the  ratio 
of  the  number  of  grams  dissolved  in  1000  cc  to  the  number  of 
grams  in  1000  cc  of  a  normal  solution.     That  is, 

.      _  grams  per  1000  cc 
~  equivalent  weight 

(2)  A    solution    of   hydrochloric    acid   was    standardized   by 
precipitating  the  chlorine  from  40  cc,  as  silver  chloride.     The 
weight  of  silver  chloride  found  was  0.6327  gm.     Required,  the 
normality  of  the  solution. 

0.6327 
1    cc  acid   solution  =0=  —  —  —  gm  silver  chloride. 

1  cc  normal  acid  solution^  0.1433  gm  silver  chloride. 


Therefore  normality  =  °'^27  -*•  0  .  1433  =  =  0.  1  107  N. 


To  make  the  solution  decinormal  1000  cc  would  be  diluted  to 
1107  cc. 

(3)  A  similar  solution  was  standardized  by  titration  of  pure 
sodium  carbonate  in  presence  of  methyl  orange,  the  following 
reaction  being  completed: 

Na2CO3+2HCl->2NaCl+H2CO3. 

It  was  found  that  32.2  cc  acid  o  0.1638   gm  of  the  primary 
standard,  sodium  carbonate.     Required  the  normality. 

0.1638  ' 
1    cc   acldo~32~2~  gm  sodium  carbonate  and 

1    cc   normal  acido  0.053  gm  sodium  carbonate. 
Therefore  normality  =^^53  =0.9598  N. 


218  QUANTITATIVE  ANALYSIS 

(4)  Another  acid  solution  was  standardized  by  titration  against 
a  measured  volume  of  standard  potassium  hydroxide  solution 
in  presence  of  methyl  orange  according  to  the  equation: 

HC1+KOH-^KC1+H20. 

1  cc  of  the  primary  standard  contained  0.00468  gm  of  potas- 
sium hydroxide.  It  was  found  from  the  titration  that  50  cc 
of  potassium  hydroxide  solution— 43. 5  cc  of  hydrochloric  acid 
solution. 

The  weight  of  potassium  hydroxide  in  50  cc  of  solution  = 
50X0.00468  gm.  Since  this  weight  was  equivalent  to  43.5 
cc  of  acid,  the  potassium  hydroxide  equivalent  to  1  cc  acid  = 
50X0.00468 


43.5 

50X0.00468 


gm.     The  normality  of  the  hydrochloric  acid  solu- 
0.095  N. 


43.5X0.0561 

In  case  the  primary  standard  is  a  solution  already  standardized 
in  the  normal  system  the  normalities  of  the  solutions  are  inversely 
as  the  respective  volumes  that  are  equivalent  to  each  other. 

N 
(5)  30.0  cc  of  ^r  sodium   thiosulphate   solution  is  found  by 

titration  to  be  equivalent  to  29.8  cc  of  iodine  solution.     The 
normality  of  the  latter  is  required. 


If  solutions  are  to  be  standardized  in  the  decimal  system 
the  calculations  involve  nothing  more  than  finding  the  weight  of 
the  substance  in  terms  of  which  the  standardization  is  to  be  ex- 
pressed, equivalent  to  1  cc  of  the  solution  which  is  being  stand- 
ardized, always  using  as  the  starting  point  the  known  weight  of 
the  primary  standard  and  following  the  method  explained  on 
page  196.  In  many  cases  the  standardization  is  to  be  expressed 
in  terms  of  the  primary  standard  itself.  For  example,  iodine 
solution  is  to  be  standardized  against  pure  arsenic  trioxide  and 
expressed  in  terms  of  the  same  substance.  Here  we  have  the 
very  simple  method  of  weighing  a  suitable  amount  of  arsenic 
trioxide,  then  dissolving  and  titrating  by  the  iodine  solution. 

Then  gm  As2Q3 

1   cc  iodine  solution  =o  -I_Bolution' 


STANDARDIZATION  219 

Other  familiar  examples  of  this  class  of  methods  are  the 
standardization  of  permanganate  solutions  against  elementary 
iron  or  antimony  for  obtaining  the  weights  of  these  elements 
equivalent  to  one  cubic  centimeter  of  the  solution. 

The  following  example  will  serve  to  illustrate  the  first  case 
just  discussed: 

(6)  A  solution  of  potassium  permanganate  was  standardized 
against  sodium  oxalate  as  follows:  2.5340  gm  of  sodium  oxalate 
was  dissolved  and  the  solution  was  diluted  to  1000  cc.  25  cc 
portions  were  titrated  and  gave  an  average  of  24.25  cc  of  potas- 
sium permanganate  solution  equivalent  to  the  oxalate  solution 
used.  Required  the  weight  of  iron  and  of  calcium  equivalent 
to  1  cc  of  permanganate  solution. 

25    cc   of   the  oxalate  solution  contained  0.025X2.5340  gm 

0.025X2.5340 
and  1  cc  of  permanganate  solution  is  equivalent  to .     _ 

gm  of  sodium  oxalate.  This  weight,  multiplied  by  the  ratio 
of  the  equivalent  weight  of  iron  or  of  calcium  to  that  of  sodium 
oxalate,  will  give  the  weights  of  these  substances  that  are 
equivalent  to  1  cc  of  the  standard  solution.  Then 

0.025X2.5340X55.88 
Ice  solution^        24.25X67.005        =0-00218  Sm  Fe 

or 

0.025X2.5340X20.035 

24.25X67.QQT~     =0-00078  gm  Ca. 

Problems 

35.  30.0  cc  of  sulphuric  acid  solution  yields  0.3625  gm  of  barium  sul- 
phate.    Calculate  the  normality  and  the  dilution  necessary  to  make  the 
solution  tenth-normal. 

36.  44.6  cc  of  silver  nitrate  solution  yields  1.2870  gm  of  silver  chloride. 
Calculate  the  normality  and  the  dilution  necessary  to  make  the  solution 
fifth-normal. 

37.  39.7  cc  of  barium  chloride  solution  yields  2.5346  gm  of  barium 
chromate.     Calculate   the   normality    and   dilute   to    make   the   solution 
half-normal. 

38.  An  acid  solution  is  standardized  by  titrating  pure  sodium  carbonate, 
using  methyl  orange  as  indicator.      45.1  cc=c=  2.4065  gm  of  sodium  car- 
bonate.    Calculate  the  normality  and  dilution  necessary  to  make  the  solu- 
tion exactly  normal. 


220  QUANTITATIVE  ANALYSIS 

39.  50  cc  oi  nitric  acid  solution  is  added  to  0.4530  gm  of  pure  calcium 
carbonate.     The  unused  excess  of  acid  is  titrated  by  a  solution  of  a  base 
and  6.15  cc  of  the  latter  is  required.     The  base  is  then  titrated  against 
the  acid  in  order  to  compare  their  concentrations  and  21.3  cc  of  acid  is 
found  to  be  equivalent  to  19.2  cc  of  base.     Calculate  the  dilution  necessary 
to  make  each  solution  fifth-normal. 

40.  A  sulphuric  acid  solution  is  standardized  by  titrating  a  sample  of 
potassium  bicarbonate  which  contains  98.45  percent  of  the  pure  compound. 
35  cc  acid  =0=  0.0391  gm  of  the  sample.     Calculate  the  normality  and  the 
dilution  necessary  to  make  the  solution  hundredth-normal. 

41.  32.9  cc  of  potassium  hydroxide  solution  exactly  neutralizes  0.3118 
gm  of  pure  potassium  acid  tartrate,  KHX^H^Oe-     Calculate  the  dilution 
necessary  to  make  the  solution  twentieth-normal. 

42.  45.9  cc  of  sodium  hydroxide  solution  was  added  to  a  solution  con- 
taining 0.25  gm  of  crystallized  oxalic  acid,   H2C2O4.2H2O,  the  indicator 
being  phenolphthalein.     The  excess  of  base  was  titrated  by  1.3  cc  of  acid. 
4.9  cc  of  the  acid  is  found  to  be  equivalent  to  50  cc  of  base.     Calculate 
the  normality  of  each  solution. 

43.  A  solution  of  barium  hydroxide  was  standardized  by  titration  against 
succinic  acid,  H2C4H4C>4,  in  presence  of  phenolphthalein.     20.9  cc  of  barium 
hydroxide  solution  neutralized  a  solution  of  1.22  gm  of  succinic  acid.     Cal- 
culate the  normality. 

44.  38.1  cc  of  a  sodium  bicarbonate  solution  exactly  neutralizes  36.7 
cc  of  a  tenth-normal  solution  of  hydrochloric  acid.     What  is  the  normality 
of  the  first  solution? 


CHAPTER  VIII 
EXPERIMENTAL  VOLUMETRIC  ANALYSIS 

Standard  Acids. — It  has  already  been  shown  that  the  most 
serviceable  acids  or  bases  for  standard  solutions  are  those  that 
belong  to  the  class  of  strong  electrolytes.  For  standard  acids 
this  practically  limits  one  to  the  use  of  hydrochloric,  sulphuric 
or  nitric  acid.  On  account  of  the  ease  with  which  it  may  be  re- 
duced, nitric  acid  is  not  to  be  recommended  for  standard  solu- 
tions of  general  application.  Of  the  first  two  acids  named,  hy- 
drochloric acid  is  usually  to  be  preferred  because  it  is  monobasic 
and  cannot  form  acid  salts. 

Materials  for  Standardization. — For  standardizing  an  acid 
use  may  be  made  of  any  method  which  involves  a  definite  re- 
action with  a  pure  substance  or  which  produces  a  precipitate 
or  gas  that  may  be  weighed  or  measured.  This  makes  either 
volumetric  or  gravimetric  methods  available.  Since  most  strong 
bases  are  hygroscopic  and  also  combine  readily  with  carbon  di- 
oxide, purification  and  weighing  are  difficult  and  these  substances 
are  unsuitable  for  use  as  primary  standards  for  standardizing 
acid  solutions.  We  have  left  the  alkali  and  alkaline  earth  car- 
bonates for  this  purpose.  Calcium  carbonate  and  sodium  car- 
bonate are  suitable  and  both  substances  may  be  obtained  nearly 
pure.  Precipitated  calcium  carbonate  may  be  used  but  it  is 
seldom  free  from  other  salts  and  an  analysis  must  be  made  be- 
fore it  is  used  for  standardizing.  The  form  most  often  used  is 
the  natural  crystallized  calcite  known  as  Iceland  spar,  which  is 
often  nearly  100  percent  calcium  carbonate,  although  its  purity 
should  not  be  assumed  without  analysis.  The  chief  disadvan- 
tage in  the  use  of  any  form  of  calcium  carbonate  for  standardizing 
acids  lies  in  the  fact  that  it  is  insoluble  in  water  and  an  excess 
of  acid  must  be  used  in  order  to  hasten  the  process  of  solution. 
In  such  a  case  a  direct  titration  cannot  be  made  but  the  excess 
of  acid  must  be  determined  by  titration  with  a  solution  of  a 

221 


222  QUANTITATIVE  ANALYSIS 

base,  whose  concentration  as  compared  with  the  acid  must  be 
known. 

Several  of  these  difficulties  may  be  obviated  by  the  use  of 
sodium  carbonate,  which  is  soluble  in  water.  As  this  substance 
is  obtained  in  commerce  it  contains  variable  quantities  of  water 
and  salts  incident  to  the  process  of  manufacture,  such  as  sodium 
sulphate  and  sodium  chloride.  This  is  largely  due  to  the  com- 
paratively large  solubility  of  the  salt  and  the  consequent  diffi- 
culty in  purifying  it  by  crystallization.  At  20°  a  saturated 
solution  contains  sodium  carbonate  to  the  extent  of  17.7  per- 
cent of  its  weight.  The  pure  salt  is  best  obtained  from  the 
bicarbonate  by  heating.  Sodium  bicarbonate  dissolves  to  the 
extent  of  8.8  percent  of  the  weight  of  solution  at  20°.  It  may 
therefore  be  more  readily  purified  by  fractional  crystallization, 
especially  if  the  purest  obtainable  commercial  salt  is  used  for 
the  purpose.  When  heated  sodium  bicarbonate  decomposes 
according  to  the  equation 

2NaHCO3-^Na2C03+H2O+CO2. 

The  dissociation  tension  of  carbon  dioxide  from  sodium  bicar- 
bonate is  as  follows:1 


Temperature  

55° 

60° 

70° 

80° 

90° 

100° 

Tension  mm.  of  mercury 

19 

25 

43 

70 

125 

310 

The  tension  of  carbon  dioxide  in  the  atmosphere  is  less  than  1 
mm  and,  in  consequence,  sodium  bicarbonate  will  slowly  de- 
compose at  temperatures  below  55°.  At  100°  the  decomposition 
is  fairly  rapid  and  the  bicarbonate  is  completely  changed  into 
normal  carbonate  by  heating  for  a  short  time  at  about  300°. 
At  still  higher  temperatures  the  normal  carbonate  will  yield 
some  sodium  oxide  and  carbon  dioxide. 

Gravimetric  Standardization. — Acids  may  also  be  standardized 
gravimetrically  in  case  insoluble  salts  can  be  produced.  Such  a 
method  will  apply  to  hydrochloric  or  sulphuric  acid  but  not  to 
nitric  acid,  since  no  insoluble  nitrate  is  known.  A  point  fre- 
quently overlooked  is  that  this  method  is  really  a  standardiza- 
tion with  respect  to  the  negative  radical  and  is  an  acid  standardi- 

»Lescoeur:  Ann.  chim.  phys.  [6]  28,  423  (1892). 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  223 

zation  only  in  case  no  salts  of  the  acid  are  present.  Even  in 
the  purest  commercial  acids  ammonium  salts  are  often  present 
and  the  weighing  of  silver  chloride  or  of  barium  sulphate  will 
thus  not  give  a  basis  for  the  correct  calculation  of  acid  strength. 
Standardization  by  Direct  Weighing. — It  is  possible  to  weigh 
the  active  substances  directly  in  the  exact  amount  necessary 
for  a  solution  of  desired  strength  only  in  case  the  substance  is 
available  in  pure  form.  This  is  not  the  case  with  most  of  the 
inorganic  acids1  and  with  comparatively  few  salts.  It  then 
becomes  necessary  to  calculate  the  approximate  quantity  re- 
quired to  make  a  solution  somewhat  stronger  than  that  desired, 
to  standardize  the  solution  so  made  and  dilute  exactly  to  the 
required  strength.  Such  dilution  may  be  accomplished  with 
accuracy  in  case  the  water  to  be  added  may  be  measured  in  a 
burette,  i.e.,  if  less  than  about  10  cc  is  required.  The  final 
volume  obtained  by  dilution  is  the  sum  of  the  initial  volume 
and  that  of  the  added  water  only  in  case  no  expansion  or  contrac- 
tion occurs  upon  mixing.  This  is  practically  the  case  if  the  solu- 
tion is  already  dilute  and  the  relative  amount  of  water  to  be  added 
is  small.  Dilution  may  then  be  accomplished  by  measuring  a 
specified  amount  of  solution  in  a  flask,  running  in  the  calculated 
amount  of  water  from  a  burette  and  mixing  directly  in  the  gradu- 
ated flask.  The  neck  of  the  flask  must  be  capable  of  easily 
holding  the  required  water  above  the  graduation.  This  fact, 
together  with  considerations  of  volume  changes  already  men- 
tioned, places  a  practical  limit  upon  the  amount  of  dilution 
that  may  be  accurately  made  in  one  process.  If  more  than  10 
cc  of  water  must  be  added  to  1000  cc  of  solution  it  is  necessary 
to  dilute  to  nearly  the  required  amount,  restandardize  and 
redilute  to  the  exact  value  required.  For  example,  if  it  is  found 

N 
that  a  solution  is  1.3462  X  TT:  and  it  is  necessary  to  dilute  the 

solution  to  make  it  exactly  tenth-normal,  each  1000  cc  must  be 
diluted  to  a  volume  of  1346.2  cc.  The  addition  of  346.2  cc 
of  water  could  not  be  accurately  made  because  there  is  no  gradu- 
ated vessel  capable  of  accurately  measuring  this  quantity. 
Such  an  addition  might  also  cause  an  appreciable  volume  change. 

1  Vide,  Moody:  J.  Chem.  Soc.,  73,  658  (1898),  and  Acree:  Am.  Chem.  J., 
36,  117  (1906). 


224  QUANTITATIVE  ANALYSIS 

The  correct  procedure  would  be  to  add  first  to  each  1000  cc 
of  solution  about  335  cc  of  water,  measured  in  a  graduated 
cylinder,  restandardize  and  then  carefully  complete  the  dilution 
in  the  manner  already  explained. 

Exercise:  Preparation  of  Pure  Sodium  Carbonate — Use  the  besi 
grade  of  sodium  bicarbonate  that  is  obtainable.  Make  qualitative  tests 
for  sulphates,  chlorides  and  potassium,  using  an  approximately  5  percent 
solution  of  the  material  for  these  tests.  If  impurities  are  found,  purtfy 
by  recrystallization  as  follows:  Make  a  saturated  solution  of  sodium 
bicarbonate  by  warming  the  purest  obtainable  salt  with  distilled  water. 
Decant  from  any  undissolved  matter  remaining  and  evaporate  the 
solution  in  a  large  porcelain  or  platinum  dish,  at  a  temperature  not 
higher  than  40°,  until  crystals  begin  to  separate.  Allow  to  cool  and 
stand,  uncovered  but  in  a  place  which  is  protected  from  dust,  until 
about  25  gm  of  crystals  have  formed,  pour  off  the  solution,  wash  the 
crystals  once  with  cold  water  and  press  between  filter  paper.  Dry  at 
100°,  powder  and  heat  in  a  platinum  crucible  at  a  temperature  between 
270°  and  300°  until  the  weight  is  constant.  The  product  should  be  pure 
sodium  carbonate  but  a  test  for  sulphates  and  chlorides  should  be  made. 
Preserve  in  a  closely  stoppered  weighing  bottle. 

Exercise:  Preparation  of  Tenth-normal  Hydrochloric  Acid. — Deter- 
mine the  specific  gravity  (if  not  already  known)  and  from  this  the  per- 
cent of  hydrochloric  acid  in  the  concentrated  solution  found  in  the 
laboratory.  From  the  data  so  obtained  calculate  the  weight  or  volume 
necessary  to  make  2500  cc  of  tenth-normal  solution.  Measure  2  percent 
more  than  this  amount  into  a  1000  cc  graduated  flask  and  fill  to  the  mark 
with  water.  Empty  into  a  bottle  having  a  capacity  of  about  2500  cc 
and  add  1500  cc  more  of  water.  Stopper  and  mix  thoroughly  by 
shaking.  Since  the  solution  has  been  warmed  by  the  reaction  between 
acid  and  water  it  should  be  allowed  to  stand  until  the  temperature  of  the 
room  is  attained  before  standardizing. 

Calculate  the  weight  of  sodium  carbonate  necessary  to  make  250  cc 
of  a  tenth-normal  solution.  Weigh  this  quantity  of  the  prepared  pure 
material  on  counterpoised  glasses,  then  brush  into  a  beaker.  Dissolve 
the  weighed  carbonate  in  distilled  water  and  carefully  rinse  into  a  250  cc 
graduated  flask.  Fill  to  the  mark  and  mix  thoroughly.  Imperfect 
mixing  is  often  found  to  be  the  source  of  discrepancies  in  titrations  with 
the  acid  solution.  The  solution  will  not  remain  constant  in  concentra- 
tion and  should  not  be  kept  for  more  than  one  day.  Fill  a  burette  with 
the  solution  and  another  with  the  acid  solution.  Before  making  the 
titrations  practice  reading  the  color  change  as  follows:  Place  100  cc 
of  distilled  water  in  a  beaker  and  add  a  drop  of  methyl  orange  and  0.5  cc 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  225 

of  sodium  carbonate  solution.  Drop  in  the  acid  solution  until  the  last 
drop  changes  the  tint  from  yellow  to  pink.  Now  drop  in  sodium  car- 
bonate solution  until  the  yellow  color  reappears.  Repeat  the  alternate 
additions  of  carbonate  and  acid  until  the  color  change  can  be  observed 
when  but  one  drop  of  either  solution  is  added.  It  will  aid  in  the  next 
process  if  this  solution  is  preserved  and  another  prepared,  the  two 
showing  the  two  colors  of  methyl  orange.  These  may  be  set  aside  for 
comparison.  Measure  40  cc  of  the  carbonate  solution  into  another 
beaker  or  Erlenmeyer  flask,  dilute  to  about  100  cc  and  titrate  with  the 
acid  solution  in  presence  of  a  drop  of  methyl  orange.  Calculate  the 
normality  of  the  solution,  also  the  volume  of  water  to  be  added  to  each 
1000  cc  to  make  exactly  tenth-normal.  If  water  to  be  added  is  more 
than  10  cc  add  nearly  the  required  amount  to  each  liter  of  the  acid, 
mix,  and  restandardize.  If  the  quantity  to  be  added  is  less  than  10 
cc  the  acid  is  diluted  as  follows:  Fill  a  dry  1000  cc  graduated  flask 
to  the  mark  with  the  acid  solution.  This  flask  should  be  capable  of 
holding  the  required  amount  of  water  above  the  mark.  From  a  burette 
add  the  calculated  quantity  of  water  directly  to  the  solution  in  the 
flask  and  mix  thoroughly.  Pour  into  a  dry  bottle  and  make  a  second 
liter  of  diluted  solution  in  the  same  manner,  having  first  rinsed  and 
dried  the  graduated  flask.  Check  the  accuracy  of  the  dilution  by 
restandardization. 

Record  upon  the  label  of  the  bottle  your  name,  the  came  of  the 
standard  solution,  its  normality  and  the  date  of  standardization,  thus : 


Hydrochloric  Acid 
N/10 


Feb.  6,  1913 
FIG.  65. — Form  of  label  for  standard  solution. 

SODA  ASH 

The  commercial  grade  of  sodium  carbonate  known  as  "soda 
ash"  contains,  in  addition  to  sodium  carbonate,  considerable 
water,  some  potassium  carbonate  and  varying  quantities  of  other 
sodium  and  potassium  salts,  such  as  sulphates  and  chlorides.  A 
complete  analysis  would  include  the  determination  of  all  radicals 

15 


226  QUANTITATIVE  ANALYSIS 

but  on  account  of  the  fact  that  soda  ash  is  used  in  many  indus- 
tries because  of  its  basic  properties  its  valuation  generally  in- 
cludes a  determination  of  basicity  and  of  water  with  other 
impurities.  The  first  determination  may  be  made  by  direct 
titration  by  a  standard  .acid  in  the  presence  of  methyl  orange, 
while  the  last  may  be  determined  directly  or  taken  by  difference, 
in  which  case  the  percent  of  water  includes  all  other  impurities. 
In  view  of  the  fact  that  the  basicity  of  the  alkali  carbonates 
toward  methyl  orange  is  due  to  the  hydrolysis  of  a  salt  of  a  strong 
base  and  a  weak  acid,  it  is  obvious  that  any  salt  derived  similarly 
will  likewise  be  basic  and  that  therefore  the  titration  by  acid  is 
really  a  method  for  determining  the  radical  of  salts  of  weak  acids 
and  is  not  a  basis  for  the  calculation  of  any  particular  salt,  such 
as  sodium  carbonate.  Thus,  for  example,  if  a  mixture  of  sodium 
carbonate,  potassium  carbonate,  sodium  bicarbonale,  sodium 
silicate,  and  sodium  borate  were  being  titrated  these  salts  would 
all  be  decomposed  by  the  standard  acid  before  an  end  point  with 
methyl  orange  would  be  reached.  In  the  absence  of  a  more 
extended  analysis  it  would  be  impossible  to  calculate  the  percent 
of  any  one  of  these  radicals  or  compounds.  Since  all  are  basic 
in  solution  and  since  all  would  serve  for  most  purposes  where 
sodium  carbonate  is  used  industrially  it  is  customary  to  report  the 
percent,  arbitrarily  calculated,  of  either  sodium  carbonate  or 
sodium  oxide  (regarded  as  being  combined),  assuming  that  all 
basicity  of  soda  ash  is  due  to  sodium  carbonate.  It  should  also 
be  noted  that  sodium  carbonate  could  contain  either  alkali 
hydroxides  or  bicarbonates  but  not  both  in  the  same  sample. 
Ordinarily  no  attention  is  given  to  either. 

On  account  of  the  lack  of  uniformity  of  most  commercial  soda 
ash  it  is  necessary  to  select  a  rather  large  sample,  dissolving  in 
water  and  measuring  an  aliquot  part  for  the  titration.  The 
directions  for  sampling  on  pages  9  to  14  should  be  carefully 
followed  but  exposure  to  air  should  not  be  unduly  prolonged. 

Determination. — Fill  a  20  cc  weighing  bottle  with  soda  ash,  properly 
sampled.  Assuming  that  the  sample  is  pure  sodium  carbonate,  calcu- 
late the  approximate  weight  that  should  be  contained  in  500  cc  of 
solution  so  that  25  cc  shall  require  about  35  cc  of  tenth-normal  acid 
for  its  titration.  Weigh  this  quantity  of  soda  ash  into  a  graduated 
500  cc  flask,  dissolve  and  dilute  to  the  mark  with  water.  Mix  thor- 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  227 

oughly  by  inverting  the  flask  and  shaking.  Measure  out  25  cc  by  means 
of  a  calibrated  pipette,  allowing  this  portion  to  run  into  200  cc  breakers 
or  Erlenmeyer  flasks.  Add  just  enough  methyl  orange  to  tint  the  solu- 
tion and  then  tirate  with  the  tenth-normal  acid.  Make  at  least  two 
more  titrations  and  calculate  the  percent  of  sodium  carbonate,  also  of 
sodium  oxide,  assuming  that  the  basicity  of  the  substance  is  entirely 
due  to  the  sodium  oxide  combined  as  carbonate. 

"Pearl  ash"  (crude  potassium  carbonate)  may  be  evaluated 
in  a  similar  manner. 

MIXTURES  OF  CARBONATES  AND  BASES 

The  use  of  the  two  indicators,  methyl  orange  and  phenol- 
phthalein,  provides  a  means  for  the  determination  of  carbonates 
and  bicarbonates  when  in  mixture,  and  also  of  carbonates  and 
soluble  bases.  Bases  and  bicarbonates  (acid  salts)  cannot  occur 
in  the  same  mixture.  If  phenolphthalein  is  added  to  a  solution 
containing  sodium  carbonate  and  sodium  hydroxide  and  the 
solution  is  titrated  by  a  standard  acid,  the  end  point  is  reached 
when  the  sodium  hydroxide  is  neutralized  and  the  sodium  car- 
bonate is  changed  to  bicarbonate: 

NaOH+  HCl-»NaCl+  H2O,  (1) 

Na2CO3+HCl-»NaHCO«+NaCl.  (2) 

Since  the  solution  has  now  become  colorless,  methyl  orange  may 
be  added  and  the  titration  continued  until  the  red  tint  appears, 
when  the  sodium  bicarbonate  has  been  completely  decomposed: 

NaHCO3+  HCl-»NaCl+ H2CO3.  (3) 

Represent  by  A  the  cubic  centimeters  of  acid  used  in  completing 
the  first  titration  and  by  B  that  used  in  the  second. 

#X  normality  XO.  106  =  gm  of  sodium  carbonate, 
(A—  B)X normality X 0.040  =  gm  of  sodium  hydroxide. 

It  must  be  noted  that  here  the  equivalent  weight  of  sodium  carbon- 
ate is  106  instead  of  53,  since  by  equation  (3)  IHCl^  !NaHCO3 
and  by  equation  (2)  lNaHCO3-lNa2C03. 

The  quantitative  conversion  of  sodium  carbonate  into  sodium 
bicarbonate  by  means  of  an  acid  can  take  place  only  when  care  is 


228  QUANTITATIVE  ANALYSIS 

taken  to  prevent  the  escape  of  carbon  dioxide.  At  the  point 
where  the  acid  enters  the  solution  there  is  at  first  complete  con- 
version of  a  part  of  the  carbonate  into  the  normal  sodium  salt 
of  the  added  acid.  If  carbon  dioxide  escapes  from  the  solution 
at  this  point  more  acid  will  be  required  to  produce  the  first 
end  point  than  would  otherwise  be  the  case.  The  escape  of  gas 
may  be  prevented  by  keeping  the  solution  at  a  temperature  near 
0°  and  by  adding  the  standard  acid  very  slowly  and  while  stirring 
vigorously.1  It  is  very  difficult  to  avoid  the  escape  of  carbonic 
acid  unless  the  relative  amount  of  carbonate  is  small.  On  this 
account  the  method  is  best  suited  to  bases,  in  which  carbonate 
occurs  as  an  impurity,  rather  than  to  materials  in  which  carbonate 
is  an  essential  constituent. 

Determination  of  Hydroxide  and  Carbonate  in  Commercial  "Caustic 
Soda"  or  "Caustic  Potash." — Arbitrarily  assuming  that  the  sample  is 
pure  sodium  or  potassium  hydroxide,  calculate  the  approximate  quan- 
tity necessary  to  dissolve  and  dilute  to  1000  cc  so  that  50  cc  shall  require 
about  40  cc  of  tenth-normal  acid.  From  a  stoppered  weighing  bottle 
weigh  this  amount  of  well-mixed  sample  into  a  1000  cc  graduated  flask. 
Dissolve  in  500  cc  of  water  and  cool  to  20°.  Dilute  to  the  mark,  mix, 
measure  out  50  cc  by  means  of  a  pipette,  add  a  drop  of  phenolphthalein, 
cool  to  0°  in  ice  water  and  titrate  to  the  disappearance  of  the  pink  color. 
Add  a  drop  of  methyl  orange  and  continue  the  titration  to  the  next 
color  change.  Calculate  the  percent  of  sodium  carbonate  and  of  sodium 
hydroxide. 

MIXTURES  OF  CARBONATES  AND  BICARBONATES 

If  a  mixture  of  a  carbonate  and  bicarbonate  is  to  be  investi- 
gated the  procedure  is  the  same  as  in  the  preceding  exercise.  In 
this  case  the  phenolphthalein  changes  color  at  the  completion 
of  the  reaction 

Na2CO3+HCl->NaHCO3+NaCl. 

When  the  color  change  of  methyl  orange  occurs  the  sodium  bi- 
carbonate so  produced,  as  well  as  that  originally  present,  has 
been  decomposed.  If  A  is  the  acid  used  in  the  first  titration  and 
B  that  used  in  the  second  titration 

A  X  normality  X  0.1 06  =  gm  of  sodium  carbonate,  and 
(B—A)  X normality  X  0.084  =  gm  of  sodium  bicarbonate. 
1  Kuster:  Z.  anorg.  Chem.,  13,  127  (1897). 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  229 

These  calculations  are  based  upon  the  same  arbitrary  assumption 
regarding  the  presence  of  other  salts  as  is  noted  in  the  discussion 
of  the  valuation  of  soda  ash. 

Determination  of  Sodium  Carbonate  and  Bicarbonate  in  a  Mixture. — 

Proceed  as  in  the  preceding  exercise,  except  that  the  calculation  is  to  be 
made  as  above  indicated.  A  thermometer  must  be  placed  in  the  solu- 
tion and  the  temperature  lowered  to  0°.  The  standard  solution  is  added 
very  slowly. 

HARDNESS  AND  ALKALINITY  OF  WATER 

The  difference  between  "hard"  and  "soft"  water  lies  in  the 
fact  that  the  former  contains  various  inorganic  compounds 
which  form  insoluble  soaps  when  used  with  soap  for  cleansing. 
The  salts  of  calcium  and  magnesium  are  chiefly  responsible  for 
this  action.  When  ordinary  soap  is  dissolved  in  such  a  water 
there  is  at  once  formed  a  precipitate  of  calcium  and  magnesium 
salts  of  the  fatty  acids. 

Temporary  Hardness. — Bicarbonates  of  the  metals  named 
above  are  quite  common  in  ground  waters.  When  the  water  is 
boiled  the  excess  of  carbonic  acid  is  expelled  and  normal  car- 
bonates are  formed: 

Ca(HCO3)2-»CaCO3+H20+CO2; 
Mg(HCO3)2^MgCO3+H2O+C02. 

Because  of  the  very  limited  solubility  of  the  normal  carbonates 
a  precipitate  is  formed  and  the  hardness  of  the  water  is  diminished 
to  this  extent.  Such  hardness  as  is  removed  in  this  manner  by 
boiling  is  known  as  "temporary  hardness"  and  the  water  so 
treated  is  partly  "softened." 

Permanent  Hardness  is  due  to  the  slight  amount  of  normal 
carbonates  (about  0.013  gm  per  liter  at  20°)  that  remains  to 
saturate  the  water,  and  to  non-decomposable,  soluble  salts  of 
calcium,  magnesium,  iron  or  other  metals  that  may  form  in- 
soluble soaps  of  fatty  acids.  The  most  common  of  such  com- 
pounds are  chlorides,  sulphates  and  nitrates. 

Alkalinity. — The  "alkalinity"  of  a  natural  water  represents  its 
content  of  carbonate,  bicarbonate,  borate,  silicate,  phosphate 
and  hydroxide,  or  such  of  these  as  may  be  present.  It  is  obvious 
that  hydroxides  and  bicarbonates  cannot  exist  in  the  same  solu- 


230  QUANTITATIVE  ANALYSIS 

tion.  Also  a  water  which  contains  sodium  or  potassium  carbon- 
ate is  not  likely  to  contain  calcium,  magnesium  or  iron  as  bi- 
carbonates, because  reactions  like  the  following  would  occur: 

Ca(HCO3)2+Na2C03^CaCO3+2NaHC03. 

Methods  for  the  Determination. — If  none  of  the  other  salts 
mentioned  as  causing  alkalinity  were  present  a  simple  titration 
with  standard  acid  in  presence  of  methyl  orange  or  erythrosine 
would  serve  as  a  determination  of  bicarbonate  hardness.  Also  if 
one  could  neglect  the  fact  that  carbonates  are  not  entirely  precipi- 
tated when  formed  by  heating  bicarbonates,  such  a  determination 
of  bicarbonate  hardness  could  be  reported  as  of  temporary 
hardness.  Because  neither  of  these  conditions  is  fulfilled  the 
most  accurate  method  for  the  determination  of  temporary  hard- 
ness is  by  making  the  titration  in  presence  of  methyl  orange 
before  and  after  boiling. 

Permanent  hardness  equals  non-carbonate  hardness  plus  hard- 
ness due  to  a  saturated  solution  of  calcium  carbonate,  mag- 
nesium carbonate,  etc.,  as  already  explained.  If  a  water  should 
have  no  temporary  hardness  its  non-carbonate  hardness  and 
permanent  hardness  would  be  identical.  Non-carbonate  hard- 
ness is  best  determined  by  the  use  of  "soda  reagent/'  a  standard 
solution  of  equal  weights  of  sodium  hydroxide  and  sodium 
carbonate.  A  measured  quantity  of  the  water  is  boiled  to 
decompose  bicarbonates  and  to  this  is  added  the  standard  solu- 
tion in  excess.  Reactions  like  the  following  take  place: 

CaS04+Na2C03-*CaCO3+Na2S04; 
MgCl2+Na2C03->MgCO3+2NaCl. 

After  filtering  to  remove  insoluble  carbonates  the  solution  is 
titrated  by  standard  acid  in  presence  of  methyl  orange  and  the 
hardness  is  calculated  from  the  amount  of  soda  reagent  that 
has  been  used  by  reactions  like  those  represented  above. 

The  use  of  sodium  hydroxide  in  the  standard  solution  appears 
to  diminish  the  solubility  of  the  normal  carbonates  that  are 
formed. 

Clark's  method  for  the  determination  of  total  hardness  is  based 
upon  the  use  of  a  standard  soap  solution,  added  until  a  permanent 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  231 

lather  is  produced.  It  was  formerly  extensively  used  but  it 
is  inaccurate  and  is  now  little  used. 

Expression  of  Results. — The  small  percent  of  dissolved  salts 
usually  found  makes  desirable  a  method  of  expressing  results 
which  is  different  from  that  used  in  other  connections.  Instead 
of  percent  it  is  customary  to  report  parts  per  hundred  thousand 
of  water,  parts  per  million  or  grains  per  gallon.  The  Imperial 
English  gallon  of  water  weighs  70,000  grains  while  the  United 
States  gallon  weighs  in  air  at  15.5°  and  760  mm  pressure,  58335+ 
grains.  This  gives  at  least  four  different  systems  which  have 
been  at  various  times  and  in  various  countries  commonly  used 
for  the  expression  of  the  results  of  water  analysis.  Hardness 
itself  has  also  been  expressed  in  Clark's  degrees  (grains  of  calcium 
carbonate  per  Imperial  gallon),  German  degrees  (parts  of  calcium 
oxide  per  100,000  parts  of  water)  and  French  degrees  (parts  of 
calcium  carbonate  per  100,000  parts  of  water).  This  has  re- 
sulted in  much  confusion  but  there  is  now  a  general  tendency 
toward  the  practice  of  expression  in  parts  per  million  or,  more 
exactly,  milligrams  per  liter,  although  in  industrial  operations 
the  report  is  often  made  as  grains  per  United  States  gallon. 
Upon  the  assumption  that  one  liter  of  water,  at  the  usual  working 
temperature,  weighs  1000  gm,  every  milligram  of  dissolved 
matter  will  represent  one  part  per  million  of  water.  This  as- 
sumption is  not  quite  correct  and  "milligrams  per  liter"  is  a 
better  expression  than  "parts  per  million." 

The  following  exercises  may  be  performed  at  this  point  by 
students  who  will  not  carry  out  a  more  extensive  analysis  of 
water  later.  (See  page  339.) 

Determination  of  Alkalinity. — Prepare  a  fifth-normal  solution  of 
sulphuric  acid,  foPowing  the  general  directions  given  on  page 
224  for  tenth-normal  hydrochloric  acid  and  standardizing  against  pure 
sodium  carbonate  in  presence  of  methyl  orange.  Dilute  100  cc  of  this 
fifth-normal  solution  to  1000  cc  with  recently  boiled  and  cooled  distilled 
water.  The  work  of  standardization  and  dilution  must  be  done  very 
carefully  in  order  to  avoid  the  necessity  for  ^standardization  of  the  last 
solution. 

.  Potassium  acid  sulphate  may  be  substituted  for  sulphuric  acid  in  this 
determination.  As  this  substance  usually  contains  considerable  water 
due  allowance  should  be  made  when  calculating  the  required  weight  for 
the  fifth-normal  solution. 


232  QUANTITATIVE  ANALYSIS 

Measure  100  cc  of  the  water  sample  in  a  volumetric  flask  and  rinse 
into  a  porcelain  dish  or  casserole  with  recently  boiled  water ;  or  measure 
the  sample  directly  into  the  porcelain  dish  by  means  of  a  pipette.  Add  2 
drops  of  methyl  orange  indicator  and  titrate  with  the  fiftieth-normal  acid 
solution  already  prepared.  Calculate  the  alkalinity  in  the  conventional 
way  as  milligrams  of  calcium  carbonate  per  liter. 

Determination  of  Temporary  Hardness. — Determine  the  alkalinity 
of  the  original  sample  as  already  directed.  Measure  a  second  portion  of 
200  cc  of  the  sample  and  rinse  into  a  500  cc  Erlenmeyer  flask.  Boil 
gently  for  10  minutes,  cover  and  cool  to  room  temperature  and  immedi- 
ately rinse  into  the  200  cc  volumetric  flask.  Dilute  to  the  mark  with 
recently  boiled  and  cooled  distilled  water  and  mix  thoroughly.  It 
is  important  to  have  distilled  water  which  is  quite  free  from  carbonic 
acid  for  this  determination,  as  otherwise  part  of  the  precipitated  carbon- 
ates will  be  redissolved  and  later  titrated  as  alkalinity.  On  this  account 
the  wash  bottle  must  not  be  used  in  the  ordinary  manner  *by  blowing 
into  it. 

Stopper  the  flask  containing  the  boiled  sample  and  allow  to  stand 
until  the  precipitate  has  settled,  then  remove  100  cc  of  the  clear  liquid 
by  means  of  a  pipette  and  redetermine  the  alkalinity.  The  difference 
(if  any)  between  the  two  titrations  represents  the  temporary  hardness. 

While  temporary  hardness  is  due  to  bicarbonates  of  calcium,  mag- 
nesium or  iron  (and  sometimes  other  metals  of  the  earth  or  alkaline- 
earth  groups)  it  is  customary  to  calculate  it  in  terms  of  calcium  carbon- 
ate, the  most  common  of  the  products  of  boiling  hard  water.  The 
convenience  of  a  fiftieth-normal  acid  for  this  use  should  be  noted.  Since 
the  equivalent  weight  of  calcium  carbonate  is  almost  exactly  50, 1  cc  of 
fiftieth-normal  acid  is  equivalent  to  1  mg  of  calcium  carbonate.  Report 
the  temporary  hardness  in  milligrams  of  calcium  carbonate  per  liter. 

Determination  of  Non-carbonate  Hardness. — Prepare  an  approxi- 
mately tenth-normal  (to  methyl  orange)  solution  of  soda  reagent,  using 
equal  weights  of  sodium  carbonate  and  sodium  hydroxide,  standardizing 
against  tenth  normal  hydrochloric  acid  and  using  recently  boiled  water 
for  dilutions  during  the  titration. 

Fill  a  dry  200  cc  volumetric  flask  (or  one  that  has  been  rinsed  with  the 
sample)  with  the  sample  of  water.  Rinse  this  into  a  500  cc  Erlenmeyer 
flask  of  resistance  glass,  using  200  cc  of  distilled  water.  Boil  for  15 
minutes  to  expel  carbon  dioxide  then  add  exactly  25  cc  of  standard 
soda  reagent  from  a  pipette.  Boil  for  10  minutes,  rinse  into  the  200  cc 
volumetric  flask  and  dilute  to  the  mark,  using  recently  boiled  and  cooled 
distilled  water.  Filter  through  a  dry  filter  and  discard  the  first  50  cc 
of  nitrate.  From  the  filtrate  that  is  subsequently  obtained  measure 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  233 

50  cc  portions  by  means  of  a  pipette  into  Erlenmeyer  flasks  and  titrate 
at  once  with  fiftieth-normal  sulphuric  acid  or  potassium  acid  sulphate, 
using  methyl  orange  or  erythrosine.  If  erythrosine  is  used,  add  1  cc 
of  the  indicator  solution  and  5  cc  of  neutral  chloroform,  titrating  until 
the  pink  color  just  disappears  from  the  chloroform  when  violently 
shaken. 

Calculate  the  non-carbonate  hardness  in  the  conventional  manner  by 
assuming  the  typical  calcium  sulphate  as  the  hardness-giving  agent 
and  reporting  milligrams  of  this  compound  per  liter.  In  waters  of  the 
"alkali"  type,  containing  carbonates  of  sodium  or  potassium,  a  negative 
value  will  be  found  for  non-carbonate  hardness,  i.e.,  more  standard 
acid  will  be  required  after  the  treatment  above  outlined  than  is  equiva- 
lent to  the  soda  reagent  added. 

Standard  Bases. — Standard  solutions  of  bases  are  subject  to 
change  in  basic  concentration  and  must  be  frequently  restand- 
ardized.  This  is  because  glass  is  appreciably  soluble  in  bases 
and  the  accumulation  of  alkali  silicates  in  the  solution  gives  an 
increase  in  basicity  toward  all  indicators.  Bases  also  absorb 
carbon  dioxide  when  exposed  to  the  air  and  this  results  in  a 
decreased  basicity  toward  indicators  of  the  class  of  phenol- 
phthalein.  For  this  reason  it  is  desirable  to  avoid  unnecessary 
contact  with  the  air  after  standardization. 

For  the  preparation  of  standard  basic  solutions  free  from 
carbonates  one  may  either  use  a  substance  whose  carbonate  is 
insoluble  or  one  which  contains  little  carbonate  and  whose 
carbonate  may  be  precipitated  by  the  addition  of  another  sub- 
stance. For  the  first  method  barium  hydroxide  is  generally 
used.  This  base  always  contains  some  barium  carbonate  but  this 
remains  undissolved  when  the  solution  is  made.  If,  thereafter, 
carbon  dioxide  is  absorbed  by  the  solution,  an  equivalent  amount 
of  barium  carbonate  is  precipitated  and  the  solution  remains 
free  from  carbonate,  although  it  must  be  restandardized.  Sodium 
hydroxide  or  potassium  hydroxide  may  be  obtained  nearly 
free  from  carbonates  by  dissolving  in  alcohol,  decanting  or 
filtering  from  undissolved  carbonate  and  evaporating  the 
alcohol  in  an  atmosphere  that  is  free  from  carbon  dioxide. 
Bases  so  prepared  may  now  be  obtained  from  the  manufacturers 
and  should  be  used  for  the  preparation  of  standard  solutions 
whenever  possible. 


234 


QUANTITATIVE  ANALYSIS 


For  the  second  method  potassium  hydroxide  or  sodium  hy- 
droxide may  be  used  and  a  slight  excess  of  barium  chloride  added 
to  the  solution,  barium  carbonate  being  thereby  precipitated. 
The  material  used  should  be  the  sticks  that  have  been  crystallized 
from  alcohol.  As  sodium  and  potassium  carbonates  are  only 


FIG.  66. — Burette  with  three-way  stopcock,  connected  with  reagent  bottle. 

slightly  soluble  in  alcohol  the  bases  from  this  solution  are  nearly 
free  from  carbonates.  In  either  case  reabsorption  of  carbon 
dioxide  is  prevented  by  passing  entering  air  through  a  soda  lime 
tube,  removing  the  solution  through  a  siphon  directly  to  the 
burette,  as  in  Fig.  66.  Covering  the  solution  with  a  layer  of 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  235 

toluene  or.  of  any  oily  substance  is  not  to  be  recommended 
because  this  results  in  fouling  the  burette. 

It  should  be  understood  that  these  precautions  are  unneces- 
sary in  most  titrations.  While  it  is  true  that  alkali  carbonates 
have  not  the  same  basicity  toward  some  of  the  indicators  as  have 
the  alkali  hydroxides,  a  proper  correction  is  made  by  standardiz- 
ing in  presence  of  the  indicator  that  is  to  be  used  in  the  determina- 
tions. This  is  a  principle  that  must  always  be  observed.  One 
must  not,  for  instance,  use  the  standardization  in  presence  of  methyl 
orange  as  a  basis  for  the  calculation  of  determinations  made  in 
presence  of  phenolphthalein. 

Selection  of  Base  for  Standard  Solutions. — The  proper  base 
for  a  standard  solution  will  depend  upon  the  nature  of  the  titra- 
tion  to  be  made.  For  reasons  discussed  on  page  211  one  will 
generally  select  a  highly  ionized  substance  and  from  this  stand- 
point sodium  hydroxide  and  potassium  hydroxide  are  about 
equally  good.  On  account  of  the  relative  cheapness  of  the  former 
it  should  be  given  preference,  wherever  possible.  However,  in 
certain  cases  (see  analysis  of  edible  oils,  beginning  on  page  345) 
a  standard  solution  of  a  base  is  used  for  the  saponification  of 
oils.  On  account  of  the  greater  solubility  of  potassium  soaps, 
potassium  hydroxide  is  best  for  this  purpose. 

Standardization. — The  standardization  of  solutions  of  bases 
may  be  accomplished  by  indirect  methods,  titrating  against  a 
previously  standardized  acid  solution,  or  by  direct  methods, 
titrating  against  a  weighed  substance  of  known  purity.  In 
general  the  first  method  is  to  be  recommended  because  the  availa- 
ble solid  acids  of  uniform  purity  are  limited  to  the  organic 
acids.  This  makes  necessary  the  use  of  phenolphthalein  in  the 
standardization,  which  limits  the  use  of  solutions  so  standardized 
to  determinations  that  can  be  made  by  means  of  this  indicator. 
Some  solid  substances  that  may  be  used  as  primary  standards 
are  oxalic  acid,  H2C204.2H2O,  benzoic  acid,  HC7H502  and  succinic 
acid,  H2C4H404.  No  direct  gravimetric  method  can  be  used 
because  there  is  no  precipitating  reagent  for  the  hydroxyl  radical 
and  a  determination  of  the  metal  would  not  give  the  basic  strength 
because  of  the  invariable  presence  of  salts  of  the  same  metal. 

Exercise :  Preparation  of  Tenth-normal  Sodium  Hydroxide  Solution. 

— Select  the  best  grade  of  sodium  hydroxide  obtainable,  that  purified 


236  QUANTITATIVE  ANALYSIS 

by  alcohol  being  preferable.  Calculate  the  weight  necessary  for  2500  cc 
of  tenth-normal  solution  and  weigh  out  the  quantity  with  2  percent 
added  to  compensate  for  carbonates,  water,  and  other  impurities.  Dis- 
solve in  a  2500  cc  bottle  having  a  solid  rubber  stopper.  Fill  the  bottle 
with  recently  boiled  and  cooled  distilled  water,  mix  thoroughly  and 
-allow  to  cool  to  the  temperature  of  the  room.  Titrate  portions  of  about 
30  cc  each  against  the  tenth-normal  acid,  using  methyl  orange  as 
indicator.  Continue  these  practice  titrations  until  the  results  agree 
closely. 

Dilute  to  make  the  solution  exactly  tenth-normal,  using  recently 
boiled  and  cooled  water.  Restandardize  against  the  acid,  using  methyl 
orange,  then  use,  in  turn,  phenolphthalein,  methyl  red,  cochineal  and 
lacmoid,  as  well  as  such  other  indicators  as  are  available.  The  indi- 
cators of  the  first  class  (page  211)  will  indicate  a  weaker  base  than  those 
of  the  third  class,  on  account  of  the  presence  of  small  amounts  of 
carbonates  in  the  standard  solution.  Record  the  normality  of  the  basic 
solution  according  to  each  indicator  and  use  the  proper  figure  for  sub- 
sequent titrations,  according  to  the  indicator  there  used. 

Determination  of  the  Concentration  of  the  Laboratory  Acids. — 
Use  the  "dilute"  acids,  either  sulphuric,  hydrochloric,  nitric  or  acetic. 
The  sample  should  not  be  weighed  in  the  analytical  balance  and  it  is 
better  to  determine  the  specific  gravity  and  then  take  a  measured  volume. 
Determine  the  specific  gravity  with  an  accurate  hydrometer.  From  the 
approximately  known  percent  of  acid  in  the  solution  calculate  the  dilu- 
tion required  to  make  a  solution  approximately  equivalent  in  concentra- 
tion to  the  standard  base.  Make  500  cc  of  solution,  measuring  accu- 
rately the  volumes  used.  Titrate  the  finally  diluted  solution  against  the 
standard  base,  using  methyl  orange  for  any  of  the  acids  above  named 
except  acetic  acid.  For  this  use  phenolphthalein.  Calculate  the  per- 
cent of  acid,  by  weight,  in  the  original  solution.  If  tables  are  at  hand 
compare  the  results  of  the  experiment  with  the  percent  corresponding 
to  the  specific  gravity  found. 

Determination  of  the  Purity  of  Citric  Acid. — Weigh  about  3  gm  of 
commercial  citric  acid,  dissolve  and  dilute  to  500  cc.  Titrate  portions 
of  30  cc  to  40  cc  by  the  standard  base,  using  phenolphthalein.  Calculate 
the  percent  of  the  tribasic  acid  HsCeHsOT.H^O  assuming  that  no  other 
acid  is  present. 

VINEGAR 

Vinegar  contains  from  3  to  6  percent  of  acetic  acid,  in  addi- 
tion to  coloring  matter,  dissolved  solids  and  sometimes  unfer- 
mented  sugar  or  alcohol.  Cider  vinegar  contains  also  from 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  237 

0.08  to  0.16  percent  of  malic  acid.  Vinegar  is  sometimes  adul- 
terated with  other  added  acids,  notably  sulphuric  acid.  The 
complete  analysis  will  include  the  determination  of  the  sub- 
stances just  mentioned  and  others  that  serve  to  characterize  the 
vinegar  with  respect  to  its  origin  or  quality.  The  determination 
of  total  acidity  alone  is  of  value  in  determining  the  ''  strength " 
of  the  vinegar.  This  is  practically  due  to  acetic  acid  alone  in 
pure  vinegars  other  than  those  made  from  cider.  In  cider 
vinegar  the  determination  of  malic  acid  is  also  of  importance  as 
indicating  its  origin. 

Determination  of  Total  Acidity  of  Vinegar. — Weigh  a  clean,  stoppered 
weighing  bottle  of  at  least  25  cc  capacity.  Add  about  25  cc  of  vinegar, 
stopper  and  reweigh  with  an  accuracy  of  1  mg.  Carefully  transfer  this 
sample  to  a  100  cc  volumetric  flask,  using  a  stirring  rod.  Rinse  the 
bottle  and  rod  with  recently  boiled  water  and  dilute  the  vinegar  and 
rinsings  to  the  mark  on  the  flask.  Mix  thoroughly.  By  means  of  a 
pipette  measure  two  or  three  separate  portions  to  Erlenmeyer  flasks, 
dilute  to  50  cc,  add  a  drop  of  phenolphthalein  and  titrate  with  standard 
base.  Calculate  the  total  acidity  as  percent  of  acetic  acid,  using  the 
normality  of  the  base  as  determined  in  presence  of  phenolphthalein. 

BORIC  ACID 

Boric  acid  is  one  of  the  weakest  of  all  inorganic  acids,  having 
a  percentage  ionization  not  far  above  that  of  hydrocyanic  acid. 
It  is  therefore  not  possible  to  titrate  it -by  standard  bases  in 
ordinary  solution  because  no  indicator  will  give  a  sudden  color 
change  at  any  point  in  its  neutralization.  It  can  be  determined 
by  titrating  in  presence  of  glycerine,  phenolphthalein  being  used 
as  indicator.1  Hydroxylated  organic  compounds  form  ester- 
like  compounds  with  the  various  boric  acids,  the  result  being 
substances  more  strongly  ionized  than  boric  acid.  Tartaric 
acid  forms  such  substances,  with  a  resultant  change  of  its  optical 
rotatory  power.  With  glycerine  and  orthoboric  acid  the  com- 
pound C3H5(OH).HBO3  is  probably  formed  because  the  substance 
reacts  as  a  monobasic  acid.  Boric  acid  combined  as  salts  may  be 
determined  by  first  adding,  in  presence  of  methyl  orange,  enough 
hydrochloric  or  sulphuric  acid  to  completely  decompose  the 

1  Thomson:  J.  Soc.  Chem.  Ind.,  12,  432  (1893). 


238  QUANTITATIVE  ANALYSIS 

borate.  In  case  carbonates  are  present  carbonic  acid  is  also 
produced  and  this  must  be  expelled  by  boiling  because  phenol- 
phthalein  is  to  be  used  in  the  last  titration.  If  the  more  common 
biborates  are  thus  decomposed  they  yield  orthoboric  acid,  methyl 
orange  indicating  the  end-point  at  the  completion  of  the  reaction  : 

Na2B407+5H2O+2HCl->2NaCl+4H3B03. 
The  addition  of  glycerine  produces  the  reaction: 


The  monobasic  acid  is  then  neutralized  in  the  titration: 


Commercial  borax  is  essentially  crystallized  sodium  biborate, 
Na2B407.10H20.  It  loses  water  rapidly  and  seldom*  corresponds 
to  this  formula. 

Determination.  —  Crush  and  mix  a  sample  of  a  borate  quickly.  Weigh 
about  10  gm,  dissolve  and  dilute  to  500  cc.  Titrate  measured  portions 
of  20  cc  with  tenth-normal  acid  in  presence  of  methyl  orange,  first  diluting 
the  solution  to  50  cc.  Now  add  to  other  diluted  portions  of  the  borate 
solution  the  exact  amount  of  acid  indicated  by  the  titration  but  no  indi- 
cator. Boil  for  a  few  minutes,  then  cool.  Add  a  drop  of  phenolphthal- 
ein  and  30  cc  of  glycerine.  The  glycerine  must  be  neutral.  Titrate 
with  standard  base  to  the  appearance  of  a  pink  color.  Add  10  cc  more 
of  glycerine  and  if  the  pink  disappears  add  more  base.  Continue  the 
addition  of  glycerine  and  standard  base  until  a  permanent  end-point 
is  attained.  Calculate  the  percent  of  boron  trioxide,  B203,  in  the 
original  borate.  Reference  to  the  formula  for  the  complex  of  boric  acid 
and  glycerine  will  show  that  the  equivalent  weight  of  boron  trioxide 
must  be  one-half  its  molecular  weight. 

Use  of  Two  Standards.  —  There  are  cases  where  a  direct  titra- 
tion cannot  be  made  conveniently  and  where  two  standard 
solutions  may  be  used  with  advantage.  If  the  titrated  substance 
is  nearly  insoluble  in  water  and  dissolves  only  when  the  standard 
solution  is  added  a  direct  titration  is  a  very  tedious  operation. 
This  is  due  to  the  fact  that  it  would  not  be  permissible  to  add  more 
standard  than  is  chemically  equivalent  to  the  substance  with 
which  the  standard  reacts,  so  that  the  active  mass  of  the  standard 
is,  necessarily,  always  small,  particularly  toward  the  end  of  the 
titration,  where  it  approaches  zero.  The  velocity  of  solution 


EXPERIMENTAL  VOLUMETRIC  ANALYSIS  239 

would  therefore  become  extremely  low,  this  also  approaching 
zero  toward  the  end  of  the  titration. 

For  this  reason  it  is  much  simpler  to  add  an  excess  of  the 
standard  solution,  the  total  quantity  used  being  measured 
accurately.  The  active  mass  of  the  standard  is  then  relatively 
large  throughout  the  operation  and  solution  of  the  sample  pro- 
ceeds fairly  rapidly.  After  solution  is  complete  the  unused 
excess  of  standard  is  titrated  by  means  of  a  second  standard 
solution.  This  gives  sufficient  data  for  the  calculation.  The 
volume  used  of  the  second  standard  is  calculated  to  the  equivalent 
volume  of  the  first.  This  figure  is  then  subtracted  from  the  total 
volume  used  of  the  first  standard  and  the  remainder  is  the  volume 
used  by  the  titrated  substance. 

A  familiar  example  of  the  use  of  two  standards  is  in  the  titra- 
tion of  the  acid-neutralizing  power  of  limestone  for  agricultural 
purposes.  The  first  standard  is  hydrochloric  acid  and  the 
second  is  either  sodium  hydroxide  or  potassium  hydroxide. 
A  tenth-normal  solution  of  acid  is  not  active  enough  for  this 
purpose  and  a  fifth-normal  solution  should  be  prepared.  The 
preparation  and  standardization  of  fifth-normal  solutions  will 
follow  the  same  lines  as  that  of  tenth-normal  solutions,  with 
which  the  student  is  already  familiar. 

"Alkalinity"  of  Limestone  for  Agricultural  Purposes. — Have  the 
sample  finely  powdered.  Weigh  exactly  0.5-gm  samples  on  counter- 
poised glasses  and  brush  into  Erlenmeyer  flasks.  Moisten  with  water 
then  pipette  75  cc  of  fifth-normal  hydrochloric  acid  into  each  flask. 
After  effervescence  has  become  slow,  connect  with  a  reflux  condenser 
and  heat  to  boiling.  Boil  for  1  minute  to  expel  carbon  dioxide,  then 
cool  and  rinse  down  the  condenser,  the  stopper  and  the  upper  part 
of  the  flask.  Add  a  drop  of  methyl  red  or  phenolphthalein  and  titrate 
the  unused  excess  of  acid  with  fifth-normal  base.  (In  standardizing 
the  base  against  the  acid,  use  must  be  made  of  the  same  indicator  as 
is  chosen  for  the  determination.) 

Calculate  the  percent  of  calcium  carbonate  in  the  limestone.  This 
will  give  a  fictitious  value  for  dolomitic  limestones,  which  contain 
magnesium  carbonate  having  an  equivalent  weight  of  42,  as  com- 
pared with  50  for  calcium  carbonate.  In  such  cases  the  calculated 
percent  of  calcium  carbonate  may  be  more  than  100. 


CHAPTER  IX 
OXIDATION  AND  REDUCTION 

Reactions  of  neutralization  constitute  a  very  important  group 
in  quantitative  chemistry.  Of  no  less  importance  is  that  group 
which  is  composed  of  reactions  of  oxidation  and  reduction. 
These  do  not  involve  the  use  of  the  organic  indicator  but  of  some 
inorganic  substance,  this  being,  in  some  cases,  the  standard 
reagent  itself.  While  the  quantitative  relations  Between  the 
reacting  substances  are  here  different  from  those  of  neutraliza- 
tion, the  same  principles  will  serve  for  the  calculation  of  the 
results  of  the  titration.  The  equivalent  weights  will  be  deter- 
mined by  dividing  the  molecular  weights  by  the  hydrogen 
equivalents  but  the  latter  will  be  determined,  not  by  the  valence 
of  the  reacting  parts  but  by  the  change  of  valence  in  the  reaction, 
because  oxidation  always  involves  a  change  of  valence  of  the 
oxidizing  and  reducing  agents.  In  the  reaction: 

BaCl2+H2SO4->BaSO4+2HCl 

the  hydrogen  equivalent  of  each  radical  is  plainly  represented  by 
its  valence,  for  it  is  in  this  measure  that  it  may  enter  into  the 
exchange  of  double  decomposition.  In  the  reaction: 

2KMn04+10FeS04+8H2S04-^K2SO4+2MnSO4+ 
5Fe2(SO4)3+8H20 

the  hydrogen  equivalents  of  potassium  permanganate  and  ferrous 
sulphate  cannot  be  so  represented  because  something  that  is 
different  from  ordinary  double  decomposition  has  taken  place. 
Manganese  has  left  a  negative  radical  and  entered  a  positive  one 
and  has  thereby  changed  its  valence  and  has  been  reduced.  The 
change  of  valence  will  represent  its  power  as  an  oxidizing  agent. 
Iron  has  taken  on  more  of  the  radical  with  which  it  was  already 
combined,  has  increased  its  valence  and  has  been  oxidized.  It 

240 


OXIDATION  AND  REDUCTION  241 

is  not  difficult  to  determine  the  change  of  valence  of  manganese 
if  its  state  of  oxidation  in  the  two  compounds  is  first  determined. 
Apparent  Valence.  —  For  the  purposes  of  this  inspection  the 
question  of  the  actual  valence  need  not  be  considered.  There 
has  been  much  difference  of  opinion  regarding  the  structural 
formulas  that  should  represent  inorganic  compounds  and  the 
real  valence  of  an  element  in  an  oxide  is  indicated  by  the  structural 
formula  of  that  oxide.  For  example,  the  dioxide  of  manganese 
might  be  represented  by  the 
1) 


formula  Mn^      ,  in  which  the  valence  of  manganese  is  4,  or  by 

O  • 
,0 
Mn^  |  in  which  the  valence  is  2.     If  the  element  is  regarded  as 

X0 

always  being  in  direct  combination  with  the  oxygen  the  latter 
may  be  taken  as  a  measure  of  the  apparent  valence  and  the  loss 
of  oxygen  or  its  equivalent  in  other  elements,  as  the  result 
of  a  reaction,  will  be  measured  by  the  apparent  change  of  valence. 
This  change  will  therefore  be  the  hydrogen  equivalent  of  either 
oxidizing  or  reducing  agent. 

The  apparent  valence  of  an  element  which  is  in  the  negative 
radical  of  an  oxyacid  or  of  its  salt  is  found  by  subtracting  the 
total  valence  of  the  positive  radical  from  the  total  valence  of  the 
other  element  of  the  negative  radical  and  dividing  the  result  by 
the  number  of  atoms  of  the  element  in  question.  Thus  the 
apparent  valence  of  manganese  in  potassium  permanganate  is 
(4X2)  —1  =  7;  that  of  chromium  in  potassium  dichromate  is 
(7X2)-2  _  6 

When  the  element  in  question  forms  a  simple  radical  its  valence 
is  easily  seen  from  an  inspection  of  the  formula.  The  valence  of 
chromium  in  chromium  chloride  is  3,  that  of  manganese  in  man- 
ganous  sulphate  is  2,  that  of  iron  in  ferric  sulphate  is  3,  etc. 

In  the  reaction  involving  the  reduction  of  manganese  from  a 
permanganate  to  a  manganous  salt,  the  apparent  valence  changes 
from  7  to  2  and  the  hydrogen  equivalent  of  the  compound  is 
therefore  5. 

The  equivalent  weight  of  potassium  permanganate,  containing 

16 


242  QUANTITATIVE  ANALYSIS 

one  atom  of  manganese  in  each  molecule,  is  one-fifth  of  its  molecu- 
lar weight  or  31.606.     Iron  is  oxidized  in  the  reaction.     Its  appar- 
ent change  of  valence  is  from  2  to  3,  its  hydrogen  equivalent  is 
therefore  1  and  its  equivalent  weight  is  its  atomic  weight. 
In  the  reaction: 

K2Cr2O7 + 6FeCl2 + 14HC1->2KC1 + 2CrCl3 + 6FeCl3 + 7H20, 

potassium  dichromate  is  the  oxidizing  agent.  Closer  examina- 
tion shows  that  the  element  chromium  is  the  real  cause  of  the 
oxidizing  action,  since  it  leaves  the  negative  radical  by  changing 
to  a  base-forming  lower  oxide.  The  change  of  valence  (reduction) 
of  chromium  is  from  6  to  3,  its  hydrogen  equivalent  is  3,  and  that 
of  potassium  dichromate,  containing  two  atoms  of  chromium  in 
the  molecule  is  6.  The  equivalent  weight  of  potassium  dichro- 
mate is  then  one-sixth  of  its  molecular  weight. 

The  assignment  of  apparent  valence  to  the  oxidizing  and  reduc- 
ing elements  is  a  valuable  aid  in  balancing  oxidation  and  reduc- 
tion equations.  For  example  the  equation  last  given  is  to  be 
balanced.  The  empirical  equation  is 

K2Cr2O7+FeCl2+HCl-^KCH-CrC]3+FeCl3+H20. 

First  inspect  the  equation  to  determine  the  oxidizing  and  reducing 
elements,  which  are  seen  to  be  chromium  and  iron.  Determine 
the  changes  in  apparent  valence  and  from  these  their  hydrogen 
equivalents  and  those  of  the  compounds  containing  them.  Write 
these  above  the  respective  compounds  thus: 

VI  I 

K2Cr2O7+FeCl2-f-HCl->KCl+CrCl3+FeCl34-H2O. 

The  hydrogen  equivalent  of  the  reducing  agent  is  to  be  the 
coefficient  of  the  oxidizing  agent,  and  vice  versa.  These  coeffi- 
cients will  not  thereafter  be  changed.  From  these  will  follow  the 
coefficients  of  potassium  chloride,  chromium  chloride  and  ferric 
chloride  and  from  all  of  these  will  be  calculated  the  coefficient  of 
hydrochloric  acid  From  the  latter  will  follow  the  coefficient  of 
water.  The  balanced  equation  is  then: 

K2Cr2O7 + 6FeCl2 + 14HC1-*2KC1 + 2CrCl3 + 6FeCl3 + 7H20. 


OXIDATION  AND  REDUCTION  243 

Problems 

45.  Determine  the  equivalent  weights  of  the  oxidizing  and  reducing  agents 
in  the  following  equations  and  balance  the  latter: 

(a)  KMnO4+MnSO4+KOH^K2SO4  +  MnO2+H2O. 

(b)  K2Cr2O7  +SnCl2  +HC1-»KC1  +SnCl4  +CrCl3  +H2O. 

(c)  H3AsO3+I2+H2O-»H3AsO4+HI. 

(d)  HgCl2+SnCl2-»HgCH-SnCl4. 

(e)  HgCl8+SnCl2-»Hg-+SnCl4. 

46.  How  much  potassium  permanganate  must  be  contained  in  1000  cc 
of  solution  so  that  1  cc  shall  be  equivalent  to  0.002  gm  of  iron?     To  0.002 
gm  of  manganese? 

47.  How  much  potassium  dichromate  must  be  contained  in  1000  cc  of 
solution  so  that  1  cc  shall  be  equivalent  to  0.005  gm  of  iron? 

48.  A  solution  of  potassium  permanganate  contains  2.83  gm  in  1000  cc. 
What  weight  of  pure  ferrous  ammonium  sulphate,  FeSO4(NH4)2SO4.6H2O, 
should  be  taken  for  standardization  so  as  to  require  30  cc  of  the  permanga- 
nate solution? 

49.  A  solution  of  potassium  dichromate  is  of  such  concentration  that  1  cc 
=0=0.005  gm  iron.     A  solution  of  potassium  permanganate  contains  3.26 
gm  of  the  salt  in  1000  cc  of  solution.     To  what  volume  should  1000  cc  of 
the  stronger  solution  be  diluted  to  make  the  two  equivalent  to  the  same 
weight  of  iron? 

50.  What  weight  of  iodine  is  required  for  1000  cc  of  a  tenth-normal  oxidiz- 
ing solution? 

51.  What  weight  of  arsenic  trioxide  is  equivalent  to  1  cc  of  tenth-normal 
iodine  solution? 

Potassium  Permanganate. — This  substance  is  a  very  con- 
venient one  for  use  as  a  standard  oxidizing  agent,  not  only  because 
it  is  readily  and  quantitatively  reduced  by  many  reducing  agents, 
but  because  it  serves  as  its  own  indicator.  The  color  of  even  a 
dilute  solution  is  quite  intense,  while  the  reduced  salt,  man- 
ganous  sulphate,  forms  colorless  solutions.  The  least  excess, 
after  the  oxidation  is  finished,  is  made  evident  by  the  color  of 
the  unchanged  permanganate.  Practically,  standard  potassium 
permanganate  is  most  often  used  for  the  determination  of  iron, 
less  frequently  for  the  determination  of  manganese,  and  occa- 
sionally for  the  determination  of  tin  or  antimony  and  indirectly 
for  the  determination  of  phosphorus,  calcium  and  many  thero 
elements.  Theoretically  it  could  be  used  for  the  direct  or  indirect 
determination  of  any  reducing  agent  or  oxidizing  agent  but 
other  methods  are  frequently  better  for  substances  other  than 
those  named 


244  QUANTITATIVE  ANALYSIS 

A  solution  of  potassium  permanganate,  if  made  by  simply 
dissolving  the  salt  in  water,  decomposes  slowly  and  must  be  fre- 
quently restandardized.  Morse1  has  shown  that  this  is  due  to  the 
presence  of  manganese  dioxide,  traces  of  which  may  have  been 
contained  in  the  original  salt  or  it  may  have  been  formed  by 
reduction  of  potassium  permanganate  by  dust  or  organic  impuri- 
ties. The  work  of  Morse  shows  that  if  the  solution  is  filtered 
through  asbestos,  and  is  kept  in  clean,  glass-stoppered  bottles 
its  concentration  will  remain  constant  almost  indefinitely. 

IRON 

Iron  may  be  readily  and  quickly  titrated  by  a  solution  of  po- 
tassium permanganate.  For  this  purpose  the  standard  solution 
should  be  made  in  the  decimal  system.  The  normal  system  is 
not  adapted  to  this  determination  because  the  potassium  per- 
manganate will  not  often  be  used  for  the  determination  of  sub- 
stances other  than  iron  and  the  decimal  system  provides  more  sim- 
ple calculations.  If  the  iron  compound,  as  an  ore,  is  taken  in 
such  quantity  that  the  weight  of  sample  is  a  simple  multiple 
of  the  iron  equivalent  of  the  standard,  the  burette  reading  is  a 
direct  percentage  reading.  Thus,  if  1  cc  of  standard  is  equivalent 
to  0.002  gm  of  iron  and  if  2  gm  of  iron  ore  is  taken  for  analysis 
each  cubic  centimeter  of  standard  represents  0.1  percent  of  iron. 

Reduction  of  Permanganate  by  Chlorides. — One  serious 
disadvantage  in  the  use  of  potassium  permanganate  for  the 
titration  of  iron  lies  in  the  fact  that  if  hydrochloric  acid  is  pres- 
ent, it  also  is  oxidized  by  the  standard,  the  result  being  a  ficti- 
tious value  for  the  percent  of  iron.  But  most  iron  ores  dissolve 
best  in  hydrochloric  acid,  the  solubility  in  sulphuric  acid  being 
very  slight.  After  solution  of  the  ore  is  accomplished  it  is  neces- 
sary to  remove  the  hydrochloric  acid  or  to  find  some  method  for 
avoiding  its  reducing  action  upon  the  permanganate.  To  re- 
move the  acid,  the  solution  of  the  ore  may  be  evaporated  with 
sulphuric  acid. 

Use  of  Manganous  Sulphate. — Instead  of  removing  the 
hydrochloric  acid  it  is  possible  to  minimize  its  reducing  action 
by  the  addition  of  manganous  sulphate  to  the  solution.  In 

1  Am.  Chem.  J.,  18,  401  (1896);  20,  521  (1898). 


OXIDATION  AND  REDUCTION  245 

order  to  understand  the  remarkable  action  of  manganous  salts 
in  preventing  the  oxidation  of  chlorides  in  dilute  solutions  it 
is  necessary  to  examine  more  closely  the  reaction  between 
potassium  permanganate  and  ferrous  chloride.  This  reaction, 
occurring  in  an  acid  solution,  is  usually  represented  as  follows: 

KMn04+5FeCl24-8HCl->KCl+MnCl2+5FeCl3+4H2O.     (1) 
This  reaction  really  takes  place  in  at  least  two  stages: 

KMnO4+4FeCl2+8HCl^KCl+MnCl3+4FeCl3+4H2O,     (2) 
MnCl3+FeCl2-+MnCl2+FeCl3.  (3) 

Equations  (2)  and  (3)  are  seen  by  inspection  to  indicate  the  same 
end  products  as  equation  (1)  and  if  no  side  reactions  of  any  sort 
took  place  the  ordinary  calculations  would  not  be  affected  by  the 
occurrence  of  two  stages  in  the  reaction.  But  manganese 
trichloride  is  a  very  unstable  compound  and  if  not  reduced  at 
once  by  ferrous  salts  in  the  immediate  vicinity  (a  condition  that 
especially  maintains  near  the  end  of  the  titration)  this  compound 
proceeds  to  decompose  spontaneously: 

2MnCl3±+2MnCl2+Cl2.  (4) 

Again,  if  this  liberated  chlorine  could  react  quantitatively  with 
ferrous  chloride: 

2FeCl2+Cl2-+2FeCl3  (5) 

there  would  be  no  error  involved  in  the  titration  for  equations  (2), 
(4)  and  (5),  in  sequence,  are  still  equivalent  to  equation  (1). 
This  condition  does  not  obtain.  Reaction  (5)  is  slow  and  un- 
certain and  one  of  the  reacting  bodies  (chlorine)  is  a  gas  which 
readily  escapes.  Therefore,  according  to  Barneby,1  the  function 
of  manganous  salts,  added  in  large  quantities,  is  to  increase  the 
concentration  of  the  manganous  ion,  thus  obstructing  the  progress 
of  reaction  (4).  By  this  means  the  spontaneous  decomposition 
of  manganese  trichloride  is  prevented  and  it  is  reduced  only  by 
contact  with  ferrous  chloride,  with  which  it  reacts  very  readily. 
Observation  of  End  Point. — As  ferrous  chloride  becomes 
oxidized  a  yellow  color  appears  in  the  solution,  deepening  to 
red  as  the  concentration  of  the  ferric  salt  increases.  Experience 

1  J.  Am.  Chem.  Soc.,  36,  1429  (1914). 


246  QUANTITATIVE  ANALYSIS 

shows  that  this  renders  difficult  or  impossible  the  observation 
of  the  first  faint  pink  of  the  permanganate  anion,  which 
constitutes  the  indication  of  the  end  point  of  the  reaction.  There 
is  no  corresponding  color  in  the  case  of  sulphates  of  iron  and  this 
goes  to  show  that  the  color  of  ferric  chloride  solutions  is  not  due 
to  the  ferric  ion  but  rather  to  hydrolysis.  Partial  or  complete 
hydrolysis  may  yield  any  or  all  of  the  products  indicated  by 
these  equations. 

FeCl3 + H20->FeOHCla + HC1, 
FeOHCl2 + H2O->Fe  (OH)  2C1 + HC1, 
Fe(OH)aCl+H80->Fe(OH)8+HCl. 

The  basic  chlorides  or  ferric  hydroxide  form  colloidal  solutions 
(sols)  which  are  colored.  As  these  reactions  are  reversible 
the  addition  of  an  excess  of  hydrochloric  acid  hinders  hydrolysis 
and  lightens  the  color  but  it  has  already  been  shown  that  a 
large  excess  of  this  acid  is  undesirable  because  of  its  reducing 
action  upon  permanganate.  Reinhardt1  showed  that  the 
formation  of  color  can  be  prevented  by  the  addition  of  phos- 
phoric acid  to  the  solution.  Evidently,  hydrolysis  is  here 
prevented  by  diminishing  the  concentration  of  one  of  the  re- 
acting ions  (ferric)  through  the  formation  of  the  phosphate, 
which  has  a  very  small  ionization. 

FeCl3+H3PO4->FeP04+3HCl. 

The  "Zimmermann-Reinhardt  solution"  contains  manganous 
sulphate,  phosphoric  acid  and  sulphuric  acid  and  it  is  added  to 
the  largely  diluted  ferrous  chloride  solution  just  before  titration 
of  the  latter  by  standard  permanganate  solution. 

Primary  Standards. — Potassium  permanganate  cannot  be 
obtained  in  a  sufficiently  high  state  of  purity  to  make  possible 
direct  weighing  of  the  substance  for  standardization.  Primary 
standard  reducing  agents  of  known  and  uniform  purity  must  be 
used.  Some  reducing  agents  that  may  be  used  for  this  purpose 
are  iron,  ferrous  ammonium  sulphate,  oxalic  acid  and  ammonium 
oxalate. 

Iron. — Pure  iron  is  not  obtainable  commercially  and  can  be 
prepared  only  with  great  difficulty.  Iron  wire  may  be  obtained 

1  Chem.  Zg.,  13,  323  (1889). 


OXIDATION  AND  REDUCTION  247 

having  a  fairly  high  degree  of  purity.  A  common  fallacy  has 
found  acceptance  by  many  chemists,  to  the  effect  that  the  so- 
called  "  piano  wire,"  purchased  for  standardizing,  is  uniformly 
99.6  percent  or  some  other  stated  percent  of  iron.  The  purity  of 
such  wire  varies  widely  and  this  material  cannot  be  used  as  a 
primary  standard  for  accurate  work  unless  it  has  been  carefully 
analyzed.  Such  analysis  is  best  accomplished  by  electrolysis. 
The  gravimetric  determination  of  iron  by  precipitation  as  ferric 
hydroxide  and  weighing  as  ferric  oxide  is  not  accurate  unless  made 
with  extreme  care,  on  account  of  the  difficulty  experienced  in 
purification  of  the  precipitate.  •  If  iron  is  to  be  used  for  standard- 
izing solutions,  weighed  portions  of  the  previously  analyzed 
material  are  dissolved  in  dilute  sulphuric  acid.  The  necessary 
limitations  as  to  accuracy  render  the  use  of  iron  as  a  primary 
standard  decidedly  unsatisfactory,  especially  since  it  has  become 
possible  to  obtain  iron  salts  in  a  high  state  of  purity. 

Ferrous  Ammonium  Sulphate. — Ferrous  ammonium  sulphate 
("Mohr's  salt")  may  be  purified  very  readily  by  crystallization 
and  preserved  without  oxidation,  if  kept  dry.  Many  analyses 
conducted  upon  high-grade  samples  have  shown  that  the  salt 
may  be  obtained  in  a  state  of  practical  purity,  the  percent  of 
iron  being  almost  exactly  that  calculated  from  the  formula. 
Oxidation  in  air  does  not  readily  occur,  so  that  the  standard 
sample  may  be  preserved  almost  indefinitely  if  kept  in  a  stop- 
pered bottle. 

Sodium  Oxalate. — This  is  one  of  the  best  primary  standards 
now  available  for  standardizing  oxidizing  solutions.  The 
reaction  with  potassium  permanganate  is  represented  thus: 

5Na2C2O4+2KMnO4+8H2S04->K2SO4+2MnSO4+5Na2SO4 

+  10C02+8H20 

The  salt  may  be  purified  by  ordinary  methods  of  recrystallization 
from  water  solutions  or  by  the  addition  of  alcohol  to  rather 
concentrated  water  solutions.  By  the  latter  method  crystal- 
lization is  more  rapid  and  the  crystals  are  consequently  finer. 
At  temperatures  between  240°  and  250°  the  crystals  may  be  dried 
completely  and  without  decomposition.1  The  solid  -does  not 
contain  water  of  crystallization.  The  standardization  of  potas- 
1  Bur.  Stand.,  Circ.  40. 


248  QUANTITATIVE  ANALYSIS 

slum  permanganate  solutions  is  made  by  titration  of  weighed 
portions  in  presence  of  sulphuric  acid,  at  temperatures  between 
80°  and  90°. 

Standard,  certified  samples  of  sodium  oxalate  may  be  obtained 
from  the  Bureau  of  Standards  and  these  may  be  kept  indefinitely, 
if  in  a  stoppered  bottle.  Stock,  solutions  cannot  be  preserved 
unchanged. 

Oxalic  Acid. — Oxalic  acid  and  ammonium  oxalate  may  be  puri- 
fied by  recrystallization  to  the  condition  required  for  primary 
standards.  If  previously  analyzed  they  are  nearly  as  satisfac- 
tory as  ferrous  ammonium  sulphate.  The  student  is  cautioned 
against  the  practice  of  assuming  the  purity  of  any  of  his  primary 
standards  without  an  analysis  as  the  basis  for  the  assumption. 

Reduction  of  the  Iron  Solution. — Iron  exists  in  the  ferric  con- 
dition in  most  ores  or  other  minerals.  In  order  to  reduce  the 
solution  of  ferric  salt  either  stannous  chloride,  zinc,  sulphurous 
acid  or  hydrogen  sulphide  may  be  employed.  •  The  first  two  are 
the  only  ones  now  commonly  used. 

Stannous  chloride,  in  solution,  possesses  the  advantage  of 
instantaneous  action  if  added  to  the  hot  solution  of  ferric  chloride. 
If  the  iron  is  to  be  reduced  by  stannous  chloride  an  addition  of 
this  salt  to  the  ore  during  the  process  of  solution  will  materially 
hasten  the  action.  For  the  final  reduction  the  stannoils  chloride 
solution  may  be  added  from  a  pipette,  the  disappearance  of  the 
red  color  of  basic  ferric  chloride  providing  an  approximate  indica- 
tion of  the  end-point. 

In  the  analysis  of  iron  ores  there  is  occasionally  trouble  at  this 
point  unless  certain  precautions  have  been  taken.  In  the  first 
place,  many  iron  ores  contain  appreciable  quantities  of  organic 
matter  and  this  serves  to  produce  a  yellow  color  when  the  ore  is 
dissolved.  As  color  due  to  this  cause  does  not  disappear  when 
the  iron  has  been  reduced  it  is  not  possible  to  determine  when  the 
correct  amount  of  stannous  chloride  has  been  added.  This 
trouble  may  be  avoided  by  igniting  the  weighed  sample  for  a 
short  time  in  a  porcelain  crucible,  before  dissolving. 

The  second  cause  of  irremovable  color  lies  in  fusion  of  insoluble 
residues  in  platinum  crucibles.  The  pyrosulphate  which  is  used 
as  a  flux  dissolves  traces  of  platinum  and  this,  with  stannous 
chloride,  forms  a  yellow  solution  containing  a  complex  of  tin  and 


OXIDATION  AND  REDUCTION  249 

platinum.  This  interference  is  avoided  by  the  substitution  of 
porcelain  crucibles  for  those  of  platinum. 

After  a  slight  excess  of  stannous  chloride  has  been  added  the 
solution  is  cooled  and  a  considerable  excess  of  mercuric  chloride 
is  added,  the  unused  stannous  chloride  being  thereby  oxidized: 

2HgCl2+SnCl2->SnCl4+2HgCl. 

Mercuric  chloride  will  not  oxidize  ferrous  chloride  and  hence  may 
be  left  in  the  solution.  If  an  insufficient  excess  of  mercuric 
chloride  is  used,  or  if  it  is  added  too  slowly,  free  mercury  may  be 
produced : 

HgCl2-f-SnCl2-*SnCl4+Hg. 

The  indication  of  such  action  is  the  appearance  of  a  gray  precipi- 
tate of  mercury  instead  of  the  characteristic  white  silky  crystals 
of  mercurous  chloride.  If  mercury  is  so  produced  the  deter- 
mination is  ruined  because  this  mercury  will  itself  reduce  some  of 
standard  oxidizing  solution  during  the  process  of  titration  of  the 
iron. 

Stannous  chloride  cannot  be  used  to  reduce  ferric  solutions 
previous  to  titration  by  potassium  permanganate  unless  the 
interference  of  chlorides  is  to  be  prevented  by  the  addition  of 
manganous  sulphate.  Instead,  pure  zinc  or  zinc  of  predeter- 
mined iron  content  may  be  used  to  reduce  the  iron.  For  this 
purpose  an  approximately  weighed  quantity  of  granular  zinc  or 
zinc  dust  may  be  added  directly  to  the  solution  and  dissolved  in 
it,  or  the  ferric  solution  may  be  passed  through  a  redactor.  The 
latter  is  a  tube  having  the  dimensions  of  a  burette,  filled  with 
amalgamated  zinc  and  having  a  dropping  funnel  fixed  in  the  top. 
The  acidified  solution  is  passed  through  the  tube  once  or  twice 
and  the  iron  is  thereby  reduced.  Most  zinc  contains  iron  or  other 
reducing  matter  and  if  it  is  to  be  dissolved  in  the  iron  solution, 
as  above  described,  a  blank  determination  should  be  made  to 
determine  the  amount  of  standard  that  will  be  required  to  oxidize 
the  reducing  matter  of  the  zinc. 

On  account  of  the  slow  reducing  action  of  zinc,  stannous  chloride 
is  much  to  be  preferred,  where  conditions  will  permit  its  use. 

Sulphurous  acid  and  hydrogen  sulphide  reduce  iron  solutions 
quickly  but  the  disadvantages  involved  in  their  preparation 
prevent  their  extensive  use  for  this  purpose. 


250 


QUANTITATIVE  ANALYSIS 


Exercise:  Preparation  of  Standard  Potassium  Permanganate  Solu- 
tion.— Calculate  the  weight  of  potassium  permanganate  required  for 
2500  cc  of  solution,  each  cubic  centimeter  to  be  equivalent  to  0.005  gm 
of  iron,  using  for  this  purpose  the  method  shown  on  page  196.  Weigh 
2  percent  more  than  the  calculated  weight  and  dissolve  in  about  1000 
cc  of  distilled  water.  Allow  to  stand  at  least 
an  hour,  then  filter  through  a  double  asbestos 
filter  arranged  as  in  Fig.  67,  all  vessels  hav- 
ing been  previously  cleaned  by  chromic  acid 
mixture.  Dilute  the  solution  to  2500  cc  and 
standardize  as  follows: 

Calculate  the  weight  of  ferrous  ammonium 
sulphate  that  will  be  approximately  equiva- 
lent to  35  cc  of  the  permanganate  solu- 
tion. Weigh  three  such  portions  into  Erlen- 
meyer  flasks  of  250  cc  capacity.*  Dissolve 
each  portion,  immediately  before  titrating, 
in  50  cc  of  distilled  water  and  add  10  cc  of 
dilute  sulphuric  acid.  Titrate  at  once  with 
the  potassium  permanganate  solution,  the 
first  appearance  of  a  permanent  pink  tint 
being  taken  as  the  end-point.  If  sodium 
oxalate  is  used  as  the  primary  standard, 
proceed  as  follows:  Warm  the  acid  solution 
of  oxalate  to  90°  and  titrate,  stirring  vigor- 
ously and  continuously.  The  permanganate 
must  not  be  added  more  rapidly  than  10  to 
15  cc  per  minute  and  the  last  0.5  to  1  cc 
must  be  added  drop-wise,  with  particular 
care  to  allow  the  color  from  each  drop  to 
disappear  before  another  drop  is  added.  At 
the  end  of  the  titration  the  temperature 
must  not  be  below  60°.  A  thermometer 
should  be  kept  in  the  solution  during  the 
entire  experiment. 

Calculate  the  value  of  the  permanganate 

solution  in  terms  of  iron  and  dilute  two  liters  of  the  solution  to  the  exact 
equivalence  of  0.005  gm  of  iron  per  cubic  centimeter.  From  this  iron 
value  calculate  also  the  calcium  and  manganese  values  (see  pages  252  and 
253),  and  record  all  upon  the  label  of  the  bottle  and  in  the  record  book. 
Determination  of  Iron  in  an  Ore. — Sample  the  ore  and  grind  the 
last  selection  to  pass  the  100-mesh  sieve.  Weigh  exactly  0.5  gm  of  ore 
on  the  counterpoised  glasses,  brushing  into  each  of  three  porcelain  cru- 


FIG.  67. — Apparatus  for 
filtering  potassium  per- 
manganate solution. 


OXIDATION  AND  REDUCTION  251 

cibles.  Heat  the  crucibles  without  covers  for  5  minutes,  using  the 
ordinary  desk  burner,  then  allow  the  crucibles  to  cool,  place  in  casseroles 
and  add  to  each  25  cc  of  concentrated  hydrochloric  acid.  If  method 
(b)  is  to  be  used  for  reducing  the  iron  add  also  at  this  point  5  cc  of  5 
percent  stannous  chloride  solution.  Cover  and  warm  until  solution 
is  complete  or  until  no  further  action  appears  to  take  place.  If  the 
residue  is  not  colored,  proceed,  without  nitration,  by  one  of  the  methods 
(a)  or  (b)  given  below.  If  the  residue  is  colored  it  may  contain  iron.  In 
this  case  filter  on  a  small  paper  and  wash  the  paper  free  from  iron  solu- 
tion with  hot  water.  Set  the  filtrate  and  washings  aside  and  burn  the 
paper  at  a  low  temperature  in  a  porcelain  crucible.  If  the  residue  is 
small  in  amount  and  apparently  contains  little  silicious  matter  it  may  be 
decomposed  by  fusing  with  potassium  pyrosulphate.  Cool  and  dissolve 
the  mass  in  hot  water,  adding  the  solution  to  the  former  filtrate. 
Now  proceed  by  one  of  the  methods  given  below. 

(a)  Add  4  cc  of  concentrated  sulphuric  acid  to  the  iron  solution  and 
evaporate  by  holding  the  casserole  over  a  free  flame,  keeping  in  constant 
motion  to  hasten  evaporation  and  to  prevent  bumping.     Evaporate 
until  the  characteristic  white  fumes  of  sulphuric  acid  appear,  this  being 
the  point  at  which  all  water  and  hydrochloric  acid  have  been  expelled. 
Cool  and  dilute  to  50  cc,  rinsing  the  solution  into  a  250-cc  Erlenmeyer 
flask.     Add  2  gm  of  granular  zinc,  as  free  as  possible  from  iron,  place  a 
funnel  with  a  short  stem  in  the  flask  and  warm  until  the  zinc  is  dissolved. 
The  iron  solution  should  now  be  quite  colorless  or  faintly  green.     Cool 
and  titrate  at  once  with  standard  potassium  permanganate  solution. 
Make  a  blank  determination  without  the  ore,  to  determine  the  amount 
of  iron  and  other  reducing  matter  in  the  zinc,  by  dissolving  1  gm  of  zinc 
in  25  cc  of  dilute  sulphuric  acid,  under  the  same  conditions  as  noted 
above,  and  titrating.     Express  the  result  of  the  blank  experiment  as 
the  number  of  cubic  centimeters  of  potassium  permanganate  reduced 
by  1  gm  of  zinc.     The  proper  value  will  then  be  subtracted  from  the 
volume  of  permanganate  used  in  the  iron  titration  and  the  percent  of 
iron  then  calculated. 

If  the  zinc  is  nearly  pure  it  will  dissolve  very  slowly.  Solution  may 
be  hastened  by  dropping  a  coil  of  platinum  wire  into  the  flask,  and  keep- 
ing it  in  contact  with  the  zinc. 

(b)  Concentrate  the  iron  solution,  if  necessary,  to  about  50  cc  and 
transfer  to  a  1000  cc  Erlenmeyer  flask.     While  the  solution  is  nearly  boil- 
ing add,  drop  by  drop  from  a  pipette,  a  5  percent  solution  of  stannous 
chloride  until  the  ferric  chloride  has  just  been  reduced,  this  being  made 
evident  by  the  disappearance  of  the  red  color.    Add  two  drops  more  of 
stannous  chloride  solution  then  cool  quickly  by  immersing  the  flask 
in  running  water.     When  cool  add,  all  at  once,  25  cc  of  a  5  percent 


252  QUANTITATIVE  ANALYSIS 

solution  of  mercuric  chloride  and  mix  well  with  the  solution.  The  pre- 
cipitate should  be  pure  white  mercurous  chloride  without  a  trace  of  gray 
mercury.  Dilute  to  500  cc  and  add  50  cc  of  a  solution  containing  144 
gm  of  phosphorous  pentoxide,  245  gm  of  sulphuric  acid  and  67  gm  of. 
crystallized  manganous  sulphate  in  each  liter  of  solution.  Titrate  at 
once  with  standard  potassium  permanganate  solution  and  calculate  the 
percent  of  iron  in  the  ore. 

CALCIUM 

Calcium  may  be  precipitated  as  oxalate,  filtered  and  washed 
free  from  ammonium  oxalate,  then  dissolved  in  hot,  dilute  sul- 
phuric acid  and  the  resulting  oxalic  acid  titrated  with  potassium 
permanganate. 

CaC204+  H2S04-+CaS04+  H2C204, 


10C02+8H20. 

In  order  to  determine  the  equivalent  weight  of  oxalic  acid  it  is 
necessary  here,  as  in  other  cases,  to  note  the  change  which  it 
undergoes  as  it  becomes  oxidized.  The  products  of  oxidation  are 
water  and  carbon  dioxide.  By  using  the  method  explained  on 
page  241  it  will  be  found  that  the  apparent  change  of  valence  of 
carbon  is  1.  Since  each  molecule  of  oxalic  acid  contains  two 
atoms  of  carbon  the  hydrogen  equivalent  of  oxalic  acid  as  a 
reducing  agent  is  2  and  its  equivalent  weight  is  one-half  its  mo- 
lecular weight.  When  oxalic  acid  reacts  as  an  acid  its  hydrogen 
equivalent  is  also  2,  although  there  is  no  necessary  connection 
between  the  two  cases.  Since  one  molecule  of  oxalic  acid  is 
formed  by  the  decomposition  of  one  molecule  of  calcium  oxalate, 
containing  one  atom  of  calcium,  the  equivalent  weight  of  calcium 
is  also  one-half  its  atomic  weight.  The  calcium  equivalent  of  the 
potassium  permanganate  solution  may  be  calculated  from  the 
iron  equivalent  by  the  expression: 

eq.  wt.  of  calcium        ,.  *     i  s 

-  Xwt.  iron  =  wt.  of  calcium. 
eq.  wt.  of  iron 

Problems 

62.  1  cc  of  a  solution  of  potassium  permanganate  is  equivalent  to  0.002 
gm  of  iron.  Calculate  the  weight  of  calcium,  calcium  oxalate  and  oxalic 
acid  equivalent  to  1  cc. 


OXIDATION  AND  REDUCTION  253 

63.  What  weight  of  potassium  dichromate  is  equivalent  to  4.75  gm  of 
potassium  permanganate,  both  being  used  for  the  oxidation  of  iron? 

54.  What  weight  of  crystallized  oxalic  acid,  H2C2O4.2H2O,  is  equivalent  to 
3.56  gm  of  crystallized  ferrous  ammonium  sulphate,  Fe(NH4)2(SO4)j.6H2O? 

65.  What  is  the  normality  of  a  solution  of  potassium  permanganate,  1  cc 
of  which  is  equivalent  to  0.005  gm  of  iron? 

56.  30  cc  of  potassium  permanganate  solution  oxidizes  0.1905  gm  of 
crystallized  oxalic  acid.  How  dilute  1000  cc  of  the  solution  to  make 
exactly  tenth-normal? 

Determination. — Use  samples  of  not  more  than  0.2  gm  of  the  calcium 
compound.  Dissolve,  precipitate,  filter  and  wash  the  calcium  oxalate 
according  to  the  method  already  learned  in  an  earlier  exercise  (page  81). 
Pierce  the  point  of  the  filter  paper  and  wash  the  precipitate  into  a  beaker 
with  the  least  possible  quantity  of  hot  water.  Thoroughly  wash  the 
paper  in  the  funnel  with  successive  portions  of  5  cc  of  hot  dilute  sulphuric 
acid  until  any  remaining  precipitate  shall  have  been  dissolved.  Again 
wash  the  paper  with  hot  water,  then  warm  (but  not  boil)  the  acid  and 
precipitate  in  the  beaker  until  the  precipitate  is  dissolved.  Titrate 
with  standard  potassium  permanganate,  inserting  a  thermometer  and 
keeping  the  solution  at  80°  to  90°  (refer  to  page  250).  Calculate  the 
percent  of  calcium  in  the  sample. 

MANGANESE 

In  acid  solutions  potassium  permanganate  is  always  reduced 
to  the  form  of  manganous  salts,  manganese  being  thereby  reduced 
to  its  lowest  state  of  oxidation,  corresponding  to  the  monoxide. 
In  basic  solutions  the  reduction  goes  only  so  far  as  to  produce 
manganese  dioxide.  If  the  reducing  agent  is  a  manganous  salt 
it  is  also  oxidized  to  manganese  dioxide : 

2KMn04+3MnS04+2H20->K2SO4+5MnO2+2H2SO4.     (a) 

This  reaction  may  be  made  the  basis  of  a  determination  of 
manganese  by  titration  with  potassium  permanganate.1  The 
manganese  ore  is  dissolved  in  hydrochloric  acid,  manganous 
chloride  being  formed.  The  chloride  is  converted  into  sulphate 
and  the  hydrochloric  acid  removed  by  evaporation  with  sul- 
phuric acid.  When  the  titration  is  made  the  solution  must  be 
only  feebly  basic.  If  a  strong  base  is  present  a  reduction  of 

1  Volhard:  Chem.  News,  40,  207  (1879). 


254  QUANTITATIVE  ANALYSIS 

potassium  permanganate  to  potassium  manganate  occurs,  the 
reducing  agent  being  manganese  dioxide  or  manganese  hydroxide : 

2KMnO4+4KOH+Mn02^3K2Mn04+2H20,  (b) 

4KMn04+6KOH+Mn(OH)2-^5K2MnO4+4H2O.         (c) 

These  undesirable  reactions  may  be  prevented  by  having  present 
a  considerable  excess  of  a  weak  base,  such  as  is  produced  by  shak- 
ing an  excess  of  zinc  oxide  with  water,  this  giving  a  suspension  of 
zinc  oxide  in  a  saturated  solution  of  zinc  hydroxide.  The  latter 
is  so  dilute  and  so  weakly  ionized  that  the  formation  of  potassium 
manganate  does  not  take  place.  It  does  provide,  however,  suffi- 
cient base  to  neutralize  the  sulphuric  acid  produced  by  reaction 
(a),  because  the  excess  of  zinc  oxide  keeps  the  solution  saturated 
with  zinc  hydroxide. 

Manganese  dioxide  possesses,  to  a  slight  extent,  acM-forming 
properties,  since  it  is  able  to  produce  a  class  of  salts  that  are 
theoretical  derivatives  of  manganous  acid,  H2MnOs(  =  H2O.Mn02) . 
This  acid  is  not  known  in  the  free  state  but  certain  manganites,  as 
those  of  calcium  and  zinc,  CaMnO3  and  ZnMnOs,  are  produced 
when  manganese  dioxide  is  formed  in  presence  of  soluble  calcium 
or  zinc  compounds.  If  no  such  metal  is  present  at  the  moment 
of  oxidation  of  manganous  salts  to  manganese  dioxide,  manganous 
manganite,  MnMnOs,  is  precipitated,  thus  removing  a  certain 
amount  of  unoxidized  manganese  from  the  solution.  An  error 
would  thereby  be  introduced  but  this  is  prevented  by  having 
zinc  hydroxide  present.  The  saturated  solution  of  manganese 
dioxide  can  have  but  a  small  concentration  of  manganous  acid  and 
this  is  at  once  precipitated  as  zinc  manganite.  The  addition  of 
zinc  oxide  therefore  serves  a  double  purpose.  It  maintains  a 
feebly  basic  solution  throughout  the  titration  and  also  prevents 
the  precipitation  of  manganous  manganite. 

Problems 

67.  From  the  equation  for  the  reaction  between  potassium  permanganate 
and  manganous  sulphate,   calculate  the  equivalent  weight  of  potassium 
permanganate  and  of  manganese  in  manganese  sulphate. 

68.  What  weight  of  potassium  permanganate  must  be  contained  in  1000 
cc  of  a  solution,  1  cc  of  which  will  oxidize  0.002  gm  of  manganese? 

69.  If  a  solution  of  potassium  permanganate  is  fifth-normal  with  respect 


OXIDATION  AND  REDUCTION  255 

to  iron  in  acid  solution  what  is  its  normality  with  respect  to  manganese  in 
basic  solution? 

60.  1  cc  of  a  solution  of  potassium  permanganate  is  equivalent  to  0.010 
gm  of  iron.  What  weight  of  manganese  will  be  oxidized  by  1  cc? 

Determination. — Calculate  the  approximate  weight  of  pyrolusite 
or  other  manganese  compound  that  is  necessary  to  reduce  about  40  cc  of 
the  standard  potassium  permanganate  solution  already  made,  arbitrarily 
assuming  a  state  of  purity  for  the  sample  and  also  that  one-fifth  of  the 
sample  is  finally  to  be  titrated.  Dry  the  sample  to  constant  weight  at 
120°  and  weigh  the  calculated  quantity  of  dried  material,  placing  in  a 
casserole.  Dissolve  by  warming  with  concentrated  hydrochloric  acid. 
When  solution  is  complete  or  when  no  further  action  is  apparent  filter 
and  wash  the  residue  and  paper,  preserving  the  filtrate  and  washings.  If 
the  residue  contains  any  dark  material,  burn  the  paper  in  a  platinum 
crucible  and  fuse  with  1  to  2  gm  of  sodium  carbonate.  If  manganese  is 
present  the  fusion  will  be  colored  green  by  sodium  manganate.  Dis- 
solve the  fused  mass  in  hot  water  and  add  to  the  main  solution. 

Remove  hydrochloric  acid  by  evaporating  with  sulphuric  acid  (about 
2  cc)  to  the  appearance  of  fumes  of  sulphur  trioxide.  Redissolve,  adding 
a  little  nitric  acid  if  necessary,  wash  into  a  250  cc  volumetric  flask,  dilute 
to  the  mark  and  mix.  Treat  50  cc  portions  as  follows:  Measure  into  an 
Erlenmeyer  flask  of  1000  cc  capacity  and  neutralize  by  the  addition  of 
zinc  oxide  suspended  in  water,  shaking  and  continuing  the  addition  until 
any  iron  is  precipitated  as  ferric  hydroxide  and  a  sufficient  excess  of  zinc 
oxide  is  present  to  maintain  a  milky  appearance  throughout  the  subse- 
quent titration.  Dilute  to  about  300  cc,  heat  nearly  to  boiling  and 
titrate,  adding  the  potassium  permanganate  solution  5  cc  at  a  time  until 
a  permanent  color  is  produced.  Treat  a  second  50  cc  portion  of  the 
manganese  solution  in  a  similar  manner  but  adding,  all  at  once,  5  cc 
less  of  standard  solution  than  was  added  to  the  first  portion  then  adding 
1  cc  at  a  time.  To  a  third  portion  add  the  entire  quantity  used  in  the 
second,  less  1  cc,  then  complete  the  titration  by  adding  the  standard 
solution  0.1  cc  at  a  time.  Treat  the  fourth  portion  in  the  same  manner 
as  the  third.  From  the  last  two  titrations  calculate  the  percent  of 
manganese  in  the  ore. 

AVAILABLE  OXYGEN 

Manganese  dioxide  is  used  not  only  as  an  ore  of  manganese  but 
also  as  an  oxidizing  agent  in  various  laboratory  processes  and  in  a 
commercial  way,  as,  for  example,  in  the  production  of  chlorine 
from  hydrochloric  acid.  In  such  cases  the  percent  of  manganese 


256  '  QUANTITATIVE  ANALYSIS 

is  not  as  important  as  is  that  of  oxygen  available  for  oxidation 
processes.  In  most  cases  the  available  oxygen  may  be  calculated 
with  sufficient  accuracy  for  commercial  requirements  from  the 
percent  of  manganese.  This  can  be  accurately  done  only  in  case 
no  other  manganese  compound  and  no  other  peroxide  is  present. 
Generally  no  other  peroxide  is  present,  although  manganese  fre- 
quently occurs  in  pyrolusite  in  small  quantities  as  other  com- 
pounds than  the  peroxide.  If  an  accurate  determination  of  availa- 
ble oxygen  is  required  it  may  be  made  by  reducing  a  weighed  sample 
of  the  manganese  dioxide  by  a  measured  amount  of  a  reducing 
agent,  titrating  the  excess  of  the  latter  by  standard  potassium 
permanganate.  The  reducing  agent  may  be  any  of  those  already 
discussed  in  connection  with  standardization  of  potassium  per- 
manganate. Ferrous  ammonium  sulphate  or  oxalic  k  acid  is  to 
be  preferred.  The  reaction  between  manganese  dioxide  and  these 
reducing  agents  in  presence  of  sulphuric  acid  is  represented  by 
the  following  equations,  which  should  be  balanced  by  the  student 
as  an  exercise  in  calculation  of  hydrogen  equivalents.  Determine 
also  what  fraction  of  the  total  oxygen  of  manganese  dioxide  is 
"available." 

Mn02+ FeS04+  H2S04-+MnSO4+ Fe2(S04)3+ H20, 
Mn02+H2C204+H2S04-+MnS04+C02+H20. 

Another  method  for  determining  the  available  oxygen  is 
described  on  page  265. 

Determination. — Dry  the  sample  of  either  pyrolusite  or  commercial 
manganese  dioxide  to  constant  weight  at  120°.  Calculate  the  weight 
that  would  be  equivalent  to  approximately  40  cc  of  the  standard  potas- 
sium permanganate  already  made,  arbitrarily  assuming  that  the  sample 
is  pure  manganese  dioxide.  Weigh  samples  of  the  calculated  weight 
into  250  cc  Erlenmeyer  flasks.  Calculate  the  weight  of  crystallized 
ferrous  ammonium  sulphate  or  oxalic  acid  that  would  reduce  approxi- 
mately 50  cc  of  the  standard  solution  of  potassium  permanganate  and 
add  this  quantity  to  each  flask.  Calculate  the  approximate  volume  of 
dilute  or  concentrated  sulphuric  acid  necessary  to  enable  the  reactions 
to  proceed  and  add  three  times  this  volume  to  each  flask.  Add  50  cc 
of  water,  warm  to  70°  and  titrate  immediately  with  standard  potassium 
permanganate  solution.  Calculate  the  percent  of  available  oxygen  in 
the  sample. 


OXIDATION  AND  REDUCTION  257 

Potassium  permanganate  solution  is  also  useful  for  the  titra- 
tion  of  hydroferro cyanic  acid  and  hydroferricyanic  acid.  Ferro- 
cyanides  are  oxidized  in  acid  solution : 

10H4Fe(CN)6+2KMnO4+3H2S04->10H3Fe(CN)6+K2S04 

+2MnSO4+8H20. 

Ferri cyanides  may  be  reduced  in  basic  solution  by  ferrous  sulphate 
and  then  titrated  by  potassium  permanganate.     The  reaction 
is  as  follows: 
K3Fe(CN)6+FeSO4+3KOH->K4Fe(CN)6+Fe(OH)3+K2S04. 

Potassium  Bichromate. — The  equation  for  the  reaction  of 
potassium  dichromate  with  ferrous  salts  is  given  on  page  242. 
This  substance  possesses  several  advantages  over  potassium  per- 
manganate as  a  standard  oxidizing  agent.  It  is  relatively  more 
stable  and  therefore  may  be  obtained  in  a  state  of  uniform  purity. 
This  makes  it  possible  to  standardize  solutions  by  direct  weighing 
when  the  degree  of  purity  of  the  salt  has  been  established  by  analy- 
sis. The  relative  stability  is  the  same  with  solutions  and  the 
standard  solution  can  be  kept  almost  indefinitely  without  chang- 
ing its  concentration.  Potassium  dichromate  may  also  be  used 
for  the  titration  of  iron  and  other  reducing  agents  in  presence  of 
hydrochloric  acid  or  chlorides,  without  oxidation  of  the  latter 
taking  place.  This  is  a  very  decided  advantage  in  the  determina- 
tion of  iron  since  it  makes  possible  the  use  of  stannous  chloride  as 
a  reducing  agent  without  the  addition  of  manganous  sulphate 
and  phosphoric  acid.  There  is  no  indicator  that  can  be  added 
directly  to  the  solution  which  is  being  titrated  by  potassium 
dichromate  and  the  color  of  the  standard  solution  is  not  suffi- 
ciently intense  to  be  of  any  use  for  this  purpose.  The  indicator 
that  is  commonly  used  is  potassium  ferricyanide,  placed  in  drops 
on  a  white  porcelain  "spot  plate."  Drops  of  the  solution  are 
removed  from  time  to  time  by  means  of  a  stirring  rod  and  allowed 
to  touch  the  drops  of  ferricyanide.  So  long  as  ferrous  iron  is 
present  the  blue  of  ferrous  ferricyanide  is  apparent  on  the  spot 
plate.  When  the  last  trace  of  iron  has  been  oxidized  there  is 
produced  on  the  plate  only  the  light  brown  ferric  ferricyanide. 
There  being  nothing  in  the  appearance  of  the  solution  of  the  iron 
salt  to  -indicate  the  approach  to  the  end-point,  the  titration  is 

17 


258  QUANTITATIVE  ANALYSIS 

necessarily  somewhat  tedious  unless  a  system  is  devised  for 
rapid  readings.  Such  a  system  has  been  used  in  connection  with 
the  determination  of  manganese  and  is  indicated  in  the  next 
exercise  and  this  removes  the  last  objection  to  the  use  of  potas- 
sium dichromate  for  the  titration  of  iron. 

Problems 

61.  A  solution  of  potassium  permanganate  contains  25.38  gm  in  1000  cc. 
What  must  be  the  concentration  of  a  potassium  dichromate  solution  in  order 
that  it  shall  have  the  same  oxidizing  power  toward  iron? 

62.  Balance  the  following  equation  and  calculate  the  equivalent  weight 
of  tin. 

K2Cr2O7  +SnCl2  +HC1-»KC1 +CrCl3 +SnCl4 +H2O. 

Exercise:  Preparation  of  Standard  Potassium  Dichromate  Solu- 
tion.— The  solution  should  be  of  such  concentration  that  1  ccis  equivalent 
0.005  gm  of  iron.  Calculate  the  weight  of  potassium  dichromate  neces- 
sary for  2000  cc  of  such  a  solution.  If  the  salt  is  known  to  be  pure, 
weigh  exactly  the  calculated  weight  and  omit  further  standardization. 
If  it  is  not  pure  but  its  oxidizing  power  known  from  previous  determina- 
tions, calculate  the  weight  of  impure  sample  required  and  use  this  weight. 
If  nothing  is  known  of  the  purity  use  1  percent  more  than  the  weight  of 
pure  salt  required  for  2500  cc  of  solution  and  standardize  the  solution  as 
directed  below.  In  any  case  dissolve  the  weighed  salt  and  dilute  to  the 
proper  volume.  In  case  titration  for  standardization  is  to  be  omitted 
and  direct  weighing  is  to  be  made  the  basis  for  standardization,  2000  cc 
of  the  solution  should  be  accurately  made  and  poured  into  a  dry  bottle. 

Standardization,  if  this  should  be  necessary,  is  accomplished  by  titra- 
tion against  ferrous  ammonium  sulphate  or  iron  wire,  the  first  method 
being  preferable.  Write  and  balance  the  equation  for  the  oxidation  of 
ferrous  sulphate  by  potassium  dichromate  in  presence  of  sulphuric  acid 
referring,  if  necessary,  to  the  equation  for  the  oxidation  of  the  chloride, 
page  242.  Calculate  the  weight  of  crystallized  ferrous  ammonium 
sulphate  necessary  to  reduce  35  cc  of  the  dichromate  solution.  Weigh 
five  portions  of  exactly  this  weight  into  250  cc  beakers  and  dissolve  each 
in  50  cc  of  recently  boiled  and  cooled  water  just  before  titrating.  Pre- 
pare a  0.01  percent  solution  of  potassium  ferricyanide  and  place  a  drop 
in  each  of  the  depressions  of  a  white  porcelain  spot  plate.  Add  to  the 
solution  of  ferrous  ammonium  sulphate  three  times  the  calculated  amount 
of  sulphuric  acid  necessary,  as  indicated  by  the  equation,  and  titrate  at 
once,  as  follows:  To  the  first  solution  add  the  dichromate  solution  5  cc 
at  a  time,  stirring  well  after  each  addition,  and  test  by  removing  a  drop 
by  means  of  the  stirring  rod  and  touching  to  a  drop  of  potassium  ferri- 


OXIDATION  AND  REDUCTION  259 

cyanide  solution  on  the  spot  plate.  The  end-point  is  reached  when  a 
blue  color  is  no  longer  produced  on  the  plate,  after  standing  for  2 
minutes.  Dust  or  reducing  gases  will  interfere  by  reducing  traces  of  ferric 
chloride.  Titrate  the  second  solution  by  adding  5  cc  less  than  tEe 
amount  of  dichromate  solution  used  in  the  first,  then  adding  1  cc  at  a 
time.  Titrate  the  third  solution  by  adding  1  cc  less  than  the  total 
used  in  the  second,  then  adding  0. 1  cc  at  a  time.  Titrate  the  fourth 
and  fifth  solutions  in  the  same  manner  and  take  the  average  of  the 
last  three  titrations  for  permanent  record.  Calculate  the  value  of  the 
solution  in  terms  of  iron.  Dilute  to  make  1  cc  equivalent  to  0.005  gm 
of  iron. 

Instead  of  weighing  five  portions  of  ferrous  ammonium  sulphate  a 
standard  solution  may  be  made  by  dissolving  ten  times  the  required 
amount,  adding  the  necessary  sulphuric  acid  and  diluting  to  500  cc. 
Portions  of  50  cc  are  then  measured  and  titrated.  The  solution  oxidizes 
upon  exposure  to  air  and  the  method  of  weighing  separate  portions  and 
dissolving  just  before  titration  is  preferable. 

IRON 

Determination  of  Iron. — Prepare  a  sample  of  iron  ore  by  grinding 
to  pass  a  100-mesh  sieve.  Weigh  five  portions  of  exactly  0.5  gm  each, 
using  the  counterpoised  glasses  and  brushing  the  ore  into  porcelain 
crucibles.  Heat  the  inclined  crucibles  for  5  minutes  over  the  desk 
burner,  cool,  place  in  casseroles  and  dissolve  in  hydrochloric  acid,  with  or 
without  the  addition  of  stannous  chloride.  Reduce  each  solution  just 
before  titration,  following  the  directions  given  for  dissolving  and  reduc- 
ing by  method  (b)  of  the  permanganate  method.  Do  not  add  the  solu- 
tion of  manganous  sulphate  and  of  phosphoric  and  sulphuric  acids. 
Dilute  to  100  cc.  The  titration  is  carried  out  exactly  as  directed  for 
standardizing  potassium  dichromate  solution.  Calculate  the  percent 
of  iron  in  the  ore. 

CHROMIUM 

The  most  important  ore  of  chromium  is  a  compound  of  iron 
and  chromium  known  as  "chromite,"  having  a  composition 
corresponding  with  the  formula  FeO.Cr2O3.  Although  chro- 
mium is  here  in  its  lowest  state  of  oxidation  the  substance  is  thought 
to  be  a  salt  of  a  hypothetical  chromous  acid,  H2Cr204.  Chromite 
cannot  be  dissolved  in  acids  nor  is  it  possible  to  decompose  it 
easily  by  fusion  with  alkali  carbonates.  Fusion  with  sodium 
peroxide  decomposes  it,  oxidizes  the  iron  to  ferric  oxide  and  the 


260  QUANTITATIVE  ANALYSIS 

chromium  to  chromium  trioxide,  forming  then  sodium  chromate. 
Upon  dissolving  in  water  and  filtering,  ferric  oxide  is  removed. 
The  addition  of  acid  produces  sodium  dichromate.  This  can 
then  be  reduced  by  adding  an  excess  of  a  standard  reducing  agent, 
such  as  ferrous  ammonium  sulphate,  titrating  the  excess  by  stand- 
ard potassium  dichromate  or  permanganate.  The  reactions  are 
expressed  by  the  following  equations,  which  should  be  balanced 
by  the  student. 

FeCr204+Na202^Fe2O3+Na2Cr04+Na2O, 
Na2CrO4+HCl^Na2Cr2O7+NaClH-H2O, 
Na2Cr207+FeS04+HCl^NaCl+Fe2(S04)3+ 

FeCl3+CrCl3+H20. 

Iodine  and  Sodium  Thiosulphate. — Iodine  and  thiosulphates 
react  quantitatively,  forming  sodium  iodide1  and  sodium  tetra- 
thionate : 

2Na2S203+l2-»Na2S406+2NaL 

This  is  an  oxidation  of  sodium  thiosulphate  by  iodine,  which  is 
itself  reduced.  The  solution  may  be  originally  neutral  or  acid, 
or  alkali  bicarbonates  may  be  present.  Normal  carbonates  or 
hydroxides  should  not  be  present  since  they  also  combine  with 
iodine : 

2NaOH + I2-+NaIO + Nal + H2O, 
2Na2CO3+l2->NaIO+N3l+CO2. 

The  color  of  dilute  solutions  of  iodine  is  sufficiently  intense  to 
serve  as  a  fairly  accurate  indicator.  Much  more  accurate  re- 
sults are  obtained  by  the  use  of  starch  as  an  indicator,  mere  traces 
of  iodine  producing  a  visible  blue  or  rose  red  color  with  starch. 
Because  of  the  fact  that  iodine  is  an  excellent  oxidizing  agent  for 
many  substances  when  a  bicarbonate  is  present,  and  that  hydri- 
odic  acid  is  oxidized  by  many  oxidizing  agents  when  an  acid  is 
present,  free  iodine  being  liberated,  the  two  standard  solutions 
of  iodine  and  sodium  thiosulphate  form  a  most  useful  pair  for 
volumetric  analysis.  As  an  example  of  their  use  the  reactions  of 
arsenic  may  be  noticed.  Arsenious  acid  or  an  arsenite  is  oxi- 
dized by  free  iodine  thus: 

H3AsO3+I2+H2Orfl3As04+2m, 


OXIDATION  AND  REDUCTION  261 

This  reaction  does  not  take  place  quantitatively  but  is  reversible. 
If,  however,  sodium  bicarbonate  is  present  in  excess  the  hydri- 
odic  acid  is  neutralized  as  fast  as  it  is  formed  and  the  reaction  is 
completed.  Standard  iodine  solution  may,  in  this  way,  be  used 
for  the  titration  of  arsenious  acid.  On  the  other  hand  arsenic 
acid  is  reduced  by  hydriodic  acid: 

H3As04+2HI->H3AsO3+H20+l2. 

This  is  seen  to  be  the  reverse  of  the  reaction  expressed  above 
and  it  would  follow  that  it  also  is  incomplete  unless  one  of  the 
products  is  removed.  This  may  be  doiie  by  adding  sodium 
thiosulphate  to  remove  the  iodine,  in  which  case  the  standard 
sodium  thiosulphate  indirectly  titrates  the  arsenic  acid.  In  prac- 
tice hydriodic  acid  is  not  kept  as  a  reagent  because  of  its  insta- 
bility but  potassium  or  sodium  iodide  and  hydrochloric  acid  are 
used  instead,  hydriodic  acid  being  thus  made  available  in  the 
solution. 

Standardization. — By  properly  purifying  iodine  standard 
solutions  may  be  made  by  direct  weighing.  Commercial  iodine 
is  usually  not  sufficiently  pure  for  this  purpose  and  must  be 
analyzed  if  it  is  to  be  used  in  this  way.  Iodine  solutions  will  not 
remain  constant  in  oxidizing  power  because  of  interaction  between 
iodine  and  water,  and  it  is  usually  not  advisable  to  attempt  to 
dilute  solutions  to  a  definite  concentration  because  they  must  be 
restandardized  after  short  intervals  of  time.  For  this  reason 
standardization  by  direct  weighing  is  not  practicable  and  the 
iodine  need  not  be  purified  before  dissolving.  The  solution  may 
then  be  standardized  by  titrating  against  any  standard  reducing 
solution.  The  best  substances  for  this  purpose  are  sodium  thio- 
sulphate and  arsenious  oxide. 

Iodine  does  not  dissolve  easily  in  water  but  is  readily  soluble 
in  a  solution  of  potassium  iodide  or  sodium  iodide.  Such  a  solu- 
tion probably  contains  an  iodide  having  the  formula  KI3  or  Nals. 
Many  organic  liquids  are  good  solvents  for  iodine.  Examples  are 
the  alcohols  and  acetic  acid.  -  These  will  be  discussed  in  the  sec- 
tion dealing  with  the  analysis  of  fats  and  oils.  When  starch  and 
iodine  are  brought  together  a  deep,  indigo-blue  color  is  produced 
and  this  serves  as  a  very  delicate  test  for  either  starch  or  iodine. 
The  nature  of  the  blue  substance  has  long  been  the  subject  of 


262  QUANTITATIVE  ANALYSIS 

investigation  and  discussion.     It  is  probably  a  solid  solution  of 
iodine  in  starch. 

Sodium  thiosulphate  may  sometimes  be  obtained  in  a  suffi- 
ciently pure  condition  to  allow  standardization  by  direct  weigh- 
ing. It  is  better  to  make  the  solution  somewhat  more  concen- 
trated than  that  desired  and  to  standardize  and  dilute  to  a 
definite  concentration.  For  standardization  the  solution  may  be 
directly  titrated  against  standard  iodine  solution  or  indirectly 
against  potassium  dichromate  or  a  salt  of  copper.  It  has  already 
been  stated  that  potassium  dichromate  may  be  obtained  in  a 
state  of  uniform  purity.  If  to  a  standard  solution  of  this  salt 
potassium  iodide  and  hydrochloric  acid  are  added,  iodine  is 
liberated  as  follows: 

K2Cr207+6KI+14HCl->8KCl+2CrCl3+7H20^3I2. 

The  liberated  iodine  may  be  titrated  by  sodium  thiosulphate  and 
the  latter  thus  standardized.  The  solution  of  potassium  dichro- 
mate used  for  iron  determinations  may  be  used  also  for  this 
purpose.  It  was  standardized  in  the  decimal  system,  however, 
and  it  will  be  necessary  to  calculate  its  value  in  the  normal  system 
because  the  solution  of  sodium  thiosulphate  is  to  be  used  for  the 
determination  of  several  different  substances.  The  following  ex- 
ample will  illustrate  the  method  of  calculation  of  standardization. 
Example. — 40  cc  of  a  solution  of  potassium  dichromate  liberates 
iodine  equivalent  to  22  cc  of  sodium  thiosulphate  solution.  1  cc  of 
potassium  dichromate  solution  is  equivalent  to  0.005  gm  of  iron.  What 
is  the  normality  of  the  thiosulphate  solution? 

1  cc  of  sodium  thiosulphate  solution  is  equivalent  to  ~~  cc  of  potas- 

40 
sium  dichromate  solution  and  to  ^  X  0.005  gm  of  iron.     A  normal 

solution  would  be  equivalent  to  0.05584  gm  of  iron.     Therefore  the 

...     .      40X0.005      A1.OOAT 
normality  is  ^-^-0.1828  N. 

The  standardization  against  a  salt  of  copper  is  also  an  excellent 
method.  This  is  described  on  page  268  in  connection  with  the 
determination  of  copper. 

Sodium  thiosulphate  is  quite  stable  in  solution  and  may  be 
kept  for  months  without  appreciable  change  in  concentration  if 
the  water  contains  no  trace  of  acid.  Even  carbonic  acid  causes 


OXIDATION  AND  REDUCTION  263 

decomposition  and  free  sulphur  is  deposited  from  the  solution, 
sulphurous  acid  being  formed.  This  is  because  thiosulphuric 
acid  is  very  unstable  and  rapidly  decomposes : 

Na2S203+H2C03^Na2C03+H2S203, 
H2S203-»H2S03+S. 

Even  a  very  small  amount  of  carbonic  acid  is  sufficient  to  start 
the  decomposition  by  liberating  some  thiosulphuric  acid.  As 
sulphurous  acid  accumulates  it  aids  the  decomposition  which  is 
thus  progressive: 

Na2S203+H2S03-+Na2SO3+H2S203, 
H2S203-»H2S03+S. 

In  order  to  avoid  starting  this  series  of  reactions  the  water 
should  be  boiled  and  cooled  before  making  the  solution. 

Problems 

63.  A  solution  of  potassium  dichromate  contains  4.95  gm  of  the  salt  in 
1000  cc.     What  weight  of  sodium  thiosulphate  is  equivalent  to  1  cc? 

64.  A  solution  of  potassium  dichromate  contains  6.235  gm  in  1000  cc. 
30  cc  of  this  solution  is  equivalent  to  42.9  cc  of  sodium  thiosulphate  solution. 
What  is  the  normality  of  the  latter? 

65.  What  weight  of  potassium  dichromate  must  be  dissolved  in  250  cc  to 
make  a  solution,  25  cc  of  which  is  equivalent  to  35  cc  of  sodium  thiosulphate 
solution  containing  13.65  gm  of  the  crystallized  salt  in  1000  cc? 

66.  1  cc  of  potassium  dichromate  solution  is  equivalent  to  0.005  gm  of 
iron.     What  is  the  iodine  equivalent? 

67.  25  cc  of  iodine  solution  is  equivalent  to  0.125  gm  of  potassium  dichro- 
mate.    To  what  volume  should  1000  cc  be  diluted  to  make  the  solution 
tenth-normal? 

68.  20  cc  of  potassium  dichromate  solution  oxidizes  0.0240  gm  of  oxalic 
acid,  H2C2O4.2H2O.     1  cc  of  the  same  solution  oxidizes  the  same  weight  of 
iron  as  does  1.2  cc  of  potassium  permanganate  solution.     What  is  the  nor- 
mality of  the  latter  solution? 

Exercise :  Preparation  of  Tenth-normal  Sodium  Thiosulphate  Solu- 
tion.—Calculate  the  weight  of  crystallized  sodium  thiosulphate, 
Na2S203.5H20,  required  for  2500  cc  of  tenth-normal  solution.  Crush 
the  salt  and  dissolve  2  percent  more  than  this  weight  in  cold,  recently 
boiled  water  and  dilute  to  2500.  cc.  Keep  the  bottle  well  stoppered 
and  out  of  direct  sunlight. 

Make  200  cc  of  a  solution  containing  30  gm  of  potassium  iodide. 
The  starch  solution  is  made  as  follows:  Moisten  1  gm  of  starch  with 


264  QUANTITATIVE  ANALYSIS 

cold  water  to  make  a  thick  paste.  Heat  200  cc  of  water  to  boiling 
and  pour  it  into  the  starch  paste.  Boil,  with  constant  stirring,  for  one 
minute.  The  solution  does  not  keep  well  and  should  be  made  each 
day  as  required.  Standardize  the  sodium  thiosulphate  solution  against 
potassium  dichromate.  If  the  solution  used  in  iron  determinations  is 
at  hand,  use  this,  otherwise  make  250  cc  of  exactly  tenth-normal 
solution  by  weighing  the  salt,  dissolving  and  diluting  to  the  required 
volume.  Measure  35  cc  of  either  solution  into  an  Erlenmeyer  flask,  add 
40  cc  of  potassium  iodide  solution  and  10  cc  of  concentrated  hydro- 
chloric acid.  Titrate  at  once  with  sodium  thiosulphate  solution, 
deferring  the  addition  of  starch  as  long  as  possible.  If  starch  is  added 
before  the  iodine  is  nearly  all  reduced  a  precipitate  of  starch  iodide 
will  form,  free  iodine  being,  in  this  way,  removed  from  the  possibility 
of  reacting  with  thiosulphate.  A  false  end  point  is  then  obtained. 

The  solution  of  chromium  chloride,  formed  by  reduction  of  potassium 
dichromate,  is  green.  The  solution  has  an  amber  tint  as  long  as  much 
free  iodine  is  present.  Upon  the  addition  of  starch  the  solution  acquires 
a  blue-green  color  and  the  change  to  pure  green  at  the  end  point  may  be 
difficult  to  detect  at  first  trial.  With  a  little  experience  the  difficulty  will 
disappear.  Make  at  least  three  titrations  and  calculate  the  normality 
of  the  sodium  thiosulphate  solution.  Dilute  to  make  tenth-normal. 

Make  a  blank  test  upon  the  potassium  iodide,  omitting  the  potassium 
dichromate  but  adding  the  hydrochloric  acid.  If  iodine  is  found, 
correct  the  observed  volume  of  sodium  thiosulphate  before  calculating 
its  concentration. 

OXIDIZING  POWER  OF  PEROXIDES 

Such  peroxides  as  those  of  manganese,  barium,  lead,  and  hy- 
drogen readily  oxidize  hydriodic  acid  and  liberate  iodine.  The 
titration  of  the  latter  by  standard  sodium  thiosulphate  solution 
constitutes  an  indirect  determination  of  the  oxidizing  power,  or 
"available  oxygen"  of  the  peroxide.  In  practice  it  is  sometimes 
not  found  convenient  to  add  an  iodide  and  hydrochloric  acid 
directly  to  the  peroxide  because  the  solution  is  usually  colored  by 
impurities  dissolving  as  chlorides.  In  such  a  case  hydrochloric 
acid  is  added  to  the  peroxide  and  the  liberated  chlorine  is  distilled 
into  potassium  iodide  solution.  In  the  case  of  manganese  perox- 
ide the  reactions  may  be  represented  thus: 

Mn02+4HCl-+MnCl2+2H2 
C12+2KI-*2KC1+I2. 


OXIDATION  AND  REDUCTION 


Problem 


265 


69.  Calculate  the  equivalent  weight  of  manganese  dioxide  and  of  available 
oxygen  and  find  the  weight  of  each  that  is  equivalent  to  1  cc  of  tenth-nornral 
sodium  thiosulphate  solution. 

These  reactions  are  analogous  to  those  occurring  with  other 
peroxides  and  the  determination  of  available  oxygen  in  manga- 
nese dioxide  istthe  most  frequently  made  of  all.  The  apparatus 
for  carrying  out  the  decomposition  arid  distillation  should  have 
ground  glass  joints  and  should  not  allow  contact  of  iodine  or 
chlorine  with  organic  matter.  The  modified  apparatus  of  Bun- 
sen,  Fig.  68,  may  be  used.  The  receiver  must  be  kept  cold  in 
order  to  avoid  loss  of  iodine. 


FIG.  68. — Modified    Bunsen's    apparatus    for    the    determination    of    available 

oxygen. 

Determination. — Dry  2  to  4  gm  of  either  commercial  manganese 
dioxide  or  pyrolusite  at  120°  until  the  weight  is  constant.  The  sample 
already  used  for  the  determination  of  manganese  may  be  used  for  this 
determination  and  the  results  obtained  by  the  two  methods  compared. 
Weigh  enough  sample  to  be  equivalent  to  about  35  cc  of  the  standard 
sodium  thiosulphate  solution  and  place  in  the  flask  of  a  Bunsen  distil- 
ling apparatus  or  of  some  other  suitable  type.1  Place  in  the  receiver  2 
gm  of  potassium  or  sodium  iodide,  dissolve  this  in  water  and  dilute  until 
the  bend  is  just  sealed  when  the  apparatus  is  in  the  proper  position. 
Immerse  the  receiver  in  ice  water,  then  add  to  the  flask  containing  the 
manganese  dioxide  30  cc  of  concentrated  hydrochloric  acid  and  quickly 
insert  the  stopper  carrying  the  delivery  tube.  Warm  the  acid  gently, 
distilling  the  chlorine  into  the  potassium  iodide  solution.  Raise  the 


266  QUANTITATIVE  ANALYSIS 

temperature  gradually  until  the  acid  is  boiling  and  boil  for  five  minutes 
after  action  is  completed.  While  the  burner  is  still  under  the  flask  lower 
the  receiver  until  the  delivery  tube  is  entirely  out  of  it,  then  remove  the 
burner.  Remove  the  delivery  tube  from  the  flask  and  rinse  it  inside 
and  outside,  the  water  flowing  back  to  the  receiver.  Rinse  the  whole 
iodine  solution  into  an  Erlenmeyer  flask  and  titrate  with  sodium  thio- 
sulphate  solution. 

Make  a  blank  test  on  the  iodide  used,  as  follows: 

Weigh  out  the  same  amour  t  as  was  used  in  the  determination  of 
available  oxygen,  the  weighing  being  accurate  to  centigrams.  Dissolve 
in  100  cc  of  distilled  water  and  add  5  cc  of  concentrated  hydrochloric 
acid.  If  a  yellow  color  appears,  indicating  the  presence  of  free  iodine, 
titrate  with  sodium  thiosulphate,  using  starch  at  the  end.  Deduct  the 
thiosulphate  used  in  the  blank  from  that  used  in  the  determination  of 
available  oxygen  and  calculate  the  percent  of  available  oxygen,  also 
the  theoretical  percent  of  manganese  and  of  manganese  dioxide.  If 
the  sample  is  the  same  as  that  used  for  the  direct  determination  of 
manganese  and  of  available  oxygen  by  potassium  permanganate  an 
interesting  comparison  of  results  of  different  methods  may  be  made, 
although  the  calculation  of  available  oxygen  from  the  percent  of  man- 
ganese may  not  check  with  the  direct  determination,  for  reasons  already 
discussed. 

Sodium  thiosulphate  may  be  used  to  titrate  the  iodine  produced 
by  the  action  of  almost  any  oxidizing  agent"  upon  a  solution  of 
potassium  iodide  and  hydrochloric  acid.  Peroxides  have  already 
been  discussed.  Other  substances  that  may  be  determined  are 
free  halogens  (chlorine  and  bromine  being  allowed  to  displace 
iodine  from  potassium  iodide),  easily  reducible  oxyacids  and  their 
salts,  as  the  halogen  oxyacids,  nitrous  acid  and  persulphuric 
acid,  oxysalts  of  metals  that  exist  in  acid  radicals,  as  dichromates, 
chromates,  permanganates  and  manganates,  and  salts  of  metals 
that  possess  more  than  one  valence,  as  iron,  copper,  mercury  and 
arsenic. 

While  sodium  thiosulphate  may  be  used  for  the  determination 
of  almost  any  oxidizing  agent  it  is  not  necessarily  true  that  this 
provides  the  best  method  for  all  such  materials.  In  many  cases 
other  methods  will  be  found  to  give  better  results  or  to  be  more 
conveniently  applied. 


OXIDATION  AND  REDUCTION  267 

Problem 

70.  Complete  the  following  equations,  balance  and  determine  the  equiva- 
lent weights  of  each  of  the  oxidizing  agents. 

Br2+KI—  » 
KBrO+KI+HCl—  » 
KBrQs+KI+HCl-* 
KC1O+KI+HC1—  > 
K2Cr2O7+KI+HCl—  > 
KMnO4+KI+HCl-> 
FeCl3+KI-» 
CuCl2+KI-» 

COPPER 

The  gravimetric  determination  of  copper  may  be  made  by 
precipitating  as  cupric  hydroxide,  heating  and  weighing  as 
cupric  oxide,  or  by  precipitating  as  cupric  sulphide,  heating  with 
sulphur  and  hydrogen  and  weighing  as  cuprous  sulphide.  Both 
methods  are  difficult  of  execution  and  are  subject  to  considerable 
errors.  Electrolytic  methods  are  more  accurate  and  more  easy 
of  accomplishment.  Copper  may  be  determined  volumetrically 
by  several  methods,  one  of  the  best  being  Low's  "iodide  method."1 
This  method  depends  upon  the  insolubility  of  cuprous  iodide  and 
the  instability  of  cupric  iodide.  If  to  a  solution  of  a  cupric 
salt,  containing  no  highly  ionized  acid  and  no  other  oxidizing 
agent,  potassium  iodide  is  added  there  is  an  immediate  precipita- 
tion of  cuprous  iodide  with  liberation  of  iodine  : 


The  iodine  may  be  titrated  by  standard  sodium  thiosulphate 
solution  and  copper  calculated. 

Problems 

71.  Calculate  the  equivalent  weight  of  copper  and  the  weight  which  is 
equivalent  to  1  cc  of  tenth-normal  sodium  thiosulphate  solution. 

72.  What  weight  of  a  copper  ore  should  be  taken  for  analysis  in  order 
that  1  cc  of  fifth-normal  thiosulphate  solution  should  indicate  1  percent  of 
copper  in  the  ore? 

1  J.  Am.  Chem.  Soc.,  18,  458  (1896);  24,  1082  (1902). 

See  also  a  comparison  of  methods  by  Fernekes  and  Koch:  Ibid.,  27, 
1224  (1905). 


268  QUANTITATIVE  ANALYSIS 

If  sodium  thiosulphate  solution  is  to  be  standardized  against 
pure  copper,  the  metal  is  dissolved  in  nitric  acid,  most  of  the 
nitrogen  oxides  are  expelled  by  boiling  and  any  remaining  trace 
of  nitrous  acid  is  oxidized  by  bromine.  The  excess  of  nitric 
acid  is  then  neutralized  by  ammonium  hydroxide,  acetic  acid 
and  potassium  iodide  are  added  and  the  free  iodine  titrated  at 
once. 

If  a  copper  ore  or  crude  copper  is  to  be  analyzed  all  metals 
whose  iodides  are  insoluble  or  whose  salts  will  oxidize  potassium 
iodide  must  first  be  removed.  The  addition  of  metallic  alu- 
minium to  the  solution  containing  sulphuric  acid  will  precipitate 
copper  and  leave  in  solution  all  other  metals  of  higher  decomposi- 
tion potentials  as  well  as  those  soluble  in  sulphurip  acid,  providing 
that  nitric  acid  be  absent.  The  latter  can  be  removed  ;by  evapo- 
rating with  sulphuric  acid.  This  treatment  also  precipitates 
lead  as  sulphate,  which  may  be  removed  by  filtration.  After 
the  copper  is  precipitated  by  aluminium  it  is  removed  by  filtration, 
washed,  dissolved  in  nitric  acid  and  determined  as  in  the  stand- 
ardization of  sodium  thiosulphate. 

During  the  process  of  filtration  and  washing,  copper  oxidizes 
and  dissolves  to  some  extent  in  the  sulphuric  acid.  This  would 
occasion  a  loss  and  to  prevent  this  a  solution  of  hydrogen  sulphide 
is  used  for  the  wash  liquid.  Any  small  amount  of  copper  that 
might  be  dissolved  is  reprecipitated  as  cupric  sulphide.  If  a 
brown  color  appears  in  the  filtrate  below  this  is  an  indication  of 
incomplete  precipitation  by  the  aluminium  or  of  resolution  during 
filtration.  In  either  case  the  determination  is  spoiled  unless 
the  copper  can  be  recovered  by  refiltration. 

Exercise:  Standardization  of  Sodium  Thiosulphate  Solution. — The 

standard  solution  already  prepared  may  be  used  and  the  copper  equiva- 
lent calculated.  In  case  it  is  desired  to  standardize  against  copper 
or  a  copper  salt,  proceed  by  one  of  the  following  methods: 

(a)  Standardization  against  Metallic  Copper  of  Known  Purity. — Weigh 
sufficient  copper  to  require  about  35  cc  of  sodium  thiosulphate  solu- 
tion. Place  in  a  250  cc  flask  and  dissolve  by  warming  with  5  cc  of 
a  mixture  of  equal  volumes  of  concentrated  nitric  acid  and  water. 
Dilute  to  25  cc  and  boil  to  expel  nitrogen  oxides.  Add  5  cc  of  bro- 
mine water  and  boil  until  all  excess  bromine  is  removed.  Cool  and  add 
strong  ammonium  hydroxide  until  a  clear  blue  solution  is  obtained,  then 


OXIDATION  AND  REDUCTION  269 

boil  until  copper  hydroxide  begins  to  precipitate.  Acidify  with  acetic 
acid  and  boil,  if  necessary,  to  dissolve  any  precipitated  cupric  hydroxide. 
Cool,  add  3  gm  of  potassium  or  sodium  iodide  and  titrate  the  liberated 
iodine  with  sodium  thiosulphate  solution.  Calculate  the  copper  equiva- 
lent of  the  solution. 

(b)  Standardization  against  a  Copper  Salt. — Weigh  the  proper  amount 
of  cupric  sulphate  of  known  purity,  dissolve  in  25  cc  of  water,  make 
slightly  basic  with  ammonium  hydroxide  and  from  this  point  proceed 
as  in  (a). 

Determination. — Dissolve  0.5  gm  of  ore  in  a  covered  casserole  with 
10  cc  of  concentrated  hydrochloric  acid  and  5  cc  of  concentrated  nitric 
acid,  warming  if  necessary  to  aid  solution.  If  the  sample  is  an  alloy  use 
enough  to  contain  0.2  to  0.4  gm  of  copper  and  dissolve  in  10  cc  of  nitric 
acid,  1:1  (specific  gravity  1.2).  Add  7  cc  of  concentrated  sulphuric 
acid  and  evaporate  until  dense  white  fumes  of  sulphur  trioxide  appear. 
Cool,  add  25  cc  of  water  and  boil  to  dissolve  the  sulphates.  Filter  to 
remove  lead  sulphate  and  gangue,  allowing  the  filtrate  to  run  into  a 
beaker.  Wash  the  residue  and  paper  and  dilute  the  filtrate  and  washings 
to  75  cc. 

Cut  a  strip  of  sheet  aluminium  about  2.5  cm  wide  and  14  cm  long, 
bend  into  a  triangle  and  stand  on  its  edge  in  the  solution.  Cover  and 
boil  until  all  copper  is  precipitated  and  the  solution  is  colorless  or  green 
from  ferrous  sulphate.  If  this  condition  cannot  be  attained  it  is  because 
nitric  acid  was  not  completely  removed  when  evaporating  with  sul- 
phuric acid.  When  all  copper  is  precipitated,  wash  down  the  sides  of 
the  beaker  with  a  jet  of  hydrogen  sulphide  solution,  pour  the  solution 
into  a  filter  paper  and  filter  quickly. 

Transfer  the  copper  to  the  filter,  washing  the  aluminium  with  hydro- 
gen sulphide  solution  while  still  in  the  beaker.  Wash  thoroughly  with 
hydrogen  sulphide  solution  and  then  place  a  clean  flask  under  the  filter. 
Add  to  the  beaker  containing  the  aluminium  6  cc  of  nitric  acid,  sp.  gr. 
1.2.  Boil  shortly  to  dissolve  adhering  copper  then  pour  the  acid  slowly 
over  the  filter  to  dissolve  the  copper  on  the  paper.  When  all  copper 
seems  to  be  dissolved  pour  over  the  paper  5  cc  of  bromine  water. 
Wash  beaker  and  paper  thoroughly  with  hot  water  then  open  the  paper 
and  wash  into  the  flask  any  particles  of  copper  that  have  escaped  the 
action  of  the  acid.  Boil  until  all  bromine  is  removed,  add  strong 
ammonium  hydroxide  until  a  deep  blue  is  obtained,  boil  until  copper 
hydroxide  begins  to  precipitate  and  from  this  point  proceed  as  in  the 
standardization  of  sodium  thiosulphate  solution.  Calculate  the  percent 
of  copper  in  the  ore. 


270  QUANTITATIVE  ANALYSIS 

BLEA.CHING  POWDER 

When  gaseous  chlorine  is  passed  over  slaked  lime  it  is  absorbed 
with  formation  of  an  unstable  compound  that  is  easily  made  to 
yield  chlorine  under  certain  conditions  and  the  compound  pro- 
vides a  convenient  means  for  storing  and  transporting  chlorine  to 
be  used  for  bleaching,  disinfecting,  etc.  This  compound,  known 
as  " bleaching  powder,"  is  a  double  salt  of  calcium  with  hydro- 
chloric and  hypochlorous  acids  and  may  be  represented  by  the 

/Cl 

formula  Ca<T        ,     When  dissolved    in  water  it  is   probably 

^C\0 

ionized  in  the  manner  characteristic  of  both  acids.  When  any 
acid,  even  carbonic  acid,  is  added  to  bleaching  powder  chlorine 
is  liberated:  » 

CaCl.C10+H2CO3->CaC03+H20+Cl2. 

This  is  due  to  the  fact  that  when  hydrochloric  acid  and  hypochlo- 
rous acid  come  together,  even  in  dilute  solutions,  they  act  upon 
each  other  with  the  formation  of  chlorine : 

HC1+HC10-VE2O+C12. 

Because  of  the  easy  decomposition  of  bleaching  powder  by  car- 
bonic acid  it  rapidly  deteriorates  when  exposed  to  air,  chlorine 
escaping.  Loss  of  efficiency  also  occurs  through  loss  of  oxygen : 

2CaCl.ClO->2CaCl2+02> 

and  through  a  decomposition  such  that  calcium  chlorate  is 
formed : 

6CaCl.ClO->Ca(ClO3)2+5CaCl2. 

The  decompositions  represented  by  the  last  two  equations  result 
in  the  formation  of  chlorine  compounds  in  which  the  chlorine  is 
not  liberated  upon  acidification.  A  determination  of  total  chlo- 
rine would  therefore  be  of  little  value  as  an  estimate  of  the  useful- 
ness of  bleaching  powder.  " Available  chlorine"  is  better 
determined  by  a  volumetric  process.  For  this  purpose  the  acidi- 
fied solution  may  be  treated  with  potassium  iodide  and  the  liber- 
ated iodine  titrated  with  standard  sodium  thiosulphate  solution, 
or  the  solution  may  be  titrated  directly  by  a  standard  solution  of 
sodium  arsenite.  For  the  last  titration  the  indicator  is  a  paste  of 
starch  and  potassium  iodide  used  on  a  porcelain  plate  or  absorbed 


OXIDATION  AND  REDUCTION  271 

by  filter  paper  and  dried.  This  method  of  reading  the  end  point 
is  inconvenient  and  the  first  method  of  titration  is  the  better  one. 

If  calcium  chlorate  is  present  in  bleaching  powder  and  a  strong 
acid  is  used  for  liberating  the  chlorine,  the  chlorate  will  be  de- 
composed, though  but  slowly.  This  is  because  chloric  acid  is 
formed  by  the  reaction  of  chlorate  with  added  acid  and  chloric 
acid  is  slowly  reduced  by  hydrochloric  acid,  liberating  chlorine: 

HC103+5HCl->3H20-f3Cl2. 

During  the  titration  the  effect  of  these  reactions  is  seen  in  an 
uncertain  end  point.  As  sodium  thiosulphate  is  added  the  blue 
color  of  starch  iodide  disappears  and  then  returns  and  deepens. 
As  the  addition  of  thiosulphate  is  continued  the  blue  finally 
permanently  disappears,  but  this  end  point  does  not  represent 
the  titration  of  chlorine  really  available  in  bleaching  processes 
because  that  which  comes  from  calcium  chlorate  is  evolved  too 
slowly  to  be  of  much  use.  This  interference  with  the  titration 
may  be  almost  entirely  averted  by  using  a  weak  acid  instead  of 
a  strong  one  for  the  decomposition  of  the  chlorohypo chlorite. 
The  concentration  of  chloric  acid  does  not  then  become  suffi- 
ciently large  to  cause  more  than  slight  oxidation  of  hydriodic  acid. 
The  most  suitable  acid  for  the  purpose  is  acetic  acid. 

Determination. — If  bleaching  powder  were  pure  calcium  chloro- 
hypochlorite,  CaCl.CIO,  it  would  contain  about  56  percent  of  chlorine. 
For  reasons  already  discussed  the  amount  of  available  chlorine  is  much 
less  than  this  and  in  the  average  commercial  product  it  is  not  much  more 
than  25  percent.  Upon  this  basis  calculate  the  weight  that  should  be 
used  when  50/1000  of  the  weighed  sample  is  to  be  taken  for  the  final 
titration.  Weigh  from  a  closed  weighing  bottle  into  a  1000  cc  graduated 
flask.  Fill  to  the  mark  with  water  and  agitate  until  the  powder  is  thor- 
oughly disintegrated  and  all  soluble  matter  is  in  solution.  Measure 
50  cc  portions  into  flasks,  add  5  gm  of  potassium  iodide  and  25  cc  of 
10  percent  acetic  acid  to  each  and  titrate  with  sodium  thiosulphate 
solution.  Calculate  the  percent  of  available  chlorine  in  the  powder. 

Standard  Iodine  Solution. — Iodine  solutions  do  not  maintain 
a  constancy  of  oxidizing  power  and  standard  solutions  must  be 
restandardized  frequently  for  accurate  work. 

It  has  already  been  explained  (page  261)  that  free  iodine  is 
usually  dissolved  with  the  aid  of  an  iodide  and  that  a  molecular 
compound  with  a  formula  such  as  NaI3  is  present  in  such  solu- 


272  QUANTITATIVE  ANALYSIS 

tions.  Two-thirds  of  the  iodine  in  this  compound  (better  repre- 
sented as  NaI.I2)  is  so  loosely  bound  that  it  behaves  as  free 
iodine.  The  formula  would  indicate  a  necessary  ratio  of  150  :  254, 
sodium  iodide  to  iodine,  or  166:254,  potassium  iodide  to  iodine. 
However,  in  practice  it  is  found  necessary  to  use  a  much  higher 
ratio  (2:1)  in  order  to  dissolve  the  iodine  readily  and  to  pre- 
serve the  solution  in  a  fairly  stable  condition.  The  twentieth- 
normal  solution  is  convenient  for  the  following  determination. 

ARSENICAL  INSECTICIDES 

Two  of  the  most  important  insecticides  containing  arsenic  are 
London  purple  and  Paris  green.  The  former  is  a  waste  product 
of  certain  aniline  dye  industries  and  contains  much  dye  in  addi- 
tion to  a  fairly  large  quantity  of  arsenic.  Paris  green  is  a  fairly 
definite  compound  of  cupric  arsenite  and  cupric  acetate,  repre- 
sented by  the  formula:  Cu3(As03)2.Cu(C2H302)2-  This  com- 
pound is  decomposed  by  boiling  with  sodium  hydroxide,  pre- 
cipitating cuprous  oxide  and  forming  sodium  arsenate  and  arsenite 
in  solution.  The  formation  of  cuprous  oxide  is  due  to  the  reducing 
action  of  sodium  arsenite,  forming  sodium  arsenate.  If  the  solu- 
tion is  to  be  titrated  for  the  determination  of  total  arsenic  this 
arsenate  must  first  be  reduced.  For  this  purpose  the  solution  is 
concentrated,  then  hydrochloric  acid  and  potassium  iodide 
are  added  and  the  resulting  free  iodine  is  removed  by  sodium 
thiosulphate. 

Na3As04+2HCl+2KteNa3As03+2KCl+H2O+I2, 


The  exact  removal  of  iodine  must  be  determined  without  the 
aid  of  starch.  In  strongly  acid  solutions  starch  is  partly  inverted, 
dextrine  being  one  of  the  intermediate  products  and  dextrine 
forms  with  iodine  a  deep  red  color  which  is  not  later  removed  and 
which  interferes  in  the  titration  of  iodine  solution. 

The  first  equation  above  represents  a  reaction  that  can  be 
quantitatively  reversed  at  will.  The  complete  reduction  of 
pentavalent  arsenic  has  just  been  accomplished  in  acid  solution, 
one  of  the  products  (iodine)  being  removed.  If  the  solution  is 
now  made  basic,  thus  removing  one  of  the  products  (hydro- 
chloric acid  or  hydriodic  acid)  of  the  reverse  reaction  and  if 


OXIDATION  AND  REDUCTION  273 

standard  iodine  solution  is  added  a  quantitative  oxidation  of 
trivalent  arsenic  occurs.  The  addition  of  a  strong  base  is  not 
permissible  because  this  will  combine  with  iodine: 

2KOH+I2-»KIO+KI+H20. 

Neither  is  it  possible  to  use  a  normal  carbonate,  for  similar  rea- 
sons. Alkali  bicarbonates  may  be  present  and  are  used  in  prac- 
tice for  neutralizing  the  acid. 

The  method  that  was  formerly  given  as  "official"  by  the  Asso- 
ciation of  Official  Agricultural  Chemists1  is  based  upon  the  prin- 
ciples just  discussed.  Several  difficulties  which  are  experienced 
in  carrying  out  the  method  have  led  to  a  change  to  a  distillation 
method  as  the  official  one.  One  of  the  principal  difficulties  is  the 
formation  of  a  yellow  colloidal  solution  of  arsenious  iodide  when 
potassium  iodide  and  hydrochloric  acid  are  added  to  reduce  the 
arsenic  solution.  This  color  makes  impossible  the  exact  removal 
of  iodine  by  sodium  thiosulphate.  If*  the  analysis  is  performed 
carefully,  as  described  below,  this  difficulty  will  disappear. 

Determination  of  Total  Arsenic  and  of  Copper  in  Paris  Green. — To 

2  gm  of  Paris  green  in  a  250-cc  flask  add  about  100  cc  of  a  2-percent 
solution  of  sodium  hydroxide.  Boil  until  all  of  the  green  compound  has 
been  decomposed  and  only  red  cuprous  oxide  remains.  Cool,  filter  into 
a  250-cc  volumetric  flask,  washing  the  paper  well,  dilute  to  the  mark  and 
mix  well.  Reserve  the  cuprous  oxide  on  the  filter  for  the  copper 
determination. 

Measure  two  or  three  portions  of  50  cc  of  the  solution  into  250- 
cc  flasks  and  concentrate  by  boiling  to  about  half  the  original  volume. 
Cool  to  60°,  add  10  cc  of  concentrated  hydrochloric  acid  and  1  gm 
of  potassium  iodide.  Mix  and  allow  to  stand  for  about  ten  minutes. 
From  a  burette  carefully  add  sodium  thiosulphate  'solution  until  the 
iodine  is  all  reduced.  Starch  should  not  be  added  but  care  should  be 
exercised  in  reaching  the  end  point.  Allow  to  stand  for  5  minutes  longer 
and  if  iodine  color  reappears  carefully  add  more  thiosulphate  solution. 
Immediately  add,  as  rapidly  as  can  be  done  without  loss  by  effervescence, 
•15  gm  of  sodium  bicarbonate,  free  from  lumps.  Titrate  at  once  with 
standard  iodine  solution,  deferring  the  addition  of  starch  until  near  the 
end  point.  Calculate  the  percent  of  total  arsenic,  expressed  as  arsenious 
oxide,  in  the  Paris  green. 

The  residue  of  cuprous  oxide  is  treated  on  the  filter  with  5  cc  of  nitric 
acid,  specific  gravity  1.2,  the  solution  being  caught  in  a  250  cc  flask. 

lBur.  Chem.,  Bull.  107. 

18 


274  QUANTITATIVE  ANALYSIS 

Wash  the  paper  well  with  hot  water  and  proceed  as  directed  for  the 
standardization  of  sodium  thiosulphate  against  metallic  copper,  page 

268,  beginning  with  "Dilute  to  25  cc  and  boil ."     Calculate  the 

percent  of  copper  in  the  Paris  green.  The  result  may  also  be  expressed 
as  cupric  oxide,  if  desired. 

If  preferred,  the  solution  of  cuprous  oxide  in  nitric  acid  may  be 
diluted  and  electrolyzed,  after  boiling  to  expel  nitrous  acid.  This 
determination  is  described  on  page  156. 

The  present  official  method1  for  the  determination  of  total 
arsenic  in  Paris  green  is  based  upon  the  volatility  of  arsenic 
trichloride  with  steam.  The  sample  is  dissolved  in  concentrated 
hydrochloric  acid  in  a  distilling  flask.  Cuprous  chloride  is  added 
to  reduce  any  pentavalent  arsenic. 

The  distillate  containing  the  arsenous  chloride  and  hydrochlo- 
ric acid  is  absorbed  in  cold  water.  The  excess  of  acid  is  then 
neutralized  with  sodium  hydroxide,  sodium  bicarbonate  is  added 
in  excess  and  the  arsenic  is 'titrated  with  standard  iodine  solution. 
The  reactions  involved  have  already  been  discussed. 

Determination  of  Total  Arsenic.  Official  Method. — Prepare  the 
following  standard  solutions: 

(a)  Arsenous  Add. — Dissolve  2  gm,  accurately  weighed,  of  pure 
arsenous  oxide  in  a  beaker  by  boiling  with  about  200  cc  of  water  and 
10  cc  of  concentrated  sulphuric  acid.  Cool  the  solution,  transfer  to  a 
500  cc  volumetric  flask,  dilute  to  the  mark  and  mix  well.  Keep  in  a 
stoppered  flask  or  bottle. 

N 
(6)  Iodine  Solution,  ^Q. — Mix  by  grinding  in  a  porcelain  mortar  6.35 

gm  of  pure  iodine  with  12.5  gm  of  potassium  iodide.  Dissolve  in 
water,  filter  into  a  1000  cc  volumetric  flask,  dilute  to  the  mark  and  mix 
well.  Standardise  against  solution  (a)  as  follows: 

Pipette  50  cc  of  the  arsenous  acid  solution  into  a  1000  cc  Erlenmeyer 
flask,  add  400  cc  of  water,  then  gradually  add  10  gm  of  sodium  bicar- 
bonate. Mix  and  titrate  at  once  with  the  iodine  solution,  adding  5  cc 
of  starch  solution  when  the  slow  disappearance  of  the  iodine  color  indi- 
cates that  the  end  point  is  nearly  reached.  Calculate  the  value  of  the 
iodine  solution  in  terms  of  arsenous  oxide,  As20a. 

For  the  determination  of  total  arsenic  in  Paris  green  the  apparatus 
shown  in  Fig.  69  is  necessary. 

A  is  a  distilling  flask  having  a  capacity  of  250  cc  and  fitted  with  a  50 
cc  dropping  funnel.  The  capacities  of  the  Erlenmeyer  flasks  B,  C  and 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  1,  Pt.  2,  p.  5. 


OXIDATION  AND  REDUCTION 


275 


D  are  500  cc,  1000  cc  and  100  cc,  respectively.  B  and  C  are  surrounded 
by  cracked  ice  and  contain  40  cc  and  100  cc,  respectively,  of  water.  D 
contains  50  cc  of  water  which  serves  as  a  trap.  The  upright  tube  of 
flask  B  reaches  to  the  bottom  of  the  flask  and  acts  as  a  safety  valve, 
preventing  liquid  from  drawing  back  from  B  when  the  distillation 
slackens  or  stops. 

Place  5  gm  of  cuprous  chloride  in  the  distilling  flask.  Calculate  the 
theoretical  weight  of  Paris  green  that  would  be  equivalent  to  250  cc  of 
the  standard  iodine  solution.  Weigh  this  amount  and  rinse  into  the 
distilling  flask  by  means  of  100  cc  of  concentrated  hydrochloric  acid. 
Distill  until  only  about  40  cc  of  liquid  remains  in  the  distilling  flask, 


FIG.  69. — Apparatus  for  arsenic  distillation. 

then  add  50  cc  of  concentrated  hydrochloric  acid  through  the  dropping 
funnel  and  distill.  Continue  this  process  until  200  cc  of  distillate  has 
been  obtained.  Stop  the  distillation  and  rinse  down  the  condenser  and 
all  connecting  tubes  into  the  flasks.  Rinse  the  contents  of  all  three  of 
the  receiving  flasks  into  a  1000  cc  volumetric  flask.  Allow  the  solution 
to  attain  the  temperature  of  the  room  then  dilute  to  the  mark  and  mix 
well.  Measure  100  cc  of  this  solution  into  a  1000  cc  Erlenmeyer  flask, 
add  two  or  three  drops  of  phenolphthalein  solution  and  nearly  neu- 
tralize with  a  saturated  solution  of  sodium  hydroxide,  leaving  the  solu- 
tion slightly  acid.  Add  10  gm  of  sodium  bicarbonate  and  titrate  the 
arsenic  with  standard  iodine  solution  as  directed  for  the  standardization 
of  the  solution.  Calculate  the  percent  of  total  arsenic  in  the  sample, 
expressing  as  arsenous  oxide. 


CHAPTER  X 
TITRATIONS   INVOLVING   THE   FORMATION    OF   PRECIPITATES 

The  completion  of  the  reactions  of  neutralization  depends 
upon  the  small  ionization  of  one  of  the  products  (water).  The 
completion  of  reactions  of  oxidation  and  reduction  depends  upon 
the  relative  potentials  of  oxidizing  and  reducing  agents.  Certain 
other  reactions  are  made  the  basis  of  volumetric  determinations, 
completed  because  of  the  formation  of  a  precipitate.  In  some 
cases  an  indicator  is  added  while  in  others  the  cessation  of  pre- 
cipitation with  further  addition  of  standard  solution  is  the 
indicator. 

SILVER 

An  example  of  titration  without  an  added  indicator  is  to  be 
found  in  Gay-Lussac's1  method  for  silver.  This  method  is  one 
of  the  oldest  of  those  analytical  methods  that  have  survived  to  the 
present  day  and,  while  it  is  not  now  extensively  used  because  it 
is  somewhat  troublesome  in  the  matter  of  execution,  it  is  one  of 
the  most  exact  of  all  known  volumetric  processes.  It  depends 
upon  the  titration  of  the  solution  of  a  silver  salt  by  a  standard 
solution  of  sodium  chloride.  The  very  small  solubility  of  silver 
chloride  renders  the  reaction  practically  complete.  The  con- 
verse of  this  method  may  be  used  for  the  determination  of 
chlorine,  bromine,  or  iodine  in  soluble  halides. 

Exercise :  Preparation  of  Standard  Solutions. — Calculate  the  weight 
of  pure  sodium  chloride  that  is  equivalent  to  5  gm  of  silver,  weigh  this 
quantity,  dissolve  in  distilled  water  and  dilute  to  1000  cc  in  a  volu- 
metric flask.  Make  a  second  solution  by  diluting  100  cc  of  this  solu- 
tion to  1000  cc.  Record  the  silver  equivalent  of  1  cc  of  each  of  these 
solutions. 

Determination. — Silver  may  be  determined  in  any  alloy  that  contains 
no  other  metal  forming  insoluble  chlorides  but  the  approximate  percent 

1  Instruction  sur  1'  essai  des  matieres  d'  argent  par  la  voie  humide. 
Paris,  1832. 

276 


THE  FORMATION  OF  PRECIPITATES  277 

of  silver  should  be  known.  A  silver  coin  may  be  used.  United  States 
silver  coinage  contains  approximately  90  percent  of  silver.  Weigh 
enough  of  the  alloy  to  give  0.5  gm  of  silver,  place  in  a  250  cc  flask  having 
a  ground  glass  stopper  and  dissolve  in  10  cc  of  a  mixture  of  equal  volumes 
of  water  and  concentrated  nitric  acid.  Both  water  and  acid  must  be 
tested  and  found  free  from  chlorine.  Boil  to  expel  oxides  of  nitrogen, 
assisting  this  action  by  drawing  air  through  the  flask  by  means  of  a 
filter  pump.  Add  to  the  solution  in  the  flask  exactly  99  cc  of  the  more 
concentrated  standard  salt  solution,  stopper  and  shake  until  the  pre- 
cipitated silver  chloride  flocculates  and  settles  readily.  Add  from  a 
second  burette  the  more  dilute  standard  solution,  0.5  cc  at  a  time, 
allowing  the  solution  to  run  down  the  sides  of  the  flask  and  observing 
whether  turbidity  is  produced.  Shake  the  flask  if  more  silver  chloride  is 
formed  and  continue  the  addition  of  the  dilute  standard  solution  until 
the  last  0.5  cc  fails  to  produce  a  visible  precipitate  in  the  clear,  superna- 
tant liquid.  Do  not  use  the  last  0.5  cc  in  the  calculation. 

It  may  sometimes  happen  that  the  percent  of  silver  in  the  alloy  is  not 
known  with  sufficient  accuracy  and  either  too  much  or  too  little  of  the 
more  concentrated  solution  is  used.  In  the  first  case  the  first  addition 
of  the  dilute  solution  fails  to  produce  a  precipitate  while  in  the  second 
case  an  unduly  large  quantity  of  the  dilute  solution  is  required  to  reach 
the  end  point.  In  either  case  the  determination  should  be  begun  again, 
the  proper  alteration  being  made  in  either  the  weight  of  sample  taken 
or  the  volume  of  concentrated  standard  solution.  From  the  results 
of  the  titration  calculate  the  percent  of  silver  in  the  alloy. 

In  the  determination  of  silver  by  the  method  of  Volhard1  an 
inorganic  indicator  is  added  to  the  solution.  The  silver  should 
be  in  the  form  of  nitrate,  a  solution  of  a  ferric  salt,  acidified 
to  suppress  hydrolysis,  is  added  and  the  silver  is  titrated  by  a 
standard  solution  of  potassium  thiocyanate  or  ammonium  thio- 
cyanate.  Silver  is  precipitated  as  silver  thiocyanate: 

AgNO3+KCNS-»AgCNS+KN03. 

When  all  of  the  silver  is  removed  from  the  solution  an  additional 
drop  of  the  standard  solution  of  thiocyanate  produces  the  red 
color  of  soluble  ferric  thiocyanate: 

Fe(N03)3+3KCNS->re(CNS)3+KN03. 

Mercury  thiocyanate  is  insoluble  in  dilute  nitric  acid  and 
mercury  must  therefore  be  absent.  The  color  of  salts  of  copper, 

1  J.  prakt.  Chem.,  [2]  9,  217  (1874). 


278  QUANTITATIVE  ANALYSIS 

nickel  and  cobalt  obscures  the  end  point  and  these  metals 
should  be  absent  although  as  much  as  60  percent  of  copper  may 
be  present. 

The  converse  of  this  method  may  be  used  for  the  determina- 
tion of  the  thiocyanate  radical. 

Exercise :  Preparation  of  Solutions. — Make  a  solution  of  silver  nitrate, 
1  cc  of  which  contains  0.005  gm  of  silver.  Standardize  gravimetric- 
ally  by  precipitating  and  weighing  silver  chloride,  or  by  Gay-Lussac's 
volumetric  method. 

Make  1500  cc  of  a  solution  of  potassium  thiocyanate  or  ammonium 
thiocyanate  by  weighing  2  percent  more  than  the  calculated  quantity 
of  salt  required  to  make  1  cc  equivalent  to  0.005  gm  of  silver. 

Make  100  cc  of  a  solution  (saturated  without  heating)  of  ferric 
ammonium  sulphate,  adding  enough  nitric  acid  to  remove  turbidity 
and  to  cause  the  red  color  to  give  place  to  pale  yellow. 

Standardize  the  thiocyanate  solution  as  follows:  Measure  35  cc 
of  the  silver  nitrate  solution  into  a  beaker  or  Erlenmeyer  flask,  dilute 
to  about  75  cc,  add  1  cc  of  ferric  ammonium  sulphate  solution  and 
titrate  with  the  thiocyanate  solution  until  a  permanent  red  tint  is 
obtained. 

Determination. — Weigh  not  more  than  0.25  gm  of  a  silver  alloy  con- 
taining no  mercury,  nickel  or  cobalt  and  not  more  than  60  percent  of  cop- 
per and  place  in  a  250  cc  flask.  Dissolve  in  10  cc  of  a  mixture  of  equal 
volumes  of  concentrated  nitric  acid  and  water,  boiling  to  expel  oxides 
of  nitrogen.  Cool,  dilute  to  about  75  cc  and  titrate  exactly  as  in  the 
standardization  of  the  thiocyanate  solution.  Calculate  the  percent  of 
silver  in  the  alloy. 

HALOGENS  AND  THE  CYANIDE  RADICAL 

Volhard's  method  also  applies  to  the  determination  of  the 
halogen  hydracids  and  cyanogen.  A  measured  excess  of  stand- 
ard silver  nitrate  solution  is  added,  precipitating  all  of  the 
chlorine,  bromine,  iodine  or  cyanogen.  The  excess  of  silver 
nitrate  is  determined  by  titration  by  standard  thiocyanate 
solution  by  the  method  already  described.  In  the  original 
method  the  precipitated  silver  halide  was  not  removed  by  filtra- 
tion before  titration  of  the  excess  of  silver.  Rosanoff  and  Hill 
have  shown1  that  the  silver  chloride  reacts  with  the  red  soluble 
ferric  thiocyanate,  which  is  produced  at  the  end  point,  as  follows : 

3AgCl+Fe(CNS)3-*FeCl3+3AgCNS, 
*J.  Am.  Chem.  Soc.,  29,  269  (1907). 


THE  FORMATION  OF  PRECIPITATES  279 

This  occurs  to  an  appreciable  extent,  even  though  the  solu- 
bility of  silver  chloride  is  less  than  that  of  silver  thiocyanate. 
Rosanoff  and  Hill  found  that  as  much  as  43  percent  of  ammonium 
thiocyanate  is  changed  in  two  minutes  by  reaction  with  silver 
chloride.  It  is  therefore  necessary  to  remove  the  precipitate  by 
filtration  before  the  final  titration. 

Determination. — Use  the  standard  thiocyanate  and  silver  nitrate 
solutions  prepared  for  the  preceding  exercise.  Weigh  enough  of  a 
soluble  chloride,  bromide,  iodide  or  cyanide  to  be  equivalent  to  about 
40  cc  of  the  silver  nitrate  solution.  Dissolve  in  a  small  amount  of 
water,  acidify  with  nitric  acid  and  add  50  cc  of  the  standard  solution 
of  silver  nitrate.  Filter  and  wash  thoroughly  and  titrate  the  excess  of 
silver  nitrate  by  standard  thiocyanate  solution.  Calculate  the  percent 
of  halogen  or  cyanogen  in  the  sample. 

A  method  for  the  direct  titration  of  the  halogens  by  standard 
silver  nitrate  solution  is  described  on  page  397  in  the  discussion 
of  water  analysis. 

ZINC 

Ferrocyanide  Method. — The  ferrocyanide  titration  of  zinc  has 
been  practised  for  a  long  time,  and  many  modifications1  of  the 
details  of  the  method  have  been  published.  Concerning  the 
accuracy  of  this  method  there  has  been  considerable  controversy, 
particularly  as  it  applies  to  the  determination  of  zinc  in  ores. 
However,  most  of  the  errors  have  been  traced  to  methods  used 
in  separating  zinc  from  interfering  metals,  preceding  the  titra- 
tion. The  modified  Waring  method2  is  described  below. 

Most  of  the  important  zinc  ores  are  soluble  in  acids,  although 
the  aluminates  require  fusion  with  potassium  pyrosulphate.  In 
the  acid  solution  silica  is  rendered  insoluble  by  evaporation,  as 
otherwise  zinc  silicate  might  precipitate.  After  evaporation 
with  sulphuric  acid  to  expel  nitric  acid  the  solution  is  boiled 
with  a  piece  of  aluminium,  which  precipitates  lead,  copper,  cad- 
mium and  bismuth,  or  such  of  these  as  are  present,  these  metals 
all  lying  below  aluminium  in  the  electrochemical  series.  Iron 
is  not  precipitated  but  is  reduced  to  the  ferrous  condition.  In 

1 J.  Ind.  Eng.  Chem.,  4,  468  (1912). 

2  J.  Am.  Chem.  Soc.,  26,  4  (1904)  and  29,  262  (1907). 


280  QUANTITATIVE  ANALYSIS 

the  clear  solution  sulphuric  acid  is  neutralized  by  sodium  bicar- 
bonate and  then  formic  acid  is  added  in  slight  excess.  In  pres- 
ence of  this  weakly  ionized  acid  zinc  is  precipitated  by  hydrogen 
sulphide. 

Zinc  sulphide,  so  separated  from  interfering  metals  which 
would  form  insoluble  ferrocyanides,  is  redissolved  in  hydrochloric 
acid  and  titrated  with  a  standard  solution  of  potassium  ferro- 
cyanide, the  following  reaction  first  taking  place : 

2ZnCl2+K4Fe(CN)6->4KCl+Zn2Fe(CN)6.  (1) 

Zinc  ferrocyanide,  so  formed,  does  not  flocculate  readily  but  as 
more  ferrocyanide  is  added  to  the  hot  solution  a  potassium  zinc 
ferrocyanide  is  precipitated : 

3Zn2Fe(CN)6+K4Fe(CN)6-^2K2Zn3[Fe(CN)6]2.     (2) 
Equations  (1)  and  (2)  may  thus  be  combined: 

3ZnCl2-h2K4Fe(CN)6-^6KCl+K2Zn3[Fe(CN)6]2.     (3) 

As  indicator,  a  solution  of  uranium  acetate  or  nitrate  or  of 
ammonium  molybdate  is  used  on  an  outside  test  plate.  The 
yellow  or  brown  color  that  is  produced  by  a  slight  excess  of 
ferrocyanide  with  ammonium  molybdate  is  of  unknown  composi- 
tion. When  uranium  salts  are ,  used  a  brown  ferrocyanide  of 
uranium  is  formed. 

The  ferrocyanide  titration  of  zinc  is  easily  performed  and  is 
fairly  accurate  if  the  details  of  the  standardization  of  the  solution 
and  of  the  determination  are  watched  closely.  All  things  con- 
sidered the  gravimetric  method,  weighing  zinc  as  pyrophosphate, 
is  to  be  preferred.  This  method  is  described  on  page  507. 

Determination  of  Zinc  in  Ores. — Weigh  0.5  gm  of  powdered  ore  and 
brush  into  a  250-cc  casserole.  Add  5  cc  each  of  concentrated  nitric  and 
hydrochloric  acids,  cover  and  heat  to  decompose  the  ore.  Finally  boil 
to  expel  all  red  oxides  of  nitrogen,  then  remove  the  cover  and  rinse  this 
and  the  sides  of  the  casserole.  Cool  and  add  5  cc  of  concentrated  sul- 
phuric acid.  Evaporate  until  sulphur  trioxide  fumes  are  freely  evolved, 
the  casserole  being  held  in  the  hand  and  agitated  during  the  process  of 
evaporation. 

Cool,  add  50  cc  of  water  and  warm  until  only  insoluble  gangue 
remains.  Bend  a  strip  of  heavy  aluminium  foil,  2.5  cm  wide  and  14 
cm  long,  into  a  triangle  and  place  in  the  solution.  Boil  for  10  minutes, 
which  should  remove  all  color  except  a  faint  green  due  to  ferrous  sul- 


THE  FORMATION  OF  PRECIPITATES  281 

phate.  Filter  through  a  paper  containing  a  piece  of  aluminium,  into  a 
500-cc  flask  containing  a  rod  or  strip  of  the  same  metal,  and  wash  with 
hot  water. 

Add  a  drop  of  methyl  orange  and  neutralize  with  a  solution  of  sodium 
bicarbonate  (about  5  percent).  Barely  restore  the  pink  color  by 
adding  a  20-percent  solution  of  formic  acid,  a  drop  at  a  time,  then  add 
an  excess  of  5  drops.  Dilute  to  200  cc  and  then  add  4  gm  of  ammonium 
thiocyanate,  unless  iron  is  known  to  be  present  in  only  a  small  amount. 

Remove  the  rod  of  aluminium  and  insert  into  the  neck  of  the  flask  a 
rubber  stopper,  through  the  single  hole  of  which  passes  a  tube  leading 
to  the  bottom  of  the  flask.  Connect  the  tube  with  a  Kipp  generator 
for  hydrogen  sulphide  and  heat  the  solution  to  boiling.  With  the 
stopper  loosely  fitted,  pass  a  stream  of  gas  through  the  boiling  solution 
until  most  of  the  zinc  has  been  precipitated,  then  push  the  stopper  in 
so  that  the  pressure  of  the  gas  from  the  generator  will  aid  absorption. 

When  the  white  zinc  sulphide  settles  readily  remove  the  stopper 
and  rinse.  Filter  through  paper  and  wash  with  hot  water,  but  without 
attempting  to  remove  adhering  precipitate  from  the  flask.  Finally 
place  the  paper  and  precipitate  in  this  flask  and  add  10  cc  of  concentrated 
hydrochloric  acid  and  50  cc  of  water,  allowing  the  acid  to  act  upon  any 
precipitate  adhering  to  the  tube.  Warm  until  all  of  the  sulphide  in  the 
flask  is  in  solution  and  boil  to  remove  hydrogen  sulphide.  Drop  in  a 
bit  of  litmus  paper  and  neutralize  the  acid  with  ammonium  hydroxide, 
then  add  3  cc  excess  of  concentrated  hydrochloric  acid. 

Rinse  the  solution  into  a  500-cc  beaker,  dilute  to  about  250  cc, 
heat  nearly  to  boiling  and  titrate  with  standard  potassium  ferro- 
cyanide,  using  a  drop  of  nearly  saturated  uranium  nitrate  or  acetate 
solution  or  of  a  2-percent  ammonium  molybdate  solution,  on  a  white 
test  plate  as  indicator.  Near  the  last  the  solution  is  stirred  and  tested 
after  each  drop  of  standard  solution  is  added.  When  a  yellow  or  brown 
color  finally  appears  it  will  be  found  that  two  or  three  of  the  tests 
immediately  preceding  this  one  will  develop  a  color  after  standing  for  a 
few  minutes.  When  this  is  the  case  the  burette  reading  corresponding 
to  the  earliest  positive  test  for  ferrocyanide  is  taken  as  the  amount  of 
solution  equivalent  to  the  zinc. 

Calculate  the  percent  of  zinc  in  the  ore. 

The  solution  of  potassium  ferrocyanide  should  be  made  so  that 
1  cc  is  equivalent  to  0.005  gm  of -zinc.  It  must  be  standardized  against 
zinc,  zinc  oxide  or  zinc  sulphate  of  known  purity,  following  exactly 
the  same  method  that  has  been  outlined  for  the  ore,  beginning  with  the 
dissolving  of  zinc  sulphide.  Record  the  concentration  of  the  solution 
in  terms  of  zinc  equivalent  to  1  cc  of  ferrocyanide  solution. 


PART  II 

ANALYSIS  OF  INDUSTRIAL  PRODUCTS  AND 
RAW  MATERIALS 


In  most  of  the  exercises  in  the  preceding  portion  of  this  book 
determinations  have  been  made  of  single  constituents  of  various 
substances  and  interfering  substances  have  usually  been  either 
absent  or  capable  of  being  removed  with  comparative  ease. 
Standard  methods  have  been  employed  and  attention  has  been 
centered  upon  the  chemical  principles  underlying  the  method  and 
the  proper  manipulation.  In  the  pages  that  follow  the  student 
will  become  acquainted  with  the  application  of  these  and  other 
determinations  to  the  testing  and  analysis  of  some  materials 
which  are  of  importance  to  our  industrial  life.  Such  materials 
are  often  quite  complicated  in  composition  and  most  varied 
procedures  are  necessary  in  a  determination  of  their  industrial 
value.  The  chemist  will  then  find  it  necessary  to  have  at  his 
command  all  of  the  chemical  principles  and  methods  of  analysis 
that  have  already  been  learned  and  to  apply  these  to  an  intelli- 
gent study  of  the  material  under  examination.  He  will  also  be 
prepared  to  take  up  other  methods  of  testing.  Some  of  the  tests 
are  purely  physical  but  they  are,  in  industrial  practice,  applied 
by  the  chemist  and  not  by  the  physicist  because  the  former  is 
usually  engaged  in  the  analysis  of  the  same  or  similar  materials. 
Other  analytical  determinations  are  empirical,  rather  than  exact, 
in  their  nature  but  must  be  made  with  the  same  degree  of  care 
and  attention  as  the  determinations  involving  definite  elements 
or  compounds. 


283 


CHAPTER  XI 
ROCK  ANALYSIS 

CARBONATE  MINERALS 

The  most  important  and  abundant  of  the  carbonate  minerals 
are  the  calcites  and  the  dolomites.  The  calcites  consist  essen- 
tially of  calcium  carbonate  and  the  dolomites  of  double  carbon- 
ates of  calcium  and  magnesium  but  these  compounds  seldom  or 
never  occur  in  a  pure  state  in  nature.  Iceland  spar  i£  one  of  the 
best-known  examples  of  a  nearly  pure  natural  variety  of  calcium 
carbonate,  yet  in  many  samples  of  Iceland  spar  substances  other 
than  calcium  carbonate  occur  in  appreciable  amounts.  For  pur- 
poses of  geological  investigation  there  is  usually  required  a  com- 
plete analysis  with  the  utmost  accuracy  that  can  be  attained. 
For  technical  purposes  this  is  not  the  case.  The  mineral  is  to  be 
used  for  a  given  industrial  purpose  where  the  essential  constituent 
is  the  one  of  chief  importance  and  where  impurities  are  important 
only  to  the  extent  that  they  may  reduce  the  percentage  of  the 
essential  constituent  or  that  they  exert  an  undesirable  influence 
'in  the  industrial  operation  to  which  the  mineral  is  to  be 
submitted. 

The  particular  application  of  the  mineral  to  the  industrial  proc- 
ess will  determine  which  impurities  are  of  considerable  and  which 
are  of  minor  importance.  Those  of  minor  importance  are  fre- 
quently grouped,  with  no  attempt  at  separation,  into  certain  arbi- 
trary classes.  For  example  a  limestone  may  be  used  as  a  source 
of  quick  lime,  as  a  flux  in  iron  smelting,  as  a  paving  material,  as 
a  building  stone,  as  a  raw  material  for.  hydraulic  cements,  or  for 
any  one  of  a  variety  of  other  purposes.  All  limestone  contains 
more  or  less  of  material  insoluble  in  acids,  consisting  chiefly  of 
various  silicates  and  of  quartz.  For  the  first  purpose  named 
these  substances  are  important  only  as  they  act  as  diluents  of  the 
essential  calcium  carbonate,  unless  they  occur  in  relatively  large 

284 


ROCK  ANALYSIS  285 

quantities.  For  such  a  purpose  the  analysis  would  be  so  made  as 
to  include  all  such  materials  as  simply  " insoluble"  or  "silicious 
matter/'  no  separation  of  the  components  being  made.  If  the 
limestone  were  to  be  used  as  a  flux  in  the  smelting  of  iron  ore, 
the  nature  of  this  insoluble  material  should  be  more  exactly 
determined,  since  it  not  only  reduces  the  actual  percent  of  cal- 
cium carbonate  but  also  may  contain  substances  that  themselves 
require  a  flux  or  that  may  even  add  very  objectionable  impurities, 
such  as  sulphur  or  phosphorus,  to  the  iron  itself.  For  paving  or 
building  material  the  physical  properties  of  the  mineral  are  of 
great  importance  and  the  chemical  analysis  might  be  considerably 
condensed. 

As  another  example  of  such  empiricism  in  analysis,  may  be  men- 
tioned the  usual  report  on  calcium.  This  element  is  usually  pre- 
cipitated as  the  oxalate.  It  will  readily  be  understood,  however, 
that  if  barium  or  strontium  is  present  and  not  previously  sepa- 
rated it  will  also  precipitate  and  will  be  included  in  the  finally 
weighed  oxide.  Unless  it  is  known  that  barium  or  strontium  is 
present  in  more  than  very  small  amounts  the  percent  of  "  cal- 
cium" alone  is  made  a  part  of  the  report  for  technical  purposes, 
strontium  or  barium  serving  the  same  purpose  as  does  calcium. 
This,  obviously,  involves  a  slight  error,  not  only  in  the  naming  of 
the  element  but  in  the  percent  as  well,  because  the  factors  for 
these  three  metals  in  their  oxides  are  all  different.  For  exact 
scientific  purposes  the  separation  and  determination  of  all  ele- 
ments or  radicals  may  be  necessary  while  for  technical  purposes 
the  analysis  will  be  ordered  according  to  the  use  to  which  the  sub- 
stance is  applied.  This  is  an  example  of  the  so-called  "  proxi- 
mate" analysis,  as  distinguished  from  the  " ultimate"  analysis. 
It  is  important  to  note  that  the  term  "proximate"  does  not  imply 
carelessness  in  working  or  neglect  of  sources  of  error.  It  should 
not  even  convey  the  idea  of  inexact  figures,  but  merely  grouping 
together  of  more  than  one  substance  to  be  reported  by  one 
generic  term,  as,  for  example,  "insoluble  matter"  above.  The 
proximate  analysis  of  coal  will  include  the  determination  of 
percents  of  " volatile  combustible  matter,"  "fixed  carbon," 
"ash,"  and  "moisture,"  yet  each  one  of  these  terms  covers  many 
substances  which  are  all  determined  together  with  no  attempt  at 
a  separation  into  the  ultimate  constituents,  simply  because  the 


286  QUANTITATIVE  ANALYSIS 

figures  so  determined  serve  a  useful  purpose  in  fixing  a  valuation 
on  the  coal. 

It  was  formerly  the  custom  to  report  the  analysis  of  acids,  bases 
and  salts,  not  as  radicals  but  as  anhydrides.  Calcium  carbonate 
would  be  reported  as  calcium  oxide  and  carbon  dioxide,  sulphuric 
acid  as  water  and  sulphur  trioxide,  etc.  This  custom  has  now 
largely  fallen  into  disuse  in  most  lines  of  analytical  chemistry  but 
has  been  retained  in  the  analysis  of  minerals. 

The  ultimate  analysis  of  carbonate  minerals  is  exhaustively 
and  scientifically  treated  in  a  bulletin  of  the  U.  S.  Geological 
Survey  and  only  reference  to  this  will  be  made.1  The  exercise 
to  follow  will  deal  with  the  analysis  made  with  industrial  ends 
in  view.  This  exercise  will  be  the  student's  introduction  to 
separations  in  quantitative  analysis.  Heretofore  the  work  with 
the  solution  has  terminated  with  the  filtration  and  the  removal  of 
the  precipitate.  The  filtrate  could  contain  nothing  but  impuri- 
ties and  by-products  of  the  reaction  and  therefore  could  be  of 
no  further  importance  to  the  analyst.  In  the  next  and  in  many 
later  exercises  the  filtrate  must  be  carefully  conserved  because 
it  contains  substances  still  to  be  determined.  The  quantity  of 
wash  liquid  must  be  made  as  small  as  possible,  not  merely  to 
minimize  its  solvent  action  upon  the  precipitate  but  also  because 
the  washings  must  be  added  to  the  chief  filtrate  and  the  total 
bulk  must  not  be  excessive  for  subsequent  operations.  Even 
with  the  exercise  of  great  care  in  this  regard  an  occasional  con- 
centration of  the  solution  by  evaporation  is  necessary  in  order 
to  reduce  its  volume  to  a  workable  value. 

Another  point  that  will  here  appear  for  the  first  time  is  that 
many  of  the  elements  or  radicals  that  must  be  separated  and 
determined  are  present  in  the  mineral  in  relatively  small  quanti- 
ties. The  student  has  been  accustomed  to  a  rapid  appearance 
of  a  considerable  quantity  of  precipitate  and  if  this  should  not 
appear  when  the  appropriate  reagent  is  added  he  is  likely  to 
conclude  that  none  of  the  substance  is  present  and  to  pass  to  the 
next  determination.  None  of  the  constituents  ordinarily  present 
in  a  given  mineral  or  other  complex  material  should  be  assumed 
to  be  absent.  The  reagent  should  be  added  and  sufficient  time 
allowed  for  the  precipitation  to  become  completed,  remembering 

1  U.  S.  Geol.  Surv.,  Bull.  700,  by  W.  F.  Hillebrand. 


ROCK  ANALYSIS  287 

that  precipitation  starts  and  proceeds  slowly  from  very  dilute 
solutions.  Even  when  no  precipitate  is  finally  visible  it  is  the 
safest  plan  to  filter,  wash  and  ignite  the  paper  in  a  weighed  cruci- 
ble, when  a  small  amount  of  precipitate  will  often  be  detected^ 
when  otherwise  it  would  have  been  weighed  with  the  next 
precipitate  to  be  produced. 

Analysis  of  Carbonate  Mineral. — Read  again  the  discussion  of  sam- 
pling on  page  9  and  apply  this  to  the  preparation  of  a  sample  of  limestone, 
dolomite  or  other  carbonate  mineral,  for  analysis.  The  small  sample 
finally  used  should  weigh  about  10  gm  and  should  pass  a  sieve  having 
100  meshes  in  each  linear  inch. 

Carbon  Dioxide. — Determine  carbon  dioxide  exactly  as  directed  on 
page  134,  noting  that  if  dolomite  is  under  investigation  solution  will 
proceed  rather  slowly  while  the  acid  is  cold.  It  is  obvious  that  hydro- 
chloric acid  must  be  used  since  considerable  quantities  of  calcium  are 
present  and  the  solubility  of  calcium  sulphate  is  not  large. 

Silica  or  Insoluble  Matter. — The  residue  from  the  carbon  dioxide 
determination  may  be  used  for  this  determination  but  it  is  better  to  use 
new  samples.  Weigh  duplicate  portions  of  0.5  gm  each  into  casseroles. 
Dissolve  in  5  cc  of  concentrated  hydrochloric  acid,  covering  the  cas- 
serole while  the  mineral  is  dissolving.  Rinse  down  the  cover  glass  and 
the  sides  of  the  casserole  and  evaporate  to  dryness  on  the  steam  bath. 
Heat  the  dry  material  at  dull  redness  until  the  organic  matter  has  been 
oxidized,  leaving  the  residue  white  or  reddish  brown  from  iron  oxide. 
Cool,  add  5  cc  of  concentrated  hydrochloric  acid  and  warm  until  all 
soluble  matter  has  passed  into  solution.  Dilute  to  about  50  cc  with 
hot  water,  boil  and  filter  into  a  Pyrex  beaker,  using  particular  care  in 
removing  all  of  the  residue  to  the  filter  paper,  since  the  white  casserole 
makes  this  process  somewhat  uncertain.  Wash  the  residue  free  from 
chlorides  with  hot  water,  collecting  the  washings  and  filtrate  in  the  same 
beaker.  The  total  volume,  after  filtration  and  washing,  should  not  be 
greater  than  100  cc.  If  it  is  greater  than  this  amount  it  should  be  con- 
centrated by  evaporation.  Place  the  paper  and  residue  in  a  weighed 
platinum  crucible,  burn  the  paper  and  then  ignite  for  10  minutes  over 
the  blast  lamp.  Report  the  percent  of  silicious  matter.  If  silicious 
matter  amounts  to  more  than  0.5  percent  it  should  be  separated  into 
its  constituents.  In  this  case  add  to  the  residue  in  the  crucible  2  gm  of 
sodium  carbonate  and  fuse  over  the  blast  lamp  until  the  silicate  is  com- 
pletely decomposed,  as  shown  by  the  cessation  of  effervescence.  Cool, 
place  the  crucible  in  a  casserole  containing  50  cc  of  water  and  warm  until 
the  material  is  completely  dissolved  or  disintegrated.  Carefully  add 


288  QUANTITATIVE  ANALYSIS 

to  the  covered  casserole  concentrated  hydrochloric  acid  until  efferves- 
cence no  longer  occurs,  then  remove  the  crucible  and  rinse.  Evaporate 
the  solution  and  heat  at  about  120°  for  10  minutes.  Add  5  cc  of  con- 
centrated hydrochloric  acid  and  warm  until  soluble  matter  is  dissolved, 
then  dilute  with  5  cc  of  water  and  filter  on  an  extracted  paper.  Wash 
with  hot  water  until  chlorides  are  completely  removed,  adding  the 
filtrate  and  washings  to  the  original  solution  of  the  mineral.  Ignite  the 
residue  and  paper  in  a  weighed  platinum  crucible,  weigh  and  report  as 
silica. 

Iron  and  Aluminium. — If  the  solution  has  a  volume  greater  than  100 
cc  it  should  be  evaporated  to  concentrate  to  about  this  volume.  Drop 
into  the  solution  a  very  small  bit  of  litmus  paper  and  then  add  dilute 
ammonium  hydroxide,  stirring,  until  the  solution  is  distinctly  basic, 
avoiding  undue  excess  of  ammonium  hydroxide.  Boil  for  5  minutes 
or  until  the  odor  of  ammonia  is  faint.  Filter  through  an  extracted 
paper  and  wash  until  free  from  chlorides,  adding  the  washings  to  the 
filtrate.  Remove  the  paper  from  the  funnel,  fold  and  burn  in  a  weighed 
porcelain  crucible.  Burn  the  paper  at  a  low  temperature  in  presence 
of  an  excess  of  air,  inclining  the  crucible  to  facilitate  oxidation.  Weigh, 
and  if  the  amount  of  the  oxides  is  not  greater  than  0.5  percent  report  as 
aluminium  oxide  and  iron  oxide.  If  more  than  this  amount  is  present  a 
separation  is  usually  made.  In  this  case  add  1  gm  of  potassium  acid 
sulphate  to  the  crucible  containing  the  oxides  of  iron  and  aluminium. 
Fuse  at  a  relatively  low  temperature  until  violent  effervescence  has 
ceased  then  heat  to  redness  until  the  oxides  have  been  completely 
dissolved.  Cool  the  crucible  and  dissolve  the  mass  in  hot  water. 
Reduce  the  iron  and  titrate  with  standard  potassium  dichromate  or 
potassium  permanganate  according  to  the  methods  already  learned. 

Manganese. — Add  bromine  water  to  the  filtrate  and  washings  from 
iron  and  aluminium  until  a  yellow  color  is  produced,  then  boil.  If 
manganese  is  present  it  will  precipitate  as  brown  manganese  dioxide. 
The  quantity  is  usually  small  but  it  must  not  be  disregarded.  Filter, 
wash  free  from  chlorides  and  ignite.  Weigh  the  oxide  MnsC^  and 
calculate  as  MnC>2,  assuming  that  the  manganese  was  originally  pres- 
ent in  this  form.  (This  is  an  arbitrary  assumption  because  manganous 
carbonate  is  of  common  occurrence.) 

Calcium. — Acidify  the  solution  with  hydrochloric  acid  and  concen- 
trate to  about  100  cc,  boiling  until  all  bromine  is  removed.  Add  a 
bit  of  litmus  paper,  then  ammonium  hydroxide  until  basic.  Heat  to 
boiling  and  add,  drop  by  drop  with  stirring,  10  cc  of  a  saturated  solu- 
tion of  ammonium  oxalate,  or  enough  to  precipitate  all  of  the  calcium. 
Determine  the  calcium  as  directed  on  page  81  or  253,  with  the  following 
addition,  designed  to  complete  the  separation  of  magnesium:  Filter 


ROCK  ANALYSIS  289 

the  precipitate  of  calcium  oxalate  and  wash  once  with  hot  water  but 
without  making  any  attempt  to  transfer  completely  to  the  filter.  Place 
the  beaker  under  the  paper  and  add  to  the  precipitate  on  the  filter 
enough  concentrated  hydrochloric  acid  to  dissolve  all  calcium  oxalate^- 
2  cc  should  be  sufficient.  Wash  the  paper  thoroughly  with  hot  water, 
precipitate  the  calcium  once  more  and  determine  as  already  directed. 
Add  the  filtrate  from  the  second  filtration  to  that  from  the  first.  Calcu- 
late the  percent  of  calcium  oxide  in  the  sample. 

Magnesium. — Determine  the  magnesium  in  the  filtrate  and  washings 
as  was  done  in  the  case  of  a  magnesium  salt.  The  determination  is 
discussed  on  page  112.  Report  the  percent  of  magnesium  oxide. 

Sodium  and  Potassium. — These  elements  are  not  often  present  in 
more  than  traces  in  the  carbonate  minerals  and  their  determination  is 
not  often  required  for  industrial  purposes.  If  such  a  determination  is 
to  be  made,  a  new  sample  of  mineral  should  be  used.  Follow  the  pro- 
cedure directed  under  the  analysis  of  silicate  minerals,  page  294. 

If  the  analysis  has  been  made  with  care  and  every  substance  present 
has  been  determined  the  sum  of  the  percents  of  all  of  the  constituents 
of  the  mineral  should  be  100.  This  will  serve  as  a  check  upon  the 
accuracy  of  the  work  but  the  sum  will  rarely  be  exactly  100.  The 
omission  of  the  determination  of  other  substances  present  in  small 
quantity  will  give  rise  to  a  negative  error,  while  imperfect  washing  and 
other  experimental  errors  will  summate  as  a  positive  error,  so  that  the 
sum  of  all  percents  may  be  either  greater  or  less  than  100.  The  student 
should  be  able  to  work  so  that  the  sum  of  all  the  errors  should  not  be 
greater  than  ±  1  percent. 

SILICATE  MINERALS 

Silica,  as  a  constituent  of  various  simple  and  complex  silicates, 
is  distributed  widely  in  the  earth's  crust.  Associated  with  other 
minerals  or  in  a  nearly  pure  form  silica  itself  is  also  to  be  found. 
These  minerals  are  only  slightly  soluble  in  acids  or  bases  and  their 
analysis  requires  a  preliminary  decomposition  by  some  agent 
which  will  react  at  elevated  temperatures.  When  silicon  dioxide 
or  a  silicate  is  heated  with  an  alkali  carbonate  to  the  point  of 
fusion  the  corresponding  alkali,  silicate  is  produced,  carbon  diox- 
ide is  evolved  and  whatever  heavy  metals  may  have  been  origi- 
nally present  as  silicates  are  left  in  the  form  of  oxides.  The  alkali 
silicates  are  soluble  in  water  (as  colloids)  and  most  of  the  metallic 

oxides  so  produced  are  soluble  in  hydrochloric  acid.     The  pre- 
19 


290  QUANTITATIVE  ANALYSIS 

viously  insoluble  mineral  is,  by  this  means,  obtained  in  solution 
and  the  ordinary  analytical  processes  will  henceforth  apply. 

All  of  the  natural  silicates  may  be  regarded  as  being  derived 
from  silicon  dioxide,  the  anhydride  of  the  various  silicic  acids. 
These  acids  are  not  known  in  the  free  state  but  their  existence  may 
be  supposed  from  the  composition  of  the  salts.  Thus  H2Si03  = 
H2O.Si02,  H4SiO4  =  2H2O.Si02,  H6Si207  =  3H20.2Si02,  H4Si2O6  = 
2H2O.2SiO2,  EUSisOs  =2H20.3SiO2,  H2Si2Oc  =H2O.2Si02.  These 
may  be  taken  as  the  acids  from  which  various  natural 
silicates  are  derived.  Kaolin,  the  essential  constituent  of  the 
various  impure  clays,  is  Al2Si2O7-2H2O,  a  salt  of  H6Si207.  The 
felspars  are  double  silicates  derived  from  the  acid  H^SisOs-  As 
examples  of  the  felspars  may  be  mentioned  orthoclase,  KAlSi3Os, 
and  albite,  NaAlSi308. 

Fusion  of  orthoclase  with  sodium  carbonate  causes  reactions 
which  may  be  simply  represented  thus : 

2KAlSi308+6Na2C03-^K2SiO3+5Na2SiO3+2NaA102+6C02. 

The  completion  of  the  reaction  is  assured  by  the  presence  of  a 
considerable  excess  of  sodium  carbonate.  According  to  the 
terms  of  the  mass  law  the  reaction  should  be  completed  by  simply 
heating  at  a  sufficiently  high  temperature  to  decompose  com- 
pletely any  carbonates  that  may  be  formed  by  the  decomposition. 
The  mass  of  silicates  resulting  from  the  fusion  may  be  decom- 
posed by  hydrochloric  acid  but  if  this  is  not  preceded  by  disin- 
tegration and  solution  of  the  water  soluble  parts  by  hot  water 
the  result  of  such  treatment  will  be  to  form  a  protective  coating 
upon  lumps  of  the  fusion,  thus  retarding  the  action  of  the  acid. 
Upon  addition  of  hydrochloric  acid  to  the  mixture  of  substances 
after  treatment  with  water,  oxides  of  earth  and  alkaline  earth 
metals  form  soluble  chlorides,  while  the  alkali  silicates  are  de- 
composed with  formation  of  alkali  chlorides  and  silicic  acid. 
The  first  separation  occurs  in  the  removal  of  the  silicic  acid  which 
must  first  be  converted  into  the  less  soluble  silicon  dioxide.  This 
conversion  is  partly,  but  imperfectly,  accomplished  by  evapora- 
tion to  dryness  and  heating  to  about  120°.  Rehydration  readily 
occurs  and  the  silica  partly  redissolves  because  of  its  marked  tend- 
ency toward  the  formation  of  hydrosols.  This  tendency  is 
diminished  by  long  heating  at  high  temperatures  since  such  treat- 
ment results  in  incipient  fusion  and  change  into  the  irreversible 


ROCK  ANALYSIS  291 

colloid,  silicic  acid,  the  production  of  the  latter  being  promoted  by 
the  presence  of  strong  acids.  It  is  practically  impossible  to  com- 
pletely separate  silica  by  one  evaporation  and  filtration,  a  small 
proportion  invariably  returning  to  the  solution.  By  evaporating 
the  filtrate,  heating,  and  again  filtering,  all  but  a  trace  of  silica 
may  be  removed.  The  residue  of  silica  is  never  pure  but  con- 
tains small  amounts  of  oxides  of  iron,  aluminium  and  calcium. 
In  order  to  correct  the  error  arising  from  this  cause  the  precipitate 
is  treated  by  hydrofluoric  acid,  which  converts  silica  into  the 
gaseous  silicon  tetrafluoride.  After  volatilization  of  this  and  of 
the  hydrofluoric  acid  the  residue  is  weighed  and  its  loss  reported 
as  silica. 

After  the  separation  of  silica  the  metals  will  be  determined  by 
the  usual  methods,  such  as  those  used  in  the  analysis  of  carbon- 
ate minerals.  It  will  readily  be  seen  that,  after  the  fusion  of  the 
silicate  with  sodium  carbonate  or  potassium  carbonate,  a  deter- 
mination of  the  alkali  metals  in  this  portion  of  the  sample  will  be 
without  significance.  Other-  methods  must  be  employed  for 
decomposing  the  silicate.  Two  such  methods  are  in  general  use. 

J.  L.  Smith  Method.  —  The  method  of  J.  Lawrence  Smith1 
depends  upon  the  action  of  calcium  chloride  upon  silicates  at 
about  800°,  resulting  in  the  formation  of  alkali  chlorides  and  sili- 
cates of  calcium  and  other  metals.  The  sample  is  intimately 
mixed  with  ammonium  chloride  and  precipitated  calcium  carbon- 
ate, and  gently  heated.  The  reaction  that  occurs  might  be  repre- 
sented as  follows,  assuming  orthoclase  to  be  the  silicate. 


+2NH3+6C02. 

This  can  be  only  an  approximate  representation  of  what  has 
really  happened  during  the  heating.  Ammonium  chloride  dis- 
sociates at  about  450°  into  ammonia  and  hydrochloric  acid: 

NH4C1-»NH3+HC1. 

The  ammonia  escapes  while  the  hydrochloric  acid  combines  with 
calcium  carbonate: 

CaC03+2HCl->CaCl2+H2O+CO2. 
xAm.  J.  Sci.,  [3]  1,  269  (1871). 


292  QUANTITATIVE  ANALYSIS 

That  the  decomposition  of  silicates  is,  in  a  large  measure,  due  to 
calcium  chloride  is  undoubtedly  true.  That  calcium  carbonate, 
as  such,  also  plays  an  important  part  in  the  reactions  would  be 
inferred  from  the  above  interpretation  of  the  reaction,  since  in 
this  reaction  only  one-third  of  the  calcium  carbonate  could  form 
the  chloride.  The  significance  of  this  equation  is  lessened  by 
the  fact  that  it  is  a  representation  of  a  series  of  reactions  that 
cannot  well  be  tested.  The  silicate  resulting  from  the  decomposi- 
tion is  probably  not  calcium  metasilicate  alone  but  is  much  more 
complex  than  this. 

After  the  decomposition  is  complete  the  mass  is  treated  with 
hot  water  which  dissolves  chlorides  of  sodium,  potassium  and 
calcium,  as  well  as  those  of  other  metals  present.  After  filtra- 
tion the  most  of  the  calcium,  iron  and  aluminium  is  precipitated 
by  ammonium  carbonate  and  ammonium  oxalate;  the  solution  is 
then  evaporated  in  a  platinum  dish  and  heated  to  expel  am- 
monium salts.  If  desired,  sulphuric  acid  may  be  added  to 
convert  sodium,  potassium  and  ammonium  salts  into  sulphates, 
thus  providing  less  liability  of  loss  of  sodium  and  potassium  during 
the  heating.  In  this  case  the  Gladding  modification  of  the  Lindo 
method  must  be  used  or  else  the  sulphates  must  be  converted 
into  chlorides  by  precipitating  with  barium  chloride.  If  sul- 
phuric acid  is  not  added  the  volatilization  of  ammonium  salts 
must  be  conducted  with  greater  care,  since  the  chlorides  of  sodium 
and  potassium  are  appreciably  volatile  at  a  temperature  of  bright 
redness.  Read  again  the  discussion  of  the  determination  of 
sodium  and  potassium  on  page  96. 

Hydrofluoric  Acid  Method. — Another  method  for  treating 
silicates  without  the  addition  of  sodium  or  potassium  carbonate 
is  that  of  decomposing  by  means  of  hydrofluoric  acid.  The  finely 
powdered  silicate  is  moistened  with  concentrated  sulphuric  acid 
and  then  hydrofluoric  acid  is  added.  The  silica  is  volatilized, 
upon  warming,  as  silicon  tetrafluoride.  After  evaporation  of  the 
excess  of  hydrofluoric  acid  and  of  sulphuric  acid  the  residue  is 
dissolved  in  water  and  the  solution  analyzed  by  practically  the 
same  procedure  as  is  followed  in  the  Smith  method. 

Analysis  of  Silicate  Mineral. — To  insure  complete  decomposition  of 
the  silicate  it  must  be  ground  much  more  finely  than  is  necessary  for 
most  other  minerals.  After  it  is  pulverized  to  pass  a  100-mesh  sieve 


ROCK  ANALYSIS  293 


about  3  gm  of  the  sample  is  ground  in  an  agate  mortar  until  it  will 
a  200-mesh  sieve. 

Moisture. — Weigh  about  0.5  gm  of  the  silicate  into  a  platinum  cru- 
cible, and  dry  for  one  hour  at  100°  to  105°,  the  loss  being  calculated  as 
hygroscopic  moisture.  If  combined  water  is  also  to  be  determined  this 
can  be  done  by  heating  to  a  temperature  above  200°  in  a  combustion 
tube  in  an  atmosphere  of  dried  carbon  dioxide,  absorbing  the  moisture 
in  weighed  tubes  filled  with  calcium  chloride. 

Silica. — Mix  with  the  sample  about  5.0  gm  of  sodium  carbonate, 
approximately  weighed.  Place  the  cover  on  the  crucible  but  slightly 
toward  one  side  so  the  contents  may  be  observed.  Heat  gently  at  first 
in  order  to  avoid  violent  effervescence  and  consequent  loss  by  spatter- 
ing, gradually  raising  the  temperature  until  the  full  heat  of  the  burner 
fails  to  cause  more  than  slight  action.  Place  the  crucible  over  the  blast 
lamp  and  heat  for  15  minutes  after  carbon  dioxide  has  ceased  to  be 
evolved.  Remove  the  lamp,  lift  the  crucible  by  means  of  the  tongs, 
using  care  to  avoid  contact  of  the  latter  with  the  fusion,  and  slowly 
rotate  the  crucible  in  such  a  manner  that  the  fused  mass  will  be  spread 
over  the  sides  of  the  crucible  as  it  solidifies. 

After  cooling  place  the  crucible  and  its  contents  in  a  Pyrex  beaker; 
a  platinum  dish  or  a  porcelain  casserole  and  cover  with  hot  distilled 
water.  Digest  until  the  entire  mass  has  become  disintegrated  and  re- 
moved from  the  crucible.  Cover  the  beaker  and  add,  gradually,  from 
a  pipette,  concentrated  hydrochloric  acid  until  the  carbonates  are  com- 
pletely decomposed.  Remove  the  crucible  and  cover  by  means  of  a 
stirring  rod,  rinsing  well.  Crucible  tongs  should  not  be  used  for  this 
purpose  unless  they  are  tipped  with  platinum.  The  solution  is  now 
evaporated  to  dryness.  If  speed  is  essential  and  if  a  casserole  has  been 
used  the  greater  part  of  the  liquid  may  be  evaporated  by  holding  over 
a  free  flame,  giving  a  rotary  motion  to  the  casserole  to  prevent  spatter- 
ing or  bumping.  If  other  work  may  be  carried  on  at  the  same  time  the 
solution  should  be  evaporated  over  the  steam  bath.  When  completely 
dry  heat  for  15  minutes  at  about  120°,  cool  and  add  5  cc  of  concentrated 
hydrochloric  acid  and  50  cc  of  water,  warm  until  soluble  salts  are  dis- 
solved and  filter  the  residue  of  silica  at  once,  washing  until  free  from 
chlorides,  adding  the  washings  to  the  filtrate.  Evaporate  the  solution 
again  to  dryness,  heat  at  about  120°  and  repeat  the  treatment  with 
hydrochloric  acid  and  water,  filtering  in  a  different  paper. 

Place  both  papers  with  their  residues  in  a  platinum  crucible,  dry 
and  burn  the  papers  in  the  usual  way,  then  ignite  over  the  blast  lamp 
for  20  minutes,  cool  and  weigh.  Small  quantities  of  metal  salts  will 
have  been  retained  in  the  residue.  In  order  to  correct  for  their  presence 
the  residue  is  moistened  with  one  or  two  drops  of  sulphuric  acid  and 


294 


QUANTITATIVE  ANALYSIS 


about  5  cc  of  hydrofluoric  acid  is  added.  The  silicon  tetrafluoride  being 
volatilized,  the  acids  are  evaporated  and  the  residue,  which  should  be 
small,  is  heated  over  the  burner.  The  loss  in  weight  is  taken  to  repre- 
sent silica.  The  residue  of  oxides  is  dissolved  by  warming  with  a  few 
drops  of  hydrochloric  acid  and  is  added  to  the  filtrate  from  the  silica. 
Iron,  Aluminium,  Manganese,  Calcium,  and  Magnesium. — Concen- 
trate the  nitrate,  if  necessary,  to  about  100  cc  then  determine  iron, 
aluminium,  manganese,  calcium  and  magnesium  exactly  as  directed  for 

the  analysis  of  carbonate  minerals,  cal- 
culating each  as  the  oxide. 

Sodium  and  Potassium. — Grind  the 
sample  in  an  agate  mortar  to  pass  a 
200-mesh  sieve  and  weigh  0.5-gm 
samples  on  counterpoised  glasses. 
Place  0.5  gm  of  resublime4  ammonium 
chloride  (free  from  sodium  and  potas- 
sium) in  the  clean  agate  mortar,  add 
the  weighed  sample  and  grind  together. 
Add  3  gm  of  powdered  calcium  car- 
bonate and  grind  again.  Place  a  gram 
of  powdered  calcium  carbonate  on  the 
bottom  of  the  crucible  and  brush  in 
the  mixture  already  prepared.  Add 
another  gram  of  calcium  carbonate  to 
the  mortar,  grind  to  remove  the  last 
traces  of  sample  from  the  mortar  and' 
brush  this  in  on  top  of  the  charge. 
This  should  not  entirely  fill  the 
crucible. 

Adjust  the  crucible  in  a  hole  in  a 
piece  of  asbestos  board  in  such  a 
manner  as  that  the  board  comes  just 
to  the  upper  level  of  the  charge.  The  upper  part  of  the  crucible  will 
then  act  as  a  condenser  for  any  vapors  of  alkali  chlorides  that  might 
form  in  the  lower  part  of  the  crucible. 

Heat  the  lower  part  of  the  crucible  gradually  until  the  evolution  of 
ammonia  becomes  slow,  then  more  strongly  with  the  nearly  full  flame 
of  two  flat  burners.  This  heating  should  be  finally  applied  to  all  but  the 
upper  end.  After  45  minutes  of  such  heating  the  crucible  is  allowed  to 
cool  and  the  charge  emptied  into  a  platinum  dish.  The  materials  will 
usually  be  sintered  together  in  a  mass  that  has  shrunken  away  from  the 
crucible,  the  calcium  carbonate  in  the  bottom  of  the  crucible  serving 
to  prevent  sticking  of  the  mass  to  the  crucible.  Actual  fusion  into  a 


FIG.  70. — Smith's  crucible  for 
the  determination  of  alkali  metals 
in  silicates. 


ROCK  ANALYSIS  295 

slag  that  is  not  decomposed  by  water  is  an  indication  that  the  tempera- 
ture was  too  high  or  that  more  calcium  carbonate  should  be  used. 

Add  75  cc  of  water  to  the  materials  in  the  dish  and  warm  until  the 
mass  is  thoroughly  disintegrated.  Boil  for  a  minute,  allow  to  settle  and 
filter.  Add  50  cc  more  of  water  to  the  residue,  boil,  allow  to  settle  and 
filter  through  the  same  paper,  transferring  all  of  the  residue  to  the 
paper.  Wash  with  hot  water  until  a  few  drops  of  the  washings  show 
only  a  faint  test  for  chlorides. 

To  determine  whether  decomposition  was  complete  in  the  crucible, 
dissolve  the  washed  residue  in  hydrochloric  acid.  Any  gritty  residue 
must  be  retreated  as  before,  as  this  indicates  insufficient  heating. 
Evaporate  the  combined  filtrates  and  washings  to  a  volume  of  75  cc 
or  less,  then  add  5  cc  of  dilute  ammonium  hydroxide  and  sufficient 
ammonium  carbonate  solution  to  precipitate  all  iron,  aluminium  and 
calcium  present.  Digest  at  a  temperature  just  below  the  boiling  point 
until  the  precipitate  settles  readily,  then  filter  and  wash  once.  Dissolve 
the  precipitate  in  dilute  hydrochloric  acid  and  repeat  the  precipitation, 
washing  the  precipitate  with  hot  water.  Evaporate  the  two  filtrates 
and  all  washings  in  a  platinum  dish.  When  dry,  carefully  heat  over 
the  free  flame  of  the  burner  until  all  ammonium  salts  are  volatilized, 
but  without,  at  any  time,  allowing  the  dish  to  become  red.  Cool,  dis- 
solve in  not  more  than  10  cc  of  hot  water,  add  a  few  drops  of  dilute 
ammonium  hydroxide  solution  and  1  cc  of  saturated,  recently  prepared 
ammonium  oxalate  solution.  Digest  until  the  small  amount  of  calcium 
oxalate  settles,  then  filter  and  wash  the  paper,  collecting  filtrate  and 
washings  in  the  platinum  dish.  Add  1  cc  of  concentrated  hydro- 
chloric acid  and  evaporate  to  dryness.  Heat  carefully  to  volatilize 
ammonium  chloride.  Cool  in  a  desiccator  and  weigh  as  sodium  chloride 
and  potassium  chloride.  Dissolve  in  a  few  cubic  centimeters  of  hot 
water  and  transfer  to  another  dish.  Dry,  ignite  and  weigh  the  first 
dish  with  any  matter  that  was  not  soluble  and  subtract  this  weight 
from  the  one  already  observed.  The  difference  is  sodium  chloride  and 
potassium  chloride. 

Determine  the  potassium  in  this  mixture  as  directed  on  page  102, 
omitting  the  washing  with  ammonium  chloride  solution,  since  we  are 
here  dealing  with  chlorides  instead  of  sulphates  of  sodium  and  potassium. 

From  the  weight  of  potassium  chlorplatinate  found,  calculate  (a) 
the  percent  of  potassium  oxide  in  the  silicate  and  (b)  the  weight  of 
potassium  chloride  in  the  mixed  chlorides.  Subtract  the  latter  weight 
from  the  total  sodium  and  potassium  chlorides  and  from  the  remain- 
ing sodium  chloride  calculate  the  percent  of  sodium  oxide  in  the  silicate. 

The  sum  of  all  oxides  determined  should  approximate  100  percent. 
Since  experimental  errors  accumulate  and  since  small  quantities  of  other 


296  QUANTITATIVE  ANALYSIS 

substances  than  those  named  will  generally  remain  undetermined,  the 
sum  of  all  will  usually  be  less,  rather  than  more  than  100  percent.  It  is 
customary  with  many  chemists  to  report  this  discrepancy  as  "undeter- 
mined" but  it  will  be  remembered  that  such  a  percent  is  not  merely 
that  of  undetermined  matter  but  that  it  also  includes  the  accumulated 
errors  of  all  of  the  other  determinations. 


CHAPTER  XII 
FUELS 

COAL 

As  might  be  expected  from  a  knowledge  of  its  origin,  coal  is 
made  up  of  a  large  number  of  organic  and  some  inorganic  com- 
pounds. The  attempt  to  separate  and  identify  these  compounds 
has  long  engaged  the  attention  of  chemists  and  geologists  but 
comparatively  little  progress  has  been  made  in  this  direction. 
The  reason  for  the  lack  of  success  in  this  attempt  is  that  most 
chemical  methods  of  analysis  involve  the  breaking  up  of  organic 
substances  and  the  disappearance  of  the  original  forms.  Such 
an  analysis  would  prove  extremely  useful  from  the  stand- 
point of  geology  and  would,  no  doubt,  be  of  service  in  indus- 
trial applications.  In  the  absence  of  adequate  methods  for 
this  purpose,  the  examination  of  coal  is  made  with  one  or 
more  of  three  ends  in  view:  (1)  To  determine  the  geological 
origin  of  the  coal;  (2)  to  determine  its  adaptability  to  various 
industrial  uses,  such  as  steaming,  heating,  manufacture  of  pro- 
ducer gas  or  illuminating  gas,  coke,  tars,  etc.;  or  (3)  to  determine 
its  fuel  value  in  heat  units  per  unit  weight  of  coal.  The 
methods  used  are  classified  under  the  head  of  proximate  analysis 
or  of  ultimate  analysis.  The  proximate  analysis  of  materials 
may  be  defined  as  "the  determination,  not  of  elements  or  radicals 
but  of  groups  of  compounds  falling  within  approximate  limits 
of  composition  and  having  similar  properties."  The  ultimate 
analysis  is,  as  the  word  indicates,  a  determination  of  the  ele- 
mentary composition.  It  is  made  with  greater  difficulty, 
involves  more  expensive  apparatus  and  requires  longer  time 
than  the  proximate  analysis,  often  gives  no  more  useful  informa- 
tion than  is  given  by  the  proximate  analysis  and  is,  for  this 
reason,  less  frequently  made. 

Methods  for  the  analysis  of  coal  were  standardized  by  a  com- 

297 


298  QUANTITATIVE  ANALYSIS 

mittee  of  the  American  Chemical  Society  in  1899. 1  A  joint  com- 
mittee of  the  American  Chemical  Society  and  the  American 
Society  for  Testing  Materials  has  since  made  two  reports,2 
revising,  in  many  respects,  the  methods  of  the  original  committee. 

Sampling. — The  correct  sampling  of  coal  is  a  very  difficult 
process.  In  a  substance  showing  such  a  lack  of  uniformity  in 
composition  it  is  obvious  that  the  sample  must  be  selected  with 
extreme  care  if  the  results  of  the  analysis  are  to  express  the 
average  composition.  For  a  thorough  discussion  of  this  matter, 
and  especially  with  regard  to  the  selection  of  samples  from  cars, 
heaps,  mines,  etc.,  the  student  is  referred  to  a  paper  by  Bailey.3 
When  the  laboratory  sample  is  finally  selected  it  should  be 
sealed  in  an  air-tight  container  to  prevent  changes  in  the  moisture 
content.  This  container  may  be  a  tin  or  galvanized  can  with 
a  screw  top,  sealed  with  a  rubber  gasket  and  adhesive  tape, 
or  fruit  jars,  tightly  sealed. 

In  the  chemical  laboratory  this  sample  must  be  further  pul- 
verized and  divided  in  order  finally  to  obtain  a  sample  which  is 
small  enough  in  quantity  and  fine  enough  to  serve  for  the  actual 
analysis,  and  which  shall  also  be  representative  of  the  original 
material. 

When  the  sample  reaches  the  laboratory  it  is  treated  to 
the  usual  progressive  crushing  and  quartering  until  the  proper 
fine  sample  is  obtained.  During  this  process  there  is  a  continued 
loss  of  moisture  so  that  the  fine  sample  finally  obtained  has  not 
the  same  percentage  composition  as  the  sample  as  received. 
If  the  coal  as  received  is  taken  as  a  basis  its  composition  will  not 
be  the  same  as  that  of  another  coal  of  different  moisture  content 
but  otherwise  identical  with  the  first.  The  only  scientifically 
correct  method  is  to  calculate  all  other  percents  to  a  dry  coal 
basis,  reporting  the  moisture  in  the  sample  as  received.  It 
is  sometimes  necessary,  however,  to  base  all  calculations  upon 
the  coal  as  received.  In  this  case  moisture  must  be  determined 
at  once,  as  well  as  after  crushing  and  preparing  the  sample  for 
analysis,  as  already  mentioned.  Loss  of  moisture  occurs  at  all 
times  when  the  coal  is  exposed.  The  sample  is  ground  fine 

1  J.  Am.  Chem.  Soc.,  20,  281  (1898);  21,  1116  (1899). 

2  J.  Ind.  Eng.  Chem.,  6,  517  (1913);  9,  100  (1917). 

3  J.  Ind.  Eng.  Chem.,  1,  161  (1909). 


FUELS  299 

enough  to  serve  for  the  analysis  and  a  determination  of  moisture 
is  made  upon  this  sample.  The  difference  between  the  percent 
of  moisture  in  the  original  coarse  sample  and  the  fine  sample 
provides  a  basis  for  the  calculation  of  the  analytical  results, 
to  the  original  coal  basis. 

Proximate  Analysis. — The  proximate  analysis  of  coal  as  usually 
carried  out  includes  the  determination  of  moisture,  volatile 
combustible  matter,  coke,  " fixed  carbon"  and  ash.  The  figures 
thus  obtained  give  considerable  information  as  to  the  geological 
age  of  the  coal,  determine  its  fitness  for  industrial  uses  and 
provide  a  basis  for  an  approximate  calculation  of  fuel  value. 

Moisture. — The  accurate  determination  of  moisture  in  coal  is  a 
difficult  operation.  If  the  coal  is  heated  to  100°  or  above  this 
temperature  there  is  danger  of  loss  of  volatile  constituents  other 
than  moisture,  whether  these  were  originally  present  or  were 
formed  upon  heating.  Oxidation  also  takes  place  during  heating. 
This  may  result  in  either  gain  or  loss  in  weight,  according  to 
whether  the  greater  part  of  the  oxygen  is  retained  in  solid  com- 
pounds or  is  lost  as  volatile  ones.  The  usual  tendency  is  toward 
a  gain  in  weight  so  that  this  error  to  some  extent  compensates 
the  loss  first  mentioned.  Such  compensation  is,  however,  not 
to  be  depended  upon  and  such  a  determination  is  therefore  not 
ideal.  A  method  that  removes  some  of  these  objections  is  that 
of  drying  at  ordinary  temperatures  over  sulphuric  acid,  under 
diminished  pressure.  The  partial  pressure  of  oxygen  being 
reduced  to  a  negligible  quantity,  oxidation  is  entirely  prevented. 
There  is  also  no  tendency  toward  breaking  down  of  the  non- vola- 
tile organic  constituents  with  the  production  of  simpler  volatile 
ones.  The  method  seems  to  give  low  and  somewhat  variable 
results,  however,  due  to  the  low  rate  at  which  moisture  is  lost 
by  the  coal  toward  the  last  of  the  drying  process. 

Volatile  Combustible  Matter,  Fixed  Carbon  and  Coke. — When 
coal  is  subjected  to  dry  distillation  out  of  contact  with  air  variable 
quantities  of  volatile  products  are  expelled  and  a  residue  of 
inorganic  -matter  and  non-volatile  carbon  is  left.  This  residue  is 
the  "coke,"  the  carbonaceous  portion  of  the  coke  being  called 
"fixed  carbon."  It  should  be  understood  that  the  original  coal 
did  not  consist  of  free  carbon  and  volatile  organic  compounds,  but 
that  heating  at  high  temperatures  resulted  in  the  formation  of 


300  QUANTITATIVE  ANALYSIS 

such  substances  by  a  decomposition  of  the  non-volatile  bitumens 
composing  the  coal.  Such  a  decomposition  of  complex  com- 
pounds into  simpler  and  more  volatile  compounds  is  technically 
known  as  "cracking."  Cracking  is  a  somewhat  indefinite  proc- 
ess and  the  products  depend  to  a  large  extent  upon  the  tempera- 
ture and  time  of  heating.  The  volatile  products  of  the  distilla- 
tion of  coal  have  a  very  important  application  in  the  industries. 
The  non-volatile  coke  is  also  an  industrial  material,  being  exten- 
sively used  as  a  fuel  for  reducing  ores,  for  the  manufacture  of 
producer  gas  and  water  gas  and  for  many  other  purposes.  The 
relative  quantities  of  volatile  matter,  fixed  carbon  and  mineral 
matter  are  of  importance  in  the  determination  of  the  fitness  of 
various  coals  for  various  industrial  uses. 

On  account  of  the  variation  in  results  obtained  by  variation  in 
the  manner  of  heating  it  becomes  difficult  or  impossible  to  make 
an  intelligent  comparison  of  different  coals  unless  a  standard 
method  is  adopted  for  their  examination.  The  control  and 
measurement  of  the  temperature  at  which  they  are  heated 
involves  the  use  of  a  pyrometer  and  an  easily  controlled  furnace. 
It  is  also  difficult  entirely  to  exclude  air  during  the  heating  and  a 
variable  oxidation  occurs  The  student  is  again  reminded  that 
volatile  combustible  matter,  fixed  carbon  and  coke  are  arbitrary 
classifications  of  substances  produced  by  an  arbitrary  method 
and  that  they  have  little  scientific  meaning  except  as  they  provide 
a  basis  for  comparisons. 

Ash. — Most  of  the  inorganic  matter  of  coal  is  left  behind  as 
"ash"  when  the  coal  is  burned  but  the  compounds  so  remaining 
are  not  identical  with  those  of  the  original  coal.  Oxidizable 
substances  are  oxidized,  sometimes  to  a  variable  extent  depending 
upon  the  degree  of  excess  of  oxygen  in  the  atmosphere  in  which 
the  coal  is  burned.  Decomposable  compounds  are  also  changed 
at  the  furnace  temperatures.  The  first  class  of  changes  is 
illustrated  by  iron  disulphide,  the  essential  compound  of  iron 
pyrites.  This  is  oxidized  to  ferric  oxide  and  sulphur  dioxide, 
the  latter  escaping  with  the  other  volatile  products  of  com- 
bustion. Carbonates  of  the  earth  and  alkaline-earth  metals 
and  hydrated  silicates  are  examples  of  compounds  which  are 
decomposed  by  heating.  Such  carbonates  are  nearly  or  com- 


FUELS  301 

pletely  converted  into  oxides  and  the  hydrated  silicates  (such  as 
clay)  are  partly  deprived  of  their  water  of  hydration. 

Because  of  such  changes  as  these  the  percent  of  "ash,"  as 
determined  by  burning  a  weighed  sample  of  coal  and  weighing 
the  residue,  must  not  be  regarded  as  indicating  directly  the 
percent  of  inorganic  matter  or  "non-coal"  in  the  original  ma- 
terial, but  rather  as  the  percent  of  non-combustible  matter  that 
would  be  left  after  the  combustion  of  the  coal  in  air.  This  in- 
accuracy of  expression  affects  the  report  on  fixed  carbon  as  well 
as  that  of  ash,  since  the  former  is  obtained  by  subtracting  the 
sum  of  ash,  moisture  and  volatile  combustible  matter  from  100. 
In  the  majority  of  cases  the  error  is  comparatively  small  and 
the  correction  is  scarcely  worth  making.  But  if  coal  carries 
unduly  large  amounts  of  pyrite  or  of  carbonates  such  a  correction 
may  be  necessary,  if  a  report  on  fixed  carbon  is  to  have  any 
significance. 

When  iron  disulphide  is  burned  in  air  the  sulphur  is  lost  as 
sulphur  dioxide  and  iron  is  left  as  ferric  oxide: 


Upon  the  assumption  that  total  sulphur  represents  the  sulphur 
of  iron  pyrite,  the  loss  on  burning  each  molecule  of  iron  disul- 
phide results  from  the  substitution  of  three  atoms  of  oxygen  for 
four  of  sulphur.  That  is, 

(4X32)  -(3X16)  =80. 

80       5 
Therefore  77^  or  ~  of  the  total  sulphur  percent  should  be  added 

l^o        o 


to  the  ash  percent  as  a  correction  for  use  in  obtaining  the  correct 
percent  of  fixed  carbon. 

Such  carbonates  as  limestone  lose  carbon  dioxide  when 
strongly  heated.  It  is  not  by  any  means  certain  that  the  con- 
version to  calcium  oxide,  etc.,  is  complete  at  the  temperatures 
employed  in  burning  the  coal  and  if  a  correction  is  to  be  made 
it  is  better  to  add  a  few  drops'  of  sulphuric  acid  to  the  ash,  evapo- 
rating the  excess  of  acid  and  heating  gently  before  a  weighing 
is  made.  The  ash  which  has  been  treated  with  sulphuric  acid 
now  contains  sulphates  instead  of  carbonates,  and  it  is  heavier 
on  this  account.  Upon  a  separate  sample  of  coal  a  determination 


302  QUANTITATIVE  ANALYSIS 

of  carbon  dioxide  of  carbonates  is  made  by  any  of  the  standard 
methods  (see  pages  128  to  137). 

The  gain  in  weight  through  sulphation  is  represented  by  the 
difference  between  the  molecular  weights  of  sulphur  trioxide 
and  carbon  dioxide.  That  is, 

80-44  =  36. 

36        9 

Therefore    '  -r  or  ^-   of  the  percent   of  carbon  dioxide  of  car- 
•44        11 

bonates  should  be  subtracted  from  the  percent  of  sulphated 
ash  to  obtain  the  original  carbonated  inorganic  matter. 

It  should  be  noted  that  the  term  "ash"  is  a  misnomer  when 
applied  to  the  percent  corrected  as  above  described.  The  latter 
should  more  properly  be  called  "inorganic  matter"  or  "non- 
coal." 

Fusing  Point  of  Ash. — The  composition  of  coal  ash  has  an 
important  bearing  upon  the  industrial  uses  of  the  coal.  This 
is  particularly  true  with  regard  to  the  fusibility  of  the  ash, 
which  is  the  principal  property  upon  which  depends  the  forma- 
tion of  clinker.  If  the  inorganic  matter  of  coal  is  of  such  com- 
position as  to  produce  an  easily  fused  ash,  a  slag  will  form  in 
the  furnace  before  combustion  is  complete,  with  the  result  that 
more  or  less  coal  matter  is  so  glazed  and  protected  from  oxida- 
tion as  entirely  to  prevent  its  combustion.  Thus  there  is  a 
troublesome  clinker  formed  which  clogs  the  grates  and  interferes 
with  proper  stoking,  and  also  a  variable  waste  of  combustible 
matter.  This  action  is,  of  course,  more  pronounced  in  cases 
of  high  furnace  temperatures.  Clinker  will  often  contain  a 
very  high  percent  of  combustible  matter,  the  latter  being  deter- 
mined by  grinding  the  solid  clinker  and  burning  at  temperatures 
under  the  fusing  point  of  the  ash. 

In  most  high  grade  coals  the  inorganic  matter  consists  very 
largely  of  silicates  of  aluminium,  all  more  or  less  refractory. 
Clay  is  typical  of  these.  If  substances  capable  of  yielding  basic 
oxides  upon  ignition  are  present,  slag  formation  is  promoted 
and  clinker  forms.  The  most  abundant  of  such  base  formers 
are  iron  pyrite  and  calcium  carbonate.  On  this  account  a  high 
sulphur  content  is  usually  indicative  of  ability  to  form  a  fusible 
ash.  A  similar  indication  is  provided  by  a  red  ash,  since  most 
^of  such  color  comes  from  ferric  oxide.  The  presence  of  lime- 


FUELS  303 

stone  is  betrayed  by  neither  high  sulphur  nor  red  ash.  Con- 
sequently a  coal  may  contain  little  sulphur  and  form  a  white  ash 
and  yet  clinker  badly,  the  calcium  carbonate  (or  oxide)  readily 
uniting  with  refractory  silicates  to  produce  complex  silicates 
which  fuse  or  soften  at  the  prevailing  furnace  temperatures. 

On  account  of  the  objectionable  character  of  clinker  a  deter- 
mination of  fusibility  of  ash  often  becomes  highly  significant. 
Of  course  such  a  complex  mixture  as  that  which  composes  coal 
ash  can  have  no  very  definite  fusing  point,  so  that  observations 
of  the  behavior  of  the  ash  at  high  temperatures  must  be  made 
according  to  somewhat  arbitrary  standards.  The  ash  is  ground, 
mixed  with  water  and  (usually)  some  organic  binder,  and  molded 
into  pyramids  similar  to  the  well  known  "Seger  cones,"  which 
have  long  been  used  for  measuring  furnace  temperatures.  The 
"cone"  is  really  a  triangular  pyramid  having  a  base  perpen- 
dicular to  one  of  the  plane  sides.  When  such  a  piece  is  heated 
to  its  softening  temperature  it  leans  toward  the  vertical  side. 
The  temperature  at  which  the  apex  curves  down  to  touch  the 
supporting  plane  is  taken  as  the  "fusing  point." 

Fieldner  and  Hall1  and  Fieldner  and  Feild,2  working  in  the 
Bureau  of  Mines,  found  that  the  fusing  temperature  so  observed 
varies  somewhat  according  to  the  size,  shape  and  inclination  of 
the  test  piece,  the  fineness  of  grinding  the  ash  before  molding 
and  the  atmosphere  in  which  the  pyramid  is  heated.  The  effect 
of  the  surrounding  atmosphere  is  to  cause  changes  in  the  composi- 
tion of  the  ash,  the  latter  reacting  with  oxidizing  or  reducing 
gases.  The  nearest  imitation  of  furnace  conditions  was  found 
in  a  mixture  of  equal  volumes  of  hydrogen  and  water  vapor. 
This  keeps  the  iron  of  the  ash  in  the  ferrous  condition,  in  which 
form  it  is  usually  found  in  furnace  clinker. 

This  determination  is  described  with  great  detail  in  the  papers 
already  cited.  If  the  results  are  to  have  any  great  value  the 
determination  must  be  made  by  a  carefully  standardized  method 
and  equipment,  including  the  special  furnace  there  described, 
or  a  similar  one. 

Preparation  of  Sample,  (a)  When  Coal  Appears  Dry. — If  the  sample 
is  coarser  than  4-mesh  and  larger  in  amount  than  10  pounds  quickly 

1  J.  Ind.  Eng.  Chem.,  7,  399  and  474  (1915). 

2  Ibid.,  7,  742  and  829  (1915). 


304  QUANTITATIVE  ANALYSIS 

crush  it  with  a  jaw  crusher  to  pass  a  4-raesh  sieve  and  reduce  it  an  a 
riffle  sampler  (page  14)  to  10  pounds,  then  crush  at  once  to  20- 
mesh  by  passing  through  rolls  or  an  enclosed  grinder  and  take,  without 
sieving,  a  60-gm  total  moisture  sample,  immediately  after  crushing. 
This  sample  should  be  taken  with  a  spatula  from  various  parts  of  the 
20-mesh  product  and  should  be  placed  directly  in  a  rubber-stoppered 
bottle. 

Thoroughly  mix  the  main  portion  of  the  sample,  reduce  on  the  small 
riffle  sampler  to  about  120  gm  and  pulverize  to  60-mesh  by  any  suitable 
apparatus  without  regard  to  loss  of  moisture.  Mix  and  divide  the  60- 
mesh  sample  on  the  small  riffle  until  it  is  reduced  to  60  gm.  Preserve 
this  in  a  rubber-stoppered  bottle. 

Determine  moisture  in  both  the  60-mesh  and  the  20-mesh  samples 
by  the  methods  given  under  the  head  of  moisture. 

(6)  When  Coal  Appears  Wet.  —  Spread  the  sample  on  weighed  pans, 
weigh  and  dry  for  12  hours  at  room  temperature  or  in  a  sp'ecial  drying 
oven  through  which  air  circulates  freely  at  10°  to  15°  above  the  room 
temperature.  Reweigh  and  continue  the  drying  until  the  loss  in  weight 
is  not  more  than  0.1  per  cent  per  hour.  Complete  the  sampling  as 
with  dry  coal  and  calculate  the  percent  of  moisture  by  air  drying. 

To  find  the  total  moisture  in  wet  coal,  as  received,  compute 
as  follows: 

Let  a  =  percent  of  moisture  .by  air  drying, 

MZQ=  percent  of  moisture  in  20-mesh  coal. 


, 

Then  -  —  17^:  --  +a  =  total  moisture  as  received. 
1UU 

The  percents  of  the  various  constituents  of  the  coal  are  de- 
termined on  the  60-mesh  coaL  To  compute  these  percents  to 
the  dry-coal  basis  or  the  as  received  bases,  proceed  as  follows: 

Let  M  6o  =  per  cent  moisture  in  60-mesh  coal, 

Mr  =  total  percent  moisture  in  coal  as  received, 
PW  =  percent  of  any  constituent  in  60-mesh  coal, 
PO  =  percent  of  any  constituent  in  dry  coal, 
Pr  =  percent  of  any  constituent  in  coal  as  received. 

Then  P. 


/100-J/A  /100-Jf, 

Pr=         100    /  Po= 


FUELS  305 

Proximate  Analysis. — Procure  a  sample  as  directed  in  the  preceding 
discussion. 

Moisture. — The  oven  for  drying  the  20-mesh  and  60-mesh  samples  ' 
must  be  so  constructed  as  to  provide  a  uniform  temperature  in  all  parts 
and  a  minimum  of  air  space.     Provision  must  be  made  for  renewing 
the  air  in  the  oven  at  the  rate  of  two  to  four  times  a  minute,  with  air 
dried  by  passing  through  concentrated  sulphuric  acid. 

A  convenient  form  of  crucible  for  the  moisture  determinations  and 
one  which  allows  the  ash  determinations  to  be  made  on  the  same  sample 
is  a  flat  bottomed  porcelain  crucible.  A  fused  silica  crucible  of  similar 
shape  may  be  used.  In  either  case  a  well  fitted  aluminium  cover  should 
be  provided.  Glass  capsules,  with  covers  ground  on,  may  also  be  used 
but  ash  determinations  will  then  require  different  samples. 

Sixty-mesh  Sample. — Heat  the  empty  crucible  under  the  conditions 
under  which  the  coal  is  to  be  dried,  cover,  cool  in  a  desiccator  over  con- 
centrated sulphuric  acid  for  30  minutes  and  weigh.  Place  approxi- 
mately 1  gm  of  the  sample  in  the  crucible,  cover  and  reweigh.  Remove 
the  cover  and  place  the  crucible  in  the  oven,  which  is  maintained  at 
104°  to  110°.  Heat  for  1  hour  then  cover  the  crucible,  cool  over  sul- 
phuric acid  for  30  minutes  and  weigh.  Calculate  the  loss  as  moisture. 

Twenty-mesh  Sample. — Use  5-gm  samples  weighed  with  an  accuracy 
of  2  mg  and  heat  for  IK  hours.  The  procedure  is  otherwise  the  same 
as  for  the  60-mesh  sample. 

Permissible  differences  in  duplicate  determinations: 

Same  analyst     Different  analysts 

Moisture  under  5% 0.2%  0.3% 

Moisture  over  5% 0.3  0.5 

Ash. — Place  the  uncovered  porcelain  crucible  containing  the  dried 
60-mesh  coal  in  a  cold  muffle  furnace  through  which  air  may  be  drawn 
and  gradually  raise  the  temperature  to  between  700°  and  750°.  When 
combustion  appears  to  be  complete  remove  the  crucible,  cover,  cool  in  a 
desiccator,  and  weigh.  Repeat  the  heating  for  15-minute  periods  until 
the  change  in  weight  is  not  greater  than  1  mg.  Calculate  the  percent 
of  ash. 

Permissible  differences  in  duplicate  determinations: 

Same  analyst  Different  analysts 

No  carbonates  present    -. .        0.2%  0.3% 

Carbonates  present    0.3  0.5 

Carbonates  and  pyrite  present 

and  more  than  12 %  ash 0.5  1.0 

20 


306  QUANTITATIVE  ANALYSIS 

Volatile  Combustible  Matter. — For  this  determination  an  electri- 
cally heated,  vertical-tube  furnace  is  desirable,  although  a  muffle 
furnace,  heated  by  gas  or  electricity  may  be  used.  If  the  tube  furnace 
is  available  the  vertical  tube  should  be  about  l^j  inches  in  diameter 
and  6  inches  deep.  The  junction  of  a  thermo-couple  connected  with 
a  pyrometer  meter  should  be  placed  immediately  below  the  bottom  of 
the  crucible.  The  furnace  is  to  be  kept  covered  during  a  determination. 

If  the  determination  of  volatile  combustible  matter  is  not  an  essential 
part  of  the  specifications  under  which  the  coal  is  bought  a  No.  4  Meker 
burner  may  be  used  for  the  heating. 

The  platinum  crucible  should  have  a  capacity  between  10  and  20  cc, 
a  diameter  between  25  and  35  mm  and  a  depth  between  30  and  35  mm. 
The  cover  must  fit  closely. 

Weigh  the  crucible  then  add,  as  nearly  as  possible  without  careful 
adjustment,  1  gm  of  60-mesh  coal,  cover  the  crucible  and  reweigh.  If 
a  furnace  is  used  for  the  heating  it  should  be  brought  to  a  temperature 
of  950°  (±20°),  and  the  crucible  is  placed  on  a  support  of  platinum  or 
of  nickel-chromium  in  the  furnace.  If  a  Meker  burner  is  used,  adjust 
the  flame  so  that  its  extreme  height  is  15  cm  and  support  the  crucible 
on  a  platinum  or  a  nickel-chromium  wire  triangle  so  that  the  bottom  is 
1  cm  above  the  top  of  the  burner. 

After  the  luminous  flame  above  the  crucible  has  disappeared,  tap 
the  crucible  cover  lightly  in  order  to  seal  more  perfectly  and  thus 
guard  against  the  entrance  of  air.  Heat  for  exactly  7  minutes  then 
remove  the  crucible  from  the  furnace  or  flame  without  disturbing 
the  cover.  Cool  in  a  desiccator  and  weigh.  Calculate  the  loss  as 
percent  of  total  volatile  matter,  including  moisture.  Subtract  the 
percent  of  moisture  found  in  the  60-mesh  sample  and  report  the  percent 
of  volatile  combustible  matter. 

Svh-bituminous  coal,  lignite  or  peat  are  given  a  preliminary  gradual 
heating  for  5  minutes  to  expel  part  of  the  large  quantity  of  volatile 
matter.  They  are  then  heated  as  above  directed  for  6  minutes. 

Permissible  differences  in  duplicate  determinations: 

Same  analyst     Different  analysts 

Bituminous  coals   0.5%  1 .0% 

Lignites 1.0  2.0 

Fixed  Carbon. — Subtract  from  100,  the  sum  of  moisture,  volatile 
combustible  matter  and  ash,  corrected  to  the  basis  of  either  dry  coal 
or  as  received.  The  remainder  is  the  percent  of  fixed  carbon  on  the 
basis  considered. 


FUELS  307 

Ultimate  Analysis. — The  complete  ultimate  analysis  of  coal 
will  include  the  determination  of  all  elements.  The  com- 
plete analysis  is  not  often  made,  the  determination  of  sulphur, 
carbon,  hydrogen  and  nitrogen  being  all  that  is  usually  required. 
Sulphur. — Sulphur  may  exist  in  coal  in  one  or  more  of  four 
forms:  elementary  sulphur,  inorganic  sulphides  (principally 
iron  pyrite)  inorganic  sulphates  and  organic  compounds.  The 
accurate  determination  of  the  amount  present  in  the  different 
forms  is  difficult  and  not  often  required. 

Three  methods  have  found  considerable  use  for  determining 
total  sulphur  in  coal.  The  powdered  sample  may  be  gently 
heated  with  sodium  carbonate  (Atkinson's  method1)  or  with  a 
mixture  of  sodium  carbonate  and  magnesium  oxide  (Eschka's 
method2) ;  or  it  may  be  fused  with  sodium  peroxide.  In  the  first 
two  cases  air  is  the  oxidizing  agent  while  in  the  last  sodium 
peroxide  performs  this  function.  The  ultimate  result  in  all 
three  is  that  the  organic  portion  of  the  coal  is  burned  while 
sulphur  is  oxidized,  either  dioxide  or  trioxide  being  retained  by  the 
basic  constituents  of  the  added  reagents,  soluble  sulphites  or 
sulphates  being  produced.  In  the  Eschka  method  magnesium 
oxide  serves  to  provide  a  porous  mixture  into  which  air  readily 
penetrates. 

It  is  necessary  to  provide  against  absorption  of  sulphur  oxides 
from  the  surrounding  atmosphere.  For  this  reason  ordinary 
illuminating  or  coal  gas  cannot  be  used  for  direct  heating  of  the 
mixture  unless  it  is  first  purified.  Alcohol  burners  are  suitable 
for  this  purpose  but  a  muffle  furnace,  heated  by  electricity,  is 
preferable. 

When  the  oxidation  of  the  coal  is  complete  the  mixture  is  boiled 
with  water  and  bromine,  the  latter  to  oxidize  to  sulphates  any 
sulphites  that  may  have  been  formed.  After  filtration  the  solu- 
tion is  acidified  and  boiled,  whereby  bromates  or  hypobromites 
are  decomposed  and  the  bromine  expelled  from  the  solution: 

NaBrO3+ HCl-»NaCl + HBrO3, 

NaBrO  +  HCl-^NaCl + HBrO, 

HBr03+5HBr-»3H20+3Br2, 

HBrO + HBr-»H20  -t  Br 2. 

'J.  Soc.  Chem.  Ind.,  6,  154  (1886).     . 
'Chera.  News,  21,  261  (1870). 


308 


QUANTITATIVE  ANALYSIS 


It  is  necessary  thus  to  decompose  salts  of  oxyacids  of  bromine 
because  of  the  tendency  shown  by  barium  sulphate  to  occlude 
these  compounds  at  the  moment  of  precipitation. 

Nitrogen. — Nitrogen  may  be  determined  by  combustion  or 
by  the  Kjeldahl,  the  Gunning  or  the  Kjeldahl-Gunning  methods. 
The  principles  underlying  the  latter  are  discussed  on  page  513 
and  following.  In  the  combustion 
method  the  coal  is  mixed  with  fine  cupric 
oxide  and  is  heated  in  a  tube  closed  at 
one  end.  Oxidation  occurs  and  the  gases 
are  passed  through  more  heated  cupric 
oxide  to  complete  the  oxidation  of  carbon 
monoxide  or  of  gaseous  hydrocarbons, 
then  over  heated  copper,  the  latter  to 
reduce  oxides  of  nitrogen  to  elementary 
nitrogen.  The  mixture  of  carbon  dioxide, 
water  vapor,  sulphur  dioxide  and  nitro- 
gen is  passed  into  sodium  hydroxide 
solution  where  all  gases  except  nitrogen 
are  absorbed.  The  latter  is  collected  in 
a  eudiometer  and  measured,  the  weight 
being  then  calculated.  A  special  form 
of  eudiometer  called  the  "  nitrometer/' 
is  useful  for  this  purpose.  This  is  shown 
in  Fig.  71.  In  order  to  force  the  gases 
out  of  the  combustion  tube  a  short  space 
in  the  closed  end  is  filled  with  sodium 
bicarbonate.  Carbon  dioxide  is  evolved 
at  will  by  heating  this  and  it  causes  the 
expulsion  of  other  gases  from  the  tube. 

Carbon  and  Hydrogen. — The  deter- 
mination of  carbon  and  hydrogen  is  made 
by  combustion  and  absorption  of  water 

vapor  and  carbon  dioxide  in  weighed  tubes  containing  ap- 
propriate absorbents.  The  method  is  the  same  as  that  used  in 
other  connections  for  organic  substances  containing  sulphur  and 
nitrogen.  For  the  absorption  of  water  vapor  calcium  chloride  or 
sulphuric  acid  may  be  used,  the  latter  giving  more  nearly  complete 
absorption  while  the  former  'is  more  conveniently  used.  For 


FIG.   71. — Schiff's 
nitrometer. 


FUELS  309 

carbon  dioxide,  solid  soda  lime  or  a  solution  of  potassium  hydroxide 
is  used,  any  of  the  standard  forms  of  absorbing  tubes  or  bulbs 
serving  as  containers.  A  combustion  tube  of  glass,  silica  or 
porcelain  is  used  for  the  decomposition  of  the  coal.  The  relative 
merits  of  these  materials  were  discussed  in  an  earlier  section 
(pages  37  and  42).  For  the  present  purpose  the  silica  tube  is 
most  satisfactory  and  any  suitable  combustion  furnace  may  be 
used.  The  tube  should  be  95  cm  long  and  should  project  10  to 
15  cm  beyond  the  furnace  at  each  end.  The  combustion  is 
effected  by  heating  the  coal  in  an  atmosphere  of  oxygen.  Since 
this  also  produces  volatile  organic  compounds  and  some  carbon 
monoxide,  it  is  necessary  to  pass  the  gases  through  a  solid  oxidiz- 
ing agent  in  order  to  complete  the  oxidation.  Cupric  oxide  is 
used  for  this  purpose  in  the  analysis  of  most  organic  substances, 
but  when  sulphur  is  present,  as  in  all  coals,  sulphur  oxides  are 
not  completely  retained  by  cupric  oxide  and  are  therefore  ab- 
sorbed in  bulbs  containing  potassium  hydroxide.  In  the  com- 
bustion of  such  materials  lead  chromate  is  substituted  for  part 
of  the  cupric  oxide.  Lead  sulphate  is  formed  and  this  is  not  decom- 
posed by  heating.  A  lower  temperature  is  used  to  avoid  fusing 
the  material  into  the  tube. 

Oxygen  for  the  combustion  may  be  made  from  manganese 
dioxide  and  potassium  chlorate  and  stored  in  a  large  "gasometer" 
or  it  may  be  purchased  in  steel  cylinders.  Oxygen  made  by  the 
first  method  named  always  contains  chlorine  oxides  and  it  must 
be  purified  by  passing  through  a  concentrated  solution  of  potas- 
sium hydroxide.  Most  of  the  commercial  oxygen  formerly 
obtainable  in  compression  cylinders  contained  chlorine  oxides, 
carbon  dioxide  and  hydrogen,  and  such  oxygen  must  be  properly 
purified  before  it  enters  the  combustion  tube.  Since  the  develop- 
ment of  the  manufacture  of  oxygen  from  liquid  air  it  is  possible 
to  obtain  gas  having  a  high  degree  of  purity.  It  is  then  only 
necessary  to  pass  the  oxygen  through  potassium  hydroxide  or 
soda  lime  in  order  to  remove  traces  of  carbon  dioxide.  In  order 
to  be  able  to  control  the  flow  a  " gasometer"  should  be  filled  from 
the  high-pressure  cylinder  and  this  used  as  the,  supply  for  the 
combustion  or  else  a  special  high-pressure  control  valve  should 
be  used  on  the  tank. 

Oxygen. — The  direct  determination  of  oxygen  is  difficult  and 


310  QUANTITATIVE  ANALYSIS 

is  seldom  attempted.  The  percent  is  sometimes  estimated  by 
subtracting  the  sum  of  percents  of  all  other  elements,  water  and 
ash,  from  100.  This  is  but  a  rough  approximation  but  is  usually 
all  that  is  required. 

(Jltimate  Analysis  of  Coal:  Sulphur.  Eschka  Method. — Thoroughly 
mix  two  parts  of  light  calcined  magnesium  oxide  and  one  part  of  anhy- 
drous sodium  carbonate.  These  materials  should  be  as  free  as  possible 
from  sulphur.  Place  3  gm  of  the  mixture  on  a  sheet  of  white  glazed 
paper  and  then  add  1  gm  of  60-mesh  coal,  accurately  weighed.  Mix 
well,  then  transfer  to  a  porcelain,  silica  or  platinum  dish,  2  inches  in 
diameter  and  1  inch  deep.  Cover  with  about  1  gm  of  the  Eschka 
mixture. 

A  flame  of  illuminating  gas  cannot  be  used  for  heating  on  account 
of  absorption  of  sulphur  dioxide  from  the  flame  by  the  basic  mixture. 
A  gas  or  electric  muffle  furnace  is  best  for  the  purpose  but  an  alcohol, 
gasoline  or  natural  gas  flame  may  be  used. 

If  a  flame  is  applied  directly,  heat  the  dish  in  a  slanting  position 
on  a  triangle  over  a  low  flame  until  most  of  the  volatile  matter  is  driven 
off,  then  gradually  increase  the  temperature  and  heat  for  30  minutes 
or  more,  stirring  occasionally,  until  all  black  particles  have  disappeared. 

If  a  muffle  furnace  is  to  be  used,  place  the  crucible  in  the  cold  furnace 
and  gradually  raise  the  temperature  so  that  about  1  hour  is  required 
to  reach  870°  to  925°.  Maintain  this  temperature  for  l}4  hours,  then 
cool  in  the  furnace  or  in  air  that  is  free  from  gases  containing  sulphur 
compounds. 

After  either  treatment  rinse  the  material  into  a  200-cc  beaker,  add 
100  cc  of  nearly  boiling  water  and  digest  on  the  steam  bath  for  30  min- 
utes, stirring  frequently.  Filter  and  wash  the  insoluble  matter  thor- 
oughly with  hot  water.  The  filtrate  and  washings  should  total  about 
250  cc.  Add  20  cc  of  saturated  bromine  water  to  the  solution  and  stir, 
then  make  slightly  acid  with  dilute  hydrochloric  acid  (shown  by  the 
failure  of  more  acid  to  cause  effervescence)  and  boil  until  all  bromine 
is  removed  and  the  solution  is  colorless.  Add  two  or  three  drops  of 
methyl  orange  or  methyl  red  and  neutralize  with  10  percent  sodium 
hydroxide  solution,  then  add  1  cc  of  approximately  normal  hydrochloric 
acid,  or  an  equivalent  volume  of  a  solution  of  any  other  normality. 

Heat  to  boiling  and  add,  dropwise  and  with  stirring,  20  cc  of  5  percent 
solution  of  barium  chloride.  Digest  on  the  steam  bath  until  the  pre- 
cipitate settles  readily  and  then  filter  and  wash  free  from  chlorides  with 
hot  water.  Fold  the  paper,  place  in  a  weighed  platinum,  porcelain, 
silica  or  alundum  crucible  and  burn  the  paper  at  a  low  temperature 


FUELS  311 

and  with  free  access  of  air  (see  page  93),  finally  heating  to  dull  red- 
ness for  10  minutes.  Cool  and  weigh.  Calculate  the  percent  of  sul- 
phur in  the  60-mesh  coal. 

The  residue  of  magnesium  oxide,  ash,  etc.,  should  be  dissolved  in 
hydrochloric  acid  and  tested  for  sulphur.  If  any  is  found  this  must  be 
determined  quantitatively  and  added  to  the  percent  already  found. 

A  blank  experiment  should  be  performed,  using  all  of  the  reagents  of 
the  regular  experiment  but  omitting  the  first  heating.  Any  sulphur 
that  is  so  obtained  from  the  reagents  is  to  be  subtracted  from  that  found 
in  the  analysis  of  the  coal. 

Permissible  differences  in  duplicate  determinations: 

Same  analyst         Different  analysts 

Sulphur  less  than  2%    0.05%  0.10% 

Sulphur  more  than  2% 0.10  0.20 

Carbon  and  Hydrogen. — A  tube  combustion  furnace  of  any  of  the 
approved  types  and  about  75  to  80  cm  long  is  necessary.  The  combus- 
tion tube  may  be  of  hard  glass,  silica  or  porcelain.  It  should  be  long 
enough  to  project  for  at  least  10  cm  at  each  end  of  the  furnace,  in  order 
to  prevent  heating  of  the  rubber  stoppers  that  must  be  inserted  in 
the  ends.  The  internal  diameter  of  the  tube  should  be  12  to  15  mm. 
Since  coal  always  contains  sulphur,  lead  chromate  must  be  used  in  the 
combustion  tube  and  this  is  prepared  by  fusing,  cooling  and  crushing 
about  100  gm.  The  largest  pieces  should  be  small  enough  to  easily  enter 
the  tube.  By  sifting  the  crushed  material,  using  a  40-mesh  sieve,  a  finer 
grade  will  be  obtained  and  this  is  used  for  mixing  with  the  powdered 
coal. 

The  combustion  tube  should  have  well  rounded  ends.  It  is  filled 
according  to  the  following  directions,  assuming  that  the  length  of  the 
furnace  is  75  cm  and  that  of  the  tube  is  95  cm. 

Into  one  end  of  the  tube  insert  a  closely  fitting  roll  of  copper  gauze, 
5  cm  long,  and  push  this  in  until  a  space  of  10  cm  is  left  at  the  end 
of  the  tube.  Into  the  other  end  pour  the  coarsely  crushed  lead  chromate 
until  a  space  50  cm  long  is  filled.  The  material  should  be  well  settled 
but  not  packed  in  such  a  way  as  to  obstruct  the  passage  of  gases.  Insert 
another  roll  of  copper  gauze  like  the  first,  to  hold  the  lead  chromate  in 
place.  Another  roll  of  copper  gauze,  10  cm  long,  is  inserted  in  such  a 
way  as  to  leave  a  space  of  10  cm  at  the  end  of  the  tube,  and  a 
space  of  5  cm  between  the  two  rolls.  The  latter  space  is  for  the  boat 
containing  the  coal.  The  ends  of  the  tube  are  closed  by  rubber  stoppers 
carrying  short  glass  tubes  for  connecting  with  the  rest  of  the  apparatus. 
The  method  of  filling  the  tube  is  shown  in  Fig.  72.  Fig.  73  shows  the 


II' 


312  QUANTITATIVE  ANALYSIS 

method  of  assembling  the  complete  apparatus.     Entering  oxygen  and 
entering    air    are    passed   through   cylinders  A   and   A',   re- 
spectively, containing  a  good  quality  of  soda  lime.     The  gases 
are  next  passed  through  U-tubes,  B  and  B',  containing  fused, 
granular  calcium  chloride.      These  tubes  are  connected  with 
the  combustion   tube   by  means  of  a  three-way    stopcock. 
Gases  leaving  the  combustion  tube  first  pass   through  two 
U-tubes    C   and   D    (preferably   glass   stoppered)    containing 
calcium  chloride,  then  through  the  carbon  dioxide  absorption 
bulbs  E  containing  potassium  hydroxide  solution  and  calcium 
chloride.     Following  these  tubes  is  a  guard  tube  F  containing 
-<    calcium  chloride,  also  an  aspirator  G.     For  the  detailed  direc- 
8    tions  for  filling  and  connecting  the  various  absorption  tubes 
*S    and  aspirator,  refer  to  the  discussion  of  the  determination  of 
•§    carbon  dioxide  in  carbonates. 

^        When  the  combustion  tube  and  all  parts  of  the  apparatus  are 
§    in  order  start  a  slow  but  steady  current  (three  bubbles  per 
J3    second  in  the  bulbs)  of  air  by  means  of  the  aspirator,  then 
»    heat  gradually  the  entire  length  of  the  combustion  tube.    The 
3    drying  tubes  C  and  D  and  the  potassium  hydroxide  bulbs 
g    need  not  be  in  the  train  because  this  preliminary  heating  is 
'•§    for   the   purpose   of   thoroughly   drying   the   contents  of  the 
.g    combustion  tube  and  oxidizing  any  organic  matter  with  which 
o    the  tube  might  be   contaminated.     They  are  therefore  re- 
£    moved,   carefully  wiped  clean  and  dry,  and  are  then  closed 
"JJ    and    placed  in  the  balance  case.      After  they   have   stood 
"   for    15   minutes,   if  ready  to   proceed  with   the   blank   test, 
these  pieces  are  weighed.      The  temperature  of  the  part  of 
the  tube  containing  lead  chromate  must  not  be  higher  than 
is  indicated  by  dull  redness   although   other  parts  may  be 
heated  to  any  temperature  under  the  softening  point  of  the 
tube. 

When  moisture  has  been  expelled  from  the  tube  to  the 
extent  that  no  condensation  is  noticed  on  the  forward  end  the 
calcium  chloride  tubes  C  and  D  and  the  potassium  hydroxide 
bulbs  are  weighed  and  placed  in  the  absorption  train.  The 
flow  of  air  is  continued  for  20  minutes,  when  the  aspirator  is 
stopped  and  the  absorption  tubes  are  again  removed,  stop- 
pered, placed  in  the  balance  case  and  weighed  after  standing 
for  15  minutes.  If  there  is  a  gain  of  more  than  0.5  mg  in 
either  the  weight  of  the  potassium  hydroxide  bulbs  or  the 
combined  weights  of  the  two  calcium  chloride  tubes  the  entire 
operation  must  be  repeated  until  there  is  no  greater  gain  than  0.5  mg. 


r 


| 


FUELS  313 

When  this  is  the  case,   that  part  of  the  combustion  tube  which  is  at 
the  left  of  the  lead  chromate  is  allowed  to  cool. 

Provide  a  porcelain  or  platinum  boat,  about  5  cm  long  and  of  the 
proper  width  for  insertion  into  the  combustion  tube.  In  the  bottom 
of  this  place  a  layer  of  powdered  lead  chromate  1  mm  deep  and  then 
weigh  into  the  boat  about  0.5  gm  of  powdered  coal  which  has  been 
properly  sampled  and  dried.  Mix  with  a  platinum  wire.  Remove 
the  rubber  stopper  at  the  left  end  of  the  tube  and  quickly  remove  the 
roll  of  copper  gauze  (now  largely  oxidized  to  cupric  oxide)  by  means  of 
a  wire  hook  and  insert  the  boat,  pushing  the  latter  in  until  it  touches 
the  roll  of  cupric  oxide  which  confines  the  lead  chromate.  Replace  the 
first  roll  and  the  rubber  stopper  as  quickly  as  possible,  start  a  current  of 
oxygen  through  the  tube  and  gradually  heat  the  cooled  portion  of  the 


tnrr-tr-\ 

C     D     E        F  II 

Li 

G 


FIG.  73. — Diagram  of  connections  for  combustion  apparatus. 

tube.  Volatile  matter  will  escape  from  the  coal  but  this  will  be  com- 
pletely oxidized  by  the  lead  chromate  in  the  forward  part  of  the  tube. 
Backward  diffusion  of  volatile  combustible  matter  will  occasion  no  loss 
by  condensation  because  the  roll  of  cupric  oxide  behind  the  boat  will 
serve  to  oxidize  a  small  quantity  of  such  gases. 

When  all  glowing  of  the  coal  has  ceased,  turn  the  three-way  stopcock 
so  that  air  is  drawn  into  the  tube,  gradually  lower  the  temperature  of 
the  left  end  so  as  to  avoid  cracking  and  continue  the  passage  of  air 
until  about  1000  cc  more  of  water  has  run  out  of  the  aspirator.  Remove 
the  absorption  tubes  and  bulbs,  close  and  allow  to  stand  15  minutes  and 
then  weigh.  From  the  total  gain  in  weight  of  the  two  calcium  chloride 
tubes,  due  to  absorption  of  water  vapor,  calculate  the  percent  of  hydro- 
gen in  the  coal.  From  the  weight  of  carbon  dioxide  absorbed  in  the 
potassium  hydroxide  bulbs  calculate  the  percent  of  carbon  in  the  coal. 

Duplicate  determinations  should  be  made.  If  many  samples  are  to 
be  analyzed  much  economy  of  time  will  result  from  the  use  ofltwo  boats 
and  two  sets  of  absorption  tubes.  As  each  experiment  is  finished 
another  can  be  started  and  the  combustion  will  proceed  while  the  first 
set  of  tubes  is  standing  in  the  balance  and  being  weighed. 


314  QUANTITATIVE  ANALYSIS 

Fuel  Value. — After  a  decision  has  been  reached  as  to  the  class 
of  coal  that  is  most  suitable  for  a  given  industrial  purpose  the 
inquiry  that  is  next  in  importance  concerns  the  number  of  heat 
units  that  can  be  obtained  from  unit  weight  of  the  various  grades 
of  coal  entering  into  that  class.  The  custom  of  purchasing  coal 
upon  a  tonnage  basis  at  a  contract  price,  with  nothing  more  than 
the  variety  of  coal  named,  is  rapidly  being  displaced,  by  large 
consumers,  by  the  method  of  purchasing  upon  a  heat  unit  basis. 
A  contract  price  is  made,  based  upon  a  specified  number  of  heat 
units  per  pound  or  ton  and  any  deviation  from  this  fuel  value 
involves  a  corresponding  alteration  in  the  price. 

In  scientific  work  the  fuel  value  is  always  calculated  as  calories 
per  gram  or  kilogram  of  fuel,  while  in  industrial  work  it  is  gener- 
ally calculated  as  " British  thermal  units"  per  pouijd  of  fuel. 
The  calorie  is  the  quantity  of  heat  required  to  raise  1  gm  of  water 
1°  C.  in  temperature.  The  British  thermal  unit  (B.  T.  U.)  is  the 
quantity  of  heat  necessary  to  raise  1  Ib  of  water  1°  F.  in  tem- 
perature. The  calculation  of  fuel  values  in  either  system  in- 
volves equal  weights  of  water  and  fuel  and  the  relation 

-P   ~,  ;?., — - — 57-  is  therefore  the  relation  of  the  centigrade  degree 

9 

to  the  Fahrenheit  degree.     That  is,  cal  per  gmX •=  =  B.  T.  U.  per 

o 

Ib,  and  B.  T.  U.  per  lbXo  =  cal  per  gm. 

y 

This  may  be  demonstrated  as  follows : 
Let  a  =  temperature  equivalent  of  1°  C.. 

b  =  temperature  equivalent  of  1°  F., 

c  =  weight  equivalent  of  1  gm, 

d  =  weight  equivalent  of  1  pound. 

Then  B.  T.  U.  perlbx|  =  B.  T.  U.  per  gm; 

IB.  T.  U.=— cal, 
ac 

therefore  B.  T.  U.  per  Ib  X~  =  cal  per  gm, 

or          B.  T.  U.  per  lbXn  =  cal  per  gm. 

y 

Conversely  cal  per  gmX^B.  T.  U.  per  Ib. 

o 


FUELS  315 

Calculation  of  Fuel  Value  from  the  Ultimate  Analysis.  —  The  cal- 
culation from  the  ultimate  analysis  is  based  upon  the  assumption 
that  the  heat  of  oxidation  of  a  compound  is  equal  to  the  sum  of 
the  heats  of  oxidation  of  the  elements  composing  it.  The  ele- 
ments of  coal  that  are  oxidizable  are  carbon,  hydrogen,  and  sul- 
phur. Nitrogen  is  mostly  evolved  in  the  free  state  and  inorganic 
matter  other  than  sulphides  is  of  little  or  no  value  for  producing 
heat.  Water  is  incombustible  and  absorbs  heat  in  becoming 
vaporized,  thus  reducing  the  available  heat  energy.  Oxygen 
is  not  only  incombustible  but,  because  of  the  fact  that  it  is 
already  combined  with  carbon  and  hydrogen  it  reduces  the  per- 
cent of  these  elements  still  available  for  combustion  and  therefore 
reduces  the  amount  of  heat  that  is  available  when  the  fuel  is 
burned.  The  fuel  value  of  elementary  carbon  is  8080,  that  of 
hydrogen  is  about  34,500  and  that  of  sulphur  is  2162  calories  per 
gram,  if  it  is  understood  that  the  products  of  combustion  are 
cooled  to  ordinary  temperature  after  combustion.  This  assump- 
tion is  not  realized  in  practice  but  it  is  customary  to  make  the 
assumption  in  calculating  fuel  values.  There  is,  of  course,  no 
way  of  knowing  the  manner  in  which  the  elements  are  combined 
in  the  organic  compounds  making  up  the  coal  substances.  The 
arbitrary  assumption  is,  however,  made  that  all  oxygen  that  is 
already  contained  in  the  dry  coal  is  in  combination  with  hydrogen. 
One-eighth  of  the  percent  of  oxygen  would  then  be  the  percent 
of  hydrogen  not  available  as  fuel.  The  complete  statement  for 
calorific  value,  based  upon  these  assumptions,  is  given  in  the 
following  formula  of  Du  Long: 


34500   H-JO   +8080  C+2162  S 
q  =  —  1QQ  -  cal  per  gm, 

where  H,  O,  C  and  S  represent  percents  of  hydrogen,  oxygen, 
carbon  and  sulphur  respectively. 

However  this  formula  is  hardly  an  accurate  statement  of  avail- 
able heat.  Before  any  compound  can  be  burned  to  the  oxides  of 
the  constituent  elements  the  compound  must  be  dissociated  into 
its  elements.  This  change  will  involve  absorption  or  liberation 
(usually  absorption)  of  heat  energy  and  the  amount  of  energy 


316  QUANTITATIVE  ANALYSIS 

change  will  depend  upon  the  method  of  combustion.  This  is 
not  known  for  coal  and  the  correction  cannot  be  applied.  There 
is  also  a  fuel  value  (positive  or  negative)  for  the  mineral  matter 
contained  in  the  coal,  since  the  ash  left  upon  burning  is  not  the 
same  as  the  original  matter.  This  heat  must  also  be  omitted 
from  the  calculation  because  its  quantity  is  not  known. 

The  changes  taking  place  during  the  conversion  of  wood  into 
the  various  forms  of  coal  are  represented  by  the  following  approxi- 
mate figures  for  the  composition  of  the  combustible  portion, 
disregarding  small  quantities  of  elements  other  than  those  des- 
ignated, as  well  as  ash. 


Carbon 

Hydrogen 

Oxygen 

Wood                    

49 

6 

43 

Lignite                                  

70 

5 

25 

Bituminous  coal 

86 

4.5 

9.5 

Anthracite  coal  

95 

2 

3 

The  gradual  loss  of  volatile  matter  involves  a  relatively  large  loss 
of  oxygen.  This  fact  explains  the  steady  rise  in  fuel  value  as 
the  coals  progress  toward  the  anthracite,  although  the  decrease 
in  the  ratio  of  hydrogen  to  carbon  remaining  would  lead  one  to 
expect  a  fall  in  fuel  value. 

Calculation  of  Fuel  Value  from  the  Proximate  Analysis.— 
Many  attempts  have  been  made  to  devise  a  formula  that  will 
serve  for  calculating  fuel  value  from  the  results  of  the  proximate 
analysis.  Such  a  formula  must  necessarily  be  purely  empirical 
and  must  fail  in  many  cases  because  coals  of  practically  identical 
proximate  composition  may  vary  widely  in  ultimate  composition 
and  constitution.  The  chief  value  of  all  such  formulas  lies  in 
making  possible  an  approximate  valuation  of  the  fuel  where 
neither  the  ultimate  analysis  nor  calorimetric  determinations  can 
be  obtained.  Following  are  a  few  examples  of  these  formulas. 
The  results  of  such  calculations  must  be  used  with  great  caution. 

Formula  of  Haas: 

156.75  [100- (%  ash+%  S+%H2O)]+40.5X  %  S  = 
B.T.U.  per  Ib. 


FUELS 


317 


Formula  of  Goutal:1 

82  C  +  AM  =  cal  per  gm  when  C  =  %  fixed  carbon,  M  =  % 
volatile  combustible  matter  and  A  =  a  coefficient  whose  value  is 
fixed  by  the  volatile  combustible  matter  as  follows: 

M  =  2-15         15-30        30-35        35^0 
A  =130  100  95  90 

Formula  of  Gmelin: 

[100- (%  H20+%  ash)]  80-6CX%  H20  =  cal  per  gm, 

in  which  C  is  a  coefficient  which  varies  with  the  percent  of  mois- 
ture as  follows: 


Percent  moisture 

C 

<3 

-  4 

3-4.5 

+  6 

4.5-8.0 

+  12 

8.5-12.0 

+  10 

12-20 

+  8 

20-28 

+  6 

>28 

+  4 

If  fixed  carbon  is  calculated  upon  a  basis  of  true  coal,  dry  and 
ash  free,  the  following  table  may  be  used: 


FC 


B.T.U.  .per  Ib 


FC 


B.  T.  U.  per  Ib 


100 

14,500 

68 

15,480 

97 

14,760 

63 

15,120 

94 

15,120 

60 

14,580 

90 

15,480 

57 

14,040 

87 

15,660 

54 

13,320 

80 

15,840 

51 

12,600 

72 

15,660 

50 

12,240 

Determination  of  Fuel  Value  by  Means  of  the  Calorimeter.— 

The  best  laboratory  method  for  the  determination  of  fuel  value 
is  by'  the  use  of  one  of  the  standard  calorimeters.  Practically 
all  of  these  depend  upon  the  measurement  of  rise  in  tempera- 
ture of  water,  caused  by  the  combustion  of  fuel  within  a  closed 
vessel  or  "bomb"  immersed  in  the  water.  Combustion  is  best 


iCompt.  rend.,  135,  477  (1902). 


318  QUANTITATIVE  ANALYSIS 

effected  by  electrical  ignition  in  an  atmosphere  of  compressed 
oxygen  since  oxidation  is  complete  and  no  reactions  can  occur 
other  than  those  of  ideal  combustion.  The  original  bomb  calo- 
rimeter of  Berthelot1  has  been  improved  and  changed  in  such  a 
manner  as  to  make  it  a  practical  instrument  for  industrial  as  well 
as  for  purely  scientific  laboratories.  Successful  modifications  are 
those  of  Mahler2  and  Emerson.3  Parr4  has  also  perfected  a  fuel 
calorimeter  in  which  the  oxidizing  agent  is  sodium  peroxide.  The 
advantages  of  this  instrument  for  industrial  testing  are  chiefly 
due  to  the  fact  that  it  dispenses  with  the  use  of  compressed  oxygen 
and  much  of  the  accessory  apparatus  for  filling  the  bomb. 
Charging  and  firing  become  a  comparatively  simple  matter  and 
the  instrument  may  be  operated  by  persons  who  have  limited 
scientific  training.  The  great  fault  of  this  and  similar  instru- 
ments comes  from  the  reaction  of  the  products  of  combustion 
with  the  excess  of  sodium  peroxide  or  with  sodium  monoxide 
formed.  Such  reactions  as  the  following  occur: 

H2O+Na£0->2NaOH, 


Materials  in  the  ash  also  react  and  form  sodium  salts.  All  of 
these  reactions  involve  heat  liberation  or  absorption  and  this 
cannot  be  exactly  calculated  because  the  composition  of  the  coal 
is  never  exactly  known.  The  best  that  can  be  done  is  to  deter- 
mine experimentally  approximate  corrections  which  will  apply  to 
different  classes  of  coal.  The  sum  of  these  corrections  may  often 
be  as  large  as  10  percent  of  the  total  rise  in  temperature  during  a 
determination  and  the  uncertainty  is  so  great  as  to  render  the 
instrument  of  questionable  value  for  any  but  the  most  approxi- 
mate determinations.  The  instruments  using  compressed  oxy- 
gen, while  usually  more  expensive,  are  best  for  accurate  fuel 
testing,  even  for  the  works  laboratory. 

Fuel   Value.  —  If   a   calorimeter   is   available   the   heat  .units 
should  be   determined   experimentally.     Calculation  may  also 

1  Ann.  chim.  phys.,  [5]  23,  160  (1881);  [6]  10,  433  (1887). 

2  Chem.  Zentr.,  63,  889  (1892). 

a  J.  Ind.  Eng.  Chem.,  1,  17  (1909). 

*  J.  Am.  Chem.  Soc.,  22,  646  (1900);  29,  1606  (1907). 


FUELS  319 

be  made  from  the  analytical  results,  using  the  formulas  already 
given  as  well  as  others  that  have  been  proposed.  Comparison 
with  calorimetric  data  will  indicate  the  degree  of  usefulness  of  the 
formulas. 

Following  is  a  description  of  the  Emerson  calorimeter  and  also 
directions  for  making  the  determination  of  fuel  value. 

Bomb. — The  bomb  is  made  of  steel,  consisting  of  two  cups 
joined  by  means  of  a  heavy  steel  nut.  The  two  cups  are  machined 


FIG.  74. — Emerson's  calorimeter. 

at  their  contact  faces  with  a  tongue  and  groove,  the  joint  being 
made  tight  by  means  of  a  lead  gasket  inserted  in  the  groove.  The 
lining  is  of  sheet  nickel,  platinum  or  gold,  spun  in  to  fit.  The 
bomb  is  made  tight  with  a  milled  wrench  or  spanner.  The  pan 
holding  the  combustible  is  of  platinum  or  nickel,  and  the  support- 
ing wire  of  nickel. 

Calorimeter. — The  jacket  is  a  double  walled  copper  tank, 
the  space  between  the  walls  being  filled  with  water.  The  calo- 
rimeter can  is  made  as  light  as  is  possible,  of  sheet  brass,  nickel 
plated. 

Stirring  Device. — The  stirrer  is  directly  connected  to  a  small 
motor  and  is  enclosed  in  a  tube  to  facilitate  its  action  in  circulating 


320 


QUANTITATIVE  ANALYSIS 


the  water.     The  stirrer  is  mounted  on  a  post  on  the  calorimeter 
jacket  as  is  the  thermometer  holder. 


FIG.  75. — Section  of  Emerson  calorimeter. 

Ignition  Wire. — Unless  ignition  of  the  fuel  requires  a  very  high 
temperature  a  platinum  resistance  wire  is  suitable.  For  ignition 
of  such  substances  as  are  used  in  determining  the  water  equivalent 
of  the  calorimeter  (naphthalene,  cane  sugar,  etc.)  or  of  anthracite 


FUELS  321 

coal  an  iron  wire  is  more  certain  in  its  action  because  it  burns 
and  produces  a  higher  temperature.  When  iron  wire  is  used  a 
correction  of  1600  calories  per  gram  of  wire  is  subtracted  from 
the  total  calories  obtained  from  the  fuel  combustion.  This 
is  the  heat  of  oxidation  of  the  iron. 

Formation  of  Nitric  Acid. — When  coal  is  burned  in  air  prac- 
tically all  of  the  nitrogen  is  liberated  in  the  elementary  form. 
On  account  of  the  high  concentration  of  oxygen  in  the  calorimeter 
bomb  a  considerable  portion  of  the  nitrogen  is  oxidized  and  the 
products  dissolve  in  the  water  which  is  formed  by  the  combustion 
of  hydrogen.  A  dilute  solution  of  nitric  acid  is  thereby  formed. 
This  gives  rise  to  a  positive  error  in  the  observation  of  fuel  value, 
the  magnitude  of  the  error  depending  upon  the  extent  to  which 
nitric  acid  is  formed.  As  a  rule  the  error  is  small  and  it  may  be 
ignored  for  ordinary  fuel  testing  but  if  a  correction  is  to  be  made 
the  nitric  acid  is  titrated  by  standard  base,  at  the  end  of  the 
experiment. 

The  heat  of  formation  and  solution  of  nitric  acid  from  elemen- 
tary nitrogen  is  230  calories  per  gram.  It  is  convenient  to  use  a 
standard  solution  of  base,  1  cc  of  which  is  equivalent  to  5  calories. 
The  normality  of  such  a  solution  is 

=  0.3450  N. 


230X0.06302 

The  number  of  cubic  centimeters  of  base  required  to  titrate  the 
nitric  acid  in  the  bomb  after  the  combustion  is  multiplied  by  5, 
the  product  being  subtracted  from  the  observed  calories. 

Formation  of  Sulphuric  Acid. — A  similar  error  results  from  the 
formation  and  solution  of  sulphuric  acid.  In  ordinary  combus- 
tion of  coal,  organic  sulphur  as  well  as  the  sulphur  of  pyrites  is 
oxidized  to  sulphur  dioxide,  which  leaves  the  furnace  as  a  gas. 
On  the  other  hand,  in  the  calorimeter  some  sulphur  trioxide  is 
formed  and  this  dissolves  as  sulphuric  acid.  The  higher  degree 
of  oxidation  of  sulphur,  as  well  as  the  solution  of  the  acid  that  is 
formed,  yield  additional  heat  and  this  also  should  be  subtracted 
from  the  observed  calories. 

The  application  of  this  correction  is  not  so  simple  as  that  for 
oxidation  of  nitrogen.  A  determination  of  total  sulphur,  such 

as  is  ordinarily  made  in  the  analysis  of  coal,  does  not  give  any 
21 


322  QUANTITATIVE  ANALYSIS 

data  as  to  the  amount  existing  in  the  coal  as  inorganic  sulphates 
which,  obviously,  do  not  develop  heat  through  oxidation.  Be- 
sides, not  all  of  the  remainder  of  the  sulphur  is  oxidized  to  the 
highest  form.  As  the  correction  is  usually  small  it  is  scarcely 
advisable  to  attempt  any  calculations  upon  such  an  uncertain 
basis. 

Radiation. — Radiation  or  absorption  of  heat  by  the  calorim- 
eter may  be  avoided  by  making  the  calorimeter  "adiabatic. " 
This  may  be  done  in  a  number  of  ways,  three  of  which  will  be 
mentioned. 

(1)  The  water  in  the  surrounding  jacket  may  be  heated  by 
electrical  means,  so  as  to  keep  pace  with  the  rise  in  temperature 
of  the  calorimeter  water.     This  is  the  most  satisfactory  method, 
although   somewhat   complicated   and   expensive   apparatus   is 
required. 

(2)  The  water  in  the  jacket  may  be  warmed  by  chemical 
action.     By  Richards'  method  a  basic  solution  is  used  to  fill  the 
jacket  and  an  acid  is  run  in  from  a  burette  at  a  rate  which 
depends  upon  the  rate  of  change  in  temperature  of  the  calorim- 
eter water  and  upon  the  concentration  of  the  acid.     The  acid 
solution  may  be  standardized  in  terms  of  the  number  of  calories 
liberated  by  the  action  of  each  cubic  centimeter  upon  the  base,  in 
which  case  the  proper  rate  of  addition  is  more  easily  determined. 

(3)  The  jacket  of  the  calorimeter  may  be  evacuated,  on  the 
principle  of  the  Dewar  flask,  the  transfer  of  heat  outwardly  then 
being  limited  to  that  which  occurs  through  conductivity  of  the 
glass  of  the  jacket.     This  would  appear  to  be  the  least  trouble- 
some method  but  it  has  not  worked  well  in  practice. 

Radiation  Corrections. — If  adiabatic  conditions  cannot  be 
maintained  several  methods  for  making  radiation  corrections 
are  available. 

(1)  The  combustion  may  be  begun  as  far  below  atmospheric 
temperature  as  it  is  to  end  above  it.     By  this  means  absorption 
of  heat  in  the  first  half  of  the  experiment  would  appear  to  balance 
radiation  during  the  last  half. 

This  is  the  roughest  sort  of  approximation  and  it  would  not 
serve  for  ordinarily  accurate  work. 

(2)  The  rate  of  change  of  temperature  may  be  observed  for  a 
certain  period  before  firing  and  for  another  period  after  the 


FUELS  323 

calorimeter  water  has  absorbed  all  of  the  heat  from  the  bomb. 
The  average  of  these  rates  is  then  considered  to  be  the  mean  rate 
of  absorption  or  radiation  of  heat  for  the  entire  experiment  and 
if  this  is  multiplied  by  the  time  elapsing  between  the  firing  and 
the  maximum  absorption  the  net  gain  or  loss  during  the  entire 
observation  period  is  given. 

This  method  is  very  commonly  employed  and  it  gives  a  very 
close  approximation  to  the  true  correction. 

(3)  Observations  are  made  in  the  same  way  as  in  method  (2). 
In  addition  the  time,  a}  required  for  six-tenths  of  the  total  rise  in 
temperature  is  observed,  also  the  time,  6,  for  the  remaining  rise. 
Instead  of  averaging  the  two  radiation  (or  absorption)  rates  the 
preliminary  rate,  E\,  is  multiplied  by  a  and  the  final  rate,  Rz, 
by  b.     The  corrected  rise  is  then 

T+Ria+R2b, 

where  T  =  total  rise,  and  Ri  and  R 2  are  regarded  as  positive  for 
falling  temperatures  and  negative  for  rising  temperatures. 

The  observation  of  the  time,  a,  is  subject  to  some  uncertainty 
when  the  temperature  is  rising  rapidly  and  on  this  account 
the  method  is  not  so  easily  applied  as  is  method  (2).  It  will 
rarely  be  found  that  the  difference  between  the  corrections 
calculated  by  these  two  methods  will  differ  by  more  than  0.2 
percent  and  as  this  is  well  within  the  permissible  variation 
method  (2)  is  recommended  for  all  but  the  most  refined  work. 

(4)  The    Regnault-Pfaundler  method  approaches  theoretical 
accuracy   more  nearly  than  any  of  the  methods  already  de- 
scribed.    For  a  discussion  of  this  method,  see  White :  Gas  and 
Fuel  Analysis  (International  Chemical  Series)  page  224. 

Time -temperature  Curves. — Three  types  of  time-tempera- 
ture curves  are  produced,  according  to  whether  the  experiment  is 
(a)  begun  and  finished  below  room  temperature,  (6)  begun  below 
and  finished  above  or  (c)  begun  and  finished  above.  These 
types  are  illustrated  in  Fig.  76.  The  relative  slopes  of  the  ends 
of  the  curves  represent  RI  and  R2. 

It  will  be  observed  that  these  slopes  are  easily  determined  in 
curve  (6)  but  that  it  is  especially  difficult  to  decide  as  to  what 
temperature  should  be  taken  as  -the  maximum  produced  by  the 
fuel  combustion,  in  the  experiment  represented  by  curve  (a). 


324 


QUANTITATIVE  ANALYSIS 


Conditions  represented  by  curve  (6)  are  to  be  obtained  when 
possible. 

Determination:  Heat  of  Combustion  of  Solid  Fuels. — Place  the 
lower  hall  of  the  bomb  in  the  holder,  and  the  fuel  pan  in  the  wire  support, 
after  having  wired  the  fuse  wire  according  to  Fig.  75. 

Extend  the  wire  across  the  pan,  allowing  it  to  dip  sufficiently  to  be  in 
contact  with  the  fuel,  which  is  later  to  be  placed  in  the  pan.  The  wire 
must  in  no  case  touch  the  pan.  The  fuse  wire  should  be  placed  in  series 
with  two  32-c.p.  lamps  in  parallel  when  the  110- volt  power  circuit  is  used 
for  firing. 


Time,  Minutes 
FIG.  76. — Time-temperature  curves. 

The  fuel  used  is  sampled  and  powdered  according  to  directions  already 
given. 

Fill  a  weighing  bottle  with  the  prepared  sample,  and  weigh  accurately 
to  one-tenth  of  a  milligram.  Pour  from  this  into  the  pan  in  the  bomb, 
until  the  pan  is  approximately  half  full.  Weigh  the  bottle  again,  and 
the  difference  between  the  above  weighings  gives  the  net  quantity  of 
the  fuel  in  the  bomb.  This  weight  should  be  greater  than  0.5  gm  and 
not  more  than  1.2  gm.  For  hard  coal  the  maximum  charge  should  be 
not  greater  than  1  gm.  Hard  coal  should  not  be  as  finely  divided  as  soft 
coal. 

The  upper  half  of  the  bomb  is  now  placed  in  position  and  tbje  nut  is 
screwed  down  as  far  as  may  be  by  hand,  care  being  taken  not  to  cross 
the  threads.  The  shoulder  on  the  upper  half  of  the  bomb,  over  which 


FUELS  325 

the  nut  makes  bearing  contact,  should  be  lubricated  with  oil.    Extreme 
care  should  be  taken  that  no  oil  or  grease  is  deposited  on  the  lead  gasket. 

The  bomb  is  now  ready  to  be  filled  with  oxygen.  The  nipple  is 
coupled  to  the  oxygen  piping  by  means  of  the  attached  hand  union  and 
after  the  connection  of  the  bomb  to  the  oxygen  piping  is  accomplished 
the  hand  set  screw  on  the  table  is  tightened.  In  handling  the  bomb, 
care  should  be  taken  not  to  tip  or  jar  it,  as  fuel  may  be  thrown  from  the 
pan. 

The  spindle  valve  on  the  bomb  is  opened  one  turn  and  then  the  valve 
on  the  oxygen  supply  tank  is  very  cautiously  opened.  The  pressure 
gauge  should  be  carefully  watched  and  the  tank  valve  so  regulated 
that  the  pressure  in  the  system  shall  rise  very  gradually.  When  the 
pressure  reaches  300  Ib  per  square  inch,  the  tank  valve  is  closed  and 
the  spindle  valve  immediately  afterward.  The  bomb  should  be  im- 
mersed in  water  immediately  to  detect  any  possible  leaks.  The  bomb 
is  now  ready  for  the  calorimeter,  which  is  prepared  as  follows: 

1900  gm  of  distilled  water,  weighed  or  measured  in  a  calibrated  flask, 
is  placed  in  the  calorimeter  can  at  a  temperature  about  1.5°  below  the 
jacket  temperature  (which  should  be  in  the  proximity  of  the  room 
temperature).  The  bomb  is  then  placed  in  the  calorimeter  and  the 
stirrer  and  thermometer  are  lowered  into  position  as  indicated  by 
Fig.  75.  The  thermometer  is  immersed  about  3  inches  in  the  water. 
The  bulb  of  the  thermometer  should  not  touch  the  bomb. 

The  terminals  of  the  electric  circuit  used  for  firing  should  now  be 
attached.  Care  should  be  taken  that  neither  the  bomb  nor  the  stirrer 
touches  the  sides  of  the  can.  The  stirrer  is  now  started  and  allowed 
to  run  3  or  4  minutes  to  equalize  the  temperature  throughout  the 
calorimeter. 

Readings  of  the  thermometer  are  now  taken  for  5  minutes  (reading 
to  0.001°  or  0.002°  every  minute)  at  the  end  of  which  time  the  switch 
is  turned  on  for  an  instant  only,  which  will  be  found  sufficient  to  fire 
the  charge.  In  course  of  a  few  seconds  the  temperature  begins  to  rise 
rapidly  and  approximate  readings  are  taken  every  minute  until  the  rise 
becomes  slow,  more  accurate  readings  then  being  taken.  After  a 
maximum  temperature  is  reached  and  the  rate  of  change  of  temperature 
is  evidently  due  only  to  radiation  to  or  from  the  calorimeter,  the  readings 
are  continued  for  an  additional  5  minutes,  reading  every  minute.  These 
readings  before  the  firings  and  after  the  maximum  temperatures  are 
necessary  in  the  computation  of  the  cooling  correction.  The  time 
elapsed  from  the  time  of  firing  to  the  maximum  temperature  should  be, 
in  no  case,  more  than  6  minutes. 

When  through  with  the  run,  replace  the  bomb  in  the  holder  and  allow 
the  products  of  combustion  to  escape  through  the  valve  at  the  top  of 


326  QUANTITATIVE  ANALYSIS 

the  bomb.  Unscrew  the  large  nut  and  clean  the  interior  of  the  bomb. 
The  inside  of  the  nut  should  be  kept  greased,  also  the  threaded  part 
at  the  top  of  the  lower  cup. 

Immediately  after  each  run,  the  lining  of  the  bomb  should  be  washed 
out  with  a  cloth  moistened  with  a  dilute  solution  of  ammonium  hydrox- 
ide and  then  with  water.  When  the  apparatus,  after  using,  is  to 
be  left  for  several  hours  or  more  before  making  another  test,  the  linings 
should  be  removed  and  the  inner  surface  of  the  bomb  slightly  coated 
with  oil.  This  oil  under  the  linings  should  be  removed  when  next 
preparing  the  bomb  for  use,  as  an  excess  of  it  may  be  ignited  with  a 
possible  resulting  injury  to  the  linings. 

Heavy  Oils,  Coke,  Hard  Coal,  Etc. — The  determination  of  the  heat 
of  combustion  of  heavy  oils,  such  as  crude  petroleum,  and  also  of  coke 
and  extremely  hard  coals,  is  best  made  by  mixing  with  a  ready  burning 
combustible,  such  as  a  high-grade  bituminous  coal  or  pure  carbon. 
This  auxiliary  combustible  facilitates  the  complete  combustion  of  the 
whole  mixture  in  the  case  of  coke  and  hard  coal,  and  with  the  heavy  oil  it 
acts  as  a  holder  and  prevents  rapid  evaporation  of  the  oil.  The  auxiliary 
combustible  should  be  placed  at  the  bottom  of  the  pan  and  the  coke, 
coal  or  oil  sprinkled  over  it.  The  carbon  or  other  auxiliary  combus- 
tible should  be  dried  with  extreme  care  and  carefully  standardized  as  to 
the  resulting  rise  in  temperature  per  gram  in  the  calorimeter  when 
completely  burned. 

Calculation. — First  plot  a  smooth  curve,  using  temperatures 
as  ordinates  and  time  as  abscissas.  Use  only  the  straight  portions 
of  the  ends  of  the  graph  for  calculating  RI  and  E.2- 

The  difference  between  the  temperature  at  maximum  and  the 
temperature  at  firing  gives  directly  the  total  rise  in  temperature 
in  the  calorimeter.  To  this  rise  a  cooling  correction  must  be 
applied,  which  is  computed  as  follows: 

The  change  in  temperature  during  the  preliminary  5  minutes 
of  reading,  divided  by  the  time  (5  minutes)  gives  the  rate  of 
change  of  temperature  per  minute,  due  to  radiation  to  or  from 
the  calorimeter,  and  also  any  heating  due  to  stirring,  etc.  This 
factor  is  RI  and  in  like  manner  the  readings  taken  after  tempera- 
ture change  has  become  uniform  give  R2.  The  two  rates  of 
change  of  temperature  give  the  existing  conditions  in  the  calorim- 
eter at  the  start  and  at  the  finish  of  the  run.  The  algebraic 
signs  of  RI  and  R2  will  be  (+)  for  falling  temperatures  and  (  — ) 
for  rising  temperatures.  Therefore,  the  algebraic  sum  of  the  two 


FUELS  327 

rates,  divided  by  two,  will  give  the  mean  value  of  the  rate  of 
change  of  temperature  during  the  entire  run,  due  to  radiation  or 
absorption  by  the  calorimeter.     This  value  multiplied  by  the  time 
from  firing  to  maximum  will  give  the  total  cooling  correction. 
The  cooling  correction  is  thus  expressed  : 
•p    I  T> 

—X  time  from  firing  to  maximum  temperature. 
2 

This  quantity  is  either  added  to,  or  subtracted  from,  the  appa- 
rent rise  taken  from  the  data  of  the  run,  according  to  its 
sign. 

The  corrected  rise  of  temperature  divided  by  the  weight  of 
fuel  used,  will  give  directly  the  rise  per  gram  of  fuel. 

This  rise  per  gram  is  multiplied  by  the  weight  of  water  plus  the 
"water  equivalent."  This  figure  is  furnished  by  the  manufac- 
turers or  it  may  be  determined  by  use  of  a  standard  fuel,  as 
naphthalene  or  cane  sugar.  The  product  is  calories  per  gram  of 
fuel,  which  is  the  result  to  be  obtained.  The  result  in  calories 
per  gram  of  fuel,  multiplied  by  the  factor  1.8  gives  B.T.U.  per 
pound  of  fuel. 

The  final  expression  for  fuel  value  is  then 


-  =  cal  per  gm, 

where    T  =  total    rise   from   firing    temperature   to   maximum, 
S  =  gm  of  coal, 
e  =  water  equivalent  of  calorimeter, 

RI  and  R2  having  the  significance  already  mentioned.  In  using 
this  formula  it  must  always  be  remembered  that  RI  and  R2  are 
regarded  as  radiation  rates  and  that  if  the  temperature  is  rising 
they  must  be  given  negative  signs. 

Since  cal  per  gmX  1:8  =  B.T.U.  per  Ib,  if  the  latter  quantity 
is  desired  in  most  cases  the  product  (1900+e)X1.8  should  be 
calculated  at  the  beginning. 

Permissible  differences  in  duplicate  determinations: 

Same  analyst  Different  analysts 

0.3%  0.5% 


328  QUANTITATIVE  ANALYSIS 

GAS  MIXTURES 

The  separation  and  exact  determination  of  gases  may  be 
accomplished  by  using  various  gravimetric  and  volumetric 
methods.  Certain  gases  may  be  absorbed  in  suitable  reagents, 
the  absorption  product  being  precipitated  and  determined  gravi- 
metrically.  Examples  of  this  class  of  methods  have  been  met  in 
the  determination  of  the  halogens  (page  122).  Sulphur  dioxide 
may  be  absorbed  in  a  basic  solution,  oxidized  by  bromine  and 
precipitated  as  barium  sulphate.  Carbon  dioxide  has  already 
heen  determined  by  absorption  in  a  weighed  solution  of  potassium 
bydroxide.  Numerous  other  examples  will  suggest  themselves. 
On  the  other  hand  many  gases  can  be  absorbed  by  reagents 
in  which  they  can  be  determined  volumetrically.  For  example 
chlorine  may  be  absorbed  by  a  solution  of  potassium  iodide  and 
the  liberated  iodine  titrated  by  standard  sodium  thiosulphate 
(page  264) ;  carbon  dioxide  may  be  absorbed  in  a  standard  solu- 
tion of  a  base  and  the  solution  titrated  by  a  standard  acid  in 
presence  of  phenolphthalein ;  sulphur  dioxide  may  be  absorbed 
and  titrated  by  standard  iodine  solution,  etc. 

For  commercial  mixtures  of  gases  these  methods  are  not  often 
used  because  the  time  required  for  a  complete  analysis  is  too  long. 
The  analysis  of  such  mixtures  as  illuminating  gas,  natural  gas, 
producer  or  water  gas,  or  chimney  or  mine  gas  must  be  made  by 
more  rapid  methods  even  at  a  sacrifice  of  a  degree  of  accuracy. 
The  gases  from  a  measured  volume  of  the  original  mixture  are 
absorbed  in  suitable  reagents  and  the  volume  loss  is  measured. 
The  results  of  the  analysis  are  computed  in  percents  by  volume. 

A  standard  type  of  apparatus  for  gas  volumetry  and  one  that 
is  to  be  found  in  most  laboratories  is  that  of  Hempel. 

Gas  Burette. — The  gas  burette,  in  which  the  gas  mixture  is 
measured,  is  shown  in  Fig.  77.  The  measuring  tube  (a)  is  con- 
nected with  a  levelling  tube  (b),  the  gas  being  confined  over  water. 
In  making  a  reading  the  water  is  brought  to  the  same  level  in 
the  two  tubes  so  that  the  gas  is  measured  at  atmospheric  pressure. 
A  complete  analysis  may  usually  be  completed  in  a  time  suffi- 
ciently short  that  no  serious  error  is  caused  by  barometric 
changes.  Changes  in  temperature  during  the  course  of  an  analy- 
sis constitute  the  most  serious  sources  of  error.  To  make  the 


FUELS 


329 


method  even  commercially  accurate  great  care  must  be  exercised 
in  this  regard.  A  quiet  room  in  which  no  other  work  is  being 
performed  should  be  used.  The  operator  must  at  all  times  avoid 
touching  the  burette  or  levelling  tube  directly  with  the  hands  or 


1 


FIG.  77. — Gas  burette  with  levelling  tube. 

breathing  upon  them  more  than  is  necessary.  Sometimes  the 
burette  is  enclosed  in  a  water  jacket  to  guard  against  any  but 
very  slow  changes. 

The  burette  may  have  a  simple  rubber  tip  and  pinch  cock  at 


330 


QUANTITATIVE  ANALYSIS 


the  top  or  it  may  be  closed  by  a  glass  cock.  The  latter  is  desir- 
able but  is  liable  to  become  stuck  by  contact  with  basic  reagents. 
It  must  be  well  lubricated  and  frequently  used  or  loosened.  The 
glass  three-way  cock  at  the  bottom  of  the  tube  is  not  often  used 
and  is  not  required. 

Absorption  Pipette. — The  apparatus  in  which  the  gases  are 
absorbed  is  known  as  an  " absorption  pipette."  The  simplest 
form  of  the  Hempel  pipette  is  illustrated  in  Fig.  78.  The  reagent 
fills  the  lower  bulb,  the  bent  tube  and  the  capillary  tube.  The 
latter  is  connected  with  the  gas  burette  by  means  of  a  short  bent 
capillary  tube  and  when  the  gas  mixture  is  forced  into  the  pipette 


n 


FIG.  78. — Hempel's  simple 
absorption  pipette. 


FIG.  79. — Hempel's  double  or  "com- 
pound" absorption  pipette. 


the  absorbent  fills  the  upper  bulb.  Some  reagents  are  rapidly 
altered  and  rendered  inefficient  by  contact  with  air.  Protection 
from  such  action  is  afforded  by  the  compound  pipette  (Fig.  79), 
the  second  pair  of  bulbs  being  filled  with  water.  It  is  sometimes 
necessary  to  insert  solid  reagents,  such  as  sticks  of  yellow  phos- 
phorus, copper  wires  for  reducing  cupric  chloride,  etc.,  or  rolls  of 
iron  gauze  or  glass  tubes  for  giving  greater  absorbing  surface  to 
the  reagent.  A  pipette  for  solids  and  liquids  has  an  opening  at 
the  bottom  of  the  first  bulb  for  the  insertion  of  such  materials. 
(Fig.  80.) 

In  order  to  increase  the  rate  of  absorption  modifications  of  the 
original  pipette  have  been  introduced.     The  gas  is  caused  to 


FUELS 


331 


bubble  through  the  reagent  instead  of  being  forced  down  over 
the  latter.  After  absorption  has  been  completed  the  remaining 
gas  is  drawn  from  the  top  by  turning  the  three-way  cock  to  com- 
municate with  the  upper  part  of  the  bulb.  (See  Fig.  81.) 

When  transferring  gases  from  the  burette  to  the  pipette  it  is 
necessary  to  avoid  mixing  the  water  of  the  burette  with  the 
reagent  in  the  pipette  because  the  latter 
is  thereby  diluted.  It  is  still  more  im- 
portant that  the  entrance  of  reagents 
into  the  burette  should  be  prevented 
because  such  contamination  of  the  water 
would  cause  premature  absorption  of 
gases.  In  order  that  such  mixing  may 


FIG.  80. — Hempel's  double  pipette,  modified 
to  admit  solids. 


FIG.  81. — Bubbling  absorp- 
tion pipette. 


be  avoided  it  is  necessary  that  there  be  a  neutral  zone  in  the  con- 
necting tubes,  into  which  neither  water  nor  reagent  shall  enter. 
If  this  part  of  the  tube  has  any  but  a  very  small  capacity 
there  will  be  an  appreciable  error,  due  to  the  gas  that  is  left  in 
the  tube  each  time.  For  this  reason  the  connecting  tubes  are  of 
capillary  dimensions. 

One  of  the  most  serious  disadvantages  in  the  use  of  Hempel 
pipettes  comes  from  the  necessity  for  connecting  and  discon- 
necting each  pipette  in  turn  as  the  different  gases  are  absorbed. 
To  obviate  this  inconvenience  many  modifications  have  been 


332 


QUANTITATIVE  ANALYSIS 


made  in  the  direction  of  a  composite  apparatus  that  does  riot 
require  the  interchange.  The  most  important  feature  of  such 
forms  of  apparatus  is  a  permanent  connection  of  the  burette 
with  the  several  absorption  pipettes,  communication  being  estab- 
lished with  each  in  turn  by  special  forms  of  stop  cocks.  This 
usually  involves  the  use  of  longer  capillary  tubes  and  this  in- 
creases the  error  already  mentioned  as  inherent  in  connecting 
tubes. 

In  apparatus  designed  for  the  analysis  of  chimney  gases  the 

feature  of  permanent  connection 
must  be  combined  with  porta- 
bility because  the  analysis  must 
usually  be  conducted  at  the 
plant.  A  modification  of  the 
Orsat  apparatus  is  here  illus- 
trated (Fig.  82). 

Solubility  of  Gases  in  Rea- 
gents.— When  water  is  used  as 
the  confining  liquid  in  the  gas 
burette  and  water  solutions  are 
used  as  absorbents  in  the  absorp- 
tion pipettes  it  is  impossible  to 
avoid  small  errors,  due  to  the 
solubility  of  the  components  of 
the  gas  mixture  in  water.  If 
the  gases  are  taken  into  a 
burette  containing  pure  water, 
each  gas  dissolves  and  the 
volume  is  diminished  after  the 
total  volume  has  been  read.  In 
order  to  avoid  the  disappear- 
ance of  a  part  of  the  gases  in  this 

way,  the  water  must  have  been  previously  saturated  by  allowing 
the  gas  to  bubble  through  it.  This  does  not  entirely  obviate  the 
error  because,  as  the  mixture  is  drawn  back  into  the  burette  for 
measurement  after  the  removal  of  each  constituent,  the  partial 
pressure  of  that  constituent  being  reduced  to  zero,  a  part  passes 
out  of  the  solution  in  the  burette  and  mixes  with  the  remaining 
gases,  the  total  observed  volume  being  rendered  too  large.  To 


FIG.  82. — Orsat's   apparatus   (modi 
fied)  for  analysis  of  chimney  gases. 


FUELS  333 

illustrate  this  action,  suppose  that  a  mixture  of  oxygen,  carbon 
dioxide  and  carbon  monoxide  is  being  analyzed.  The  water  in  the 
burette  is  first  saturated  with  the  mixture  but  the  amount  of  each 
dissolved  is  a  function  of  its  partial  pressure  (concentration)  in 
the  mixture.  The  measured  gases  are  passed  into  a  pipette  con- 
taining potassium  hydroxide  where  the  carbon  dioxide  is  com- 
pletely absorbed,  its  partial  pressure  in  the  gases  being  reduced 
to  (practically)  zero.  Upon  passing  the  mixture  of  carbon 
monoxide  and  oxygen  back  into  the  burette  a  certain  amount 
of  dissolved  carbon  dioxide  will  be  given  up  by  the  water  and 
the  volume  will  be  somewhat  larger  than  the  sum  of  the  volumes 
of  the  other  two  gases.  Also  where  the  mixture  was  confined 
over  potassium  hydroxide  solution  the  latter  dissolved  small 
amounts  of  carbon  monoxide  and  oxygen  and  some  of  these 
gases  may  be  given  up  to  mixtures  later  being  analyzed.  The 
calculation  of  the  amount  of  error  may  thus  become  a  compli- 
cated matter.  The  error  is  negligible,  from  the  industrial  stand- 
point, if  the  analysis  is  completed  within  a  short  period  of  time. 

Fuel  and  Lighting  Gases. — In  illuminating  gas  the  following 
constituents  are  determined:  carbon  dioxide,  ethylene  and  its 
homologues,  oxygen,  carbon  monoxide,  hydrogen,  methane  and 
nitrogen.  They  are  absorbed,  in  the  order  named,  one  after 
another,  and  the  contraction  in  volume  noted  after  each  absorp- 
tion. Hydrogen  and  methane  are  determined  by  combustion 
and  nitrogen  is  computed  by  subtracting  the  sum  of  the  other 
gases  from  100.  The  various  absorbents  for  these  gases  will  be 
discussed. 

Carbon  Dioxide. — A  solution  of  any  of  the  strong  bases  may 
be  used  for  absorbing  carbon  dioxide.  Potassium  hydroxide 
possesses  the  advantage  of  large  solubility  and  rapid  absorption 
of  gas  and  is  almost  always  used  for  this  purpose  in  gas  analysis. 
Also  potassium  carbonate,  formed  by  absorption  of  carbon  diox- 
ide in  potassium  hydroxide,  is  more  soluble  in  the  basic  solution 
than  is  sodium  carbonate.  A  solution  made  by  dissolving  solid 
potassium  hydroxide  in  twice'  its  weight  of  water  (about  33  per- 
cent, by  weight)  is  suitable,  this  being  the  same  strength  as  that 
employed  for  gravimetric  determinations.  100  cc  of  a  33  per- 
cent solution  will  absorb  about  four  liters  of  carbon  dioxide 
before  it  becomes  inefficient.  The  potassium  hydroxide  used 


334  QUANTITATIVE  ANALYSIS 

should  not  be  that  which  has  been  purified  from  alcohol  solu- 
tion, because  traces  of  alcohol  are  retained  in  the  solid  base  and 
alcohol  vapor  or  other  organic  vapors  are  given  up  to  the  gas. 
The  solution  may  be  used  in  either  the  single  or  double  Hempel 
pipette  or  in  any  of  the  modified  pipettes.  If  the  Hempel  pipette 
is  used  it  should  contain  rolls  of  iron  gauze  in  order  to  increase 
the  surface  of  solution  exposed.  As  the  solution  is  forced  down, 
leaving  the  gauze  exposed,  the  film  of  solution  retained  upon  the 
surface  of  the  wires  greatly  increases  the  rate  of  absorption. 

Hydrogen  sulphide  will  be  included  in  the  fraction  absorbed 
by  potassium  hydroxide  unless  it  has  been  otherwise  removed. 
Its  quantity  is  usually  small. 

Carbon  Monoxide. — The  most  conveniently  used  absorbent 
for  carbon  monoxide  is  a  solution  of  cuprous  chloride.  This 
salt  is  only  slightly  soluble  in  water  and  must  be  dissolved  in 
either  hydrochloric  acid  or  ammonium  hydroxide.  Either  solu- 
tion absorbs  carbon  monoxide  with  the  formation  of  a  rather 
unstable  compound  whose  exact  nature  is  unknown.  The  acid 
solution  is  made  as  follows:  Mix  86  gm  of  cupric  oxide  and  17 
gm  of  finely  divided  copper  and  slowly  add  to  1000  cc  of  a  mix- 
ture of  equal  volumes  of  concentrated  hydrochloric  acid  and 
water.  Stir  until  the  solid  matter  has  dissolved,  then  place  in 
bottles  having  bundles  of  copper  wire  reaching  from  top  to  bot- 
tom. Stopper  the  bottles  and  allow  to  stand  until  colorless. 
Cupric  chloride,  formed  by  dissolving  cupric  oxide  in  hydro- 
chloric acid,  is  reduced  by  copper  to  cuprous  chloride : 

CuO+2HCl^CuCl2+H20, 
CuCl2+Cu-»2CuCl. 

100  cc  of  this  solution  will  efficiently  absorb  about  400  cc  of 
carbon  monoxide.  Absorption  takes  place  slowly  and  the  gas 
must  be  shaken  with  the  solution  for  some  time  or  be  allowed 
to  bubble  through  it.  The  -double  pipette  must  be  used  because 
cuprous  chloride  is  readily  oxidized  in  contact  with  air,  cupric 
chloride  being  formed. 

Oxygen. — Oxygen  is  absorbed  by  a  solution  of  potassium 
pyrogallate  or  by  yellow  phosphorus.  The  former  solution  is 
prepared  by  dissolving  120  gm  of  potassium  hydroxide  in  80  cc 


FUELS 


335 


of  water,  cooling  and  placing  in  the  double  absorption  pipette 
then  adding  15  cc  of  a  25  percent  solution  of  pyrogallic  acid  and 
mixing.  Potassium  hydroxide  purified  by  alcohol  should  not  be 
used.  The  solution  will  readily  absorb  about  200  cc  of  oxygen 
for  each  100  cc  of  solution.  It  does  not  act  rapidly  at  tempera- 
tures below  15°. 

Yellow  phosphorus  may  be  used  in  the  form  of  sticks  which  are 
placed  in  the  double  pipette  for  solids  and  liquids  and  kept 
covered  with  water.  This  absorbent  possesses  the  great  advan- 
tage of  retaining  its  capacity  for  absorbing  oxygen  until  the  sticks 
have  become  completely  used  up.  The  product  of  the  union  of 
phosphorus  and  oxygen,  phosphorus  pentoxide,  dissolves  in 
water  so  that  the  surface  of  the  sticks  is  always  fresh.  Absorp- 
tion becomes  slow  below  15°,  and  traces  of  unsaturated  hydrocar- 
bons of  the  ethylene  series  partially  inhibit  the  absorption.  For 
the  latter  reason  phosphorus  is  not  suitable  for  use  in  those 
forms  of  assembled  apparatus  for  the  analysis 
of  chimney  gases  in  which  the  ethylene  hydro- 
carbons are  not  determined  at  all. 

Ethyleue  and  Its  Homologues. — These  gases 
give  higher  illuminating  power  to  the  mixture 
of  methane,  hydrogen  and  carbon  monoxide, 
gases  which  burn  with  a  non-luminous  flame. 
For  this  reason  they  are  collectively  known  as 
"illuminants."  Fuming  sulphuric  acid  or 
bromine  water  may  be  used.  Fuming  sul- 
phuric acid  reacts  with  members  of  the 
ethylene  series  of  hydrocarbons,  forming 
addition  products  as  well  as  condensation 
products.  These  are  either  liquids  or  soluble 
solids  and  are  therefore  removed  from  the 
gas  mixture.  The  absorption  is  not  rapid  and  the  acid  should 
be  shaken  with  the  gas  if  the  Hempel  pipette,  or  one  similar  to 
it,  is  used.  The  single  pipette  is  used,  since  a  water  seal  in  the 
second  bulbs  is  inadmissible.  .  In  order  to  increase  the  contact 
of  gas  with  acid  the  pipette  contains  a  third  small  bulb  which 
is  filled  with  glass  beads.  Contact  of  the  acid  with  rubber 
connections  must  be  avoided.  100  cc  of  fuming  sulphuric  acid 
will  absorb  about  800  cc  of  ethylene. 


FIG.  83. — Fuming 
sulphuric  acid  pi- 
pette for  unsatu- 
rated hydrocarbons. 
Gill's  modification. 


336 


QUANTITATIVE  ANALYSIS 


Bromine  water  absorbs  ethylene  and  its  homologues  with 
formation  of  bromine  addition  compounds: 

C2H4+Br2-+C2H4Br2. 

Tt  is  somewhat  more  convenient  to  use  than  fuming  sulphuric 
acid  but  does  not  absorb  with  great  readiness.  If  excess  of 
bromine  is  placed  in  the  pipette  the  absorbing  power  is  undi- 
minished  until  all  of  this  bromine  has  been  dissolved. 

Hydrocarbon  Vapors. — Gases  formed  by  distilling  coal  often 
contain  vapors  of  liquid  hydrocarbons,  chiefly  benzene.  These 
are  partly  absorbed  by  fuming  sulphuric  acid  but  may  not  be 
entirely  removed.  They  may  be  absorbed  in  absolute  alcohol 
and  so  determined.  The  absorbing  power  of  absolute  alcohol  is 
not  large  and  gases  coming  from  the  ordinary  gas  burette,  being 
saturated  with  water  vapor,  soon  diminish  the  efficieticy  of  the 

alcohol  by  imparting  moisture 
to  it.  Dennis  and  O'Neill 
suggested1  the  use  of  a  solu- 
tion of  nickel  sulphate  in  am- 
monium hydroxide.  Neither 
this  solution  nor  absolute 
alcohol  is  an  entirely  satis- 
factory absorbent.  The  de- 
termination of  hydrocarbon 
vapors  is  frequently  omitted, 
these  vapors  then  being  ab- 
sorbed along  with  unsaturated 
hydrocarbons. 

Hydrogen. — The     determi- 
nation of  hydrogen  is  made 

by  burning  with  oxygen,  measuring  the  resulting  contraction  in 
volume,  or  by  absorption  in  palladium.  The  combustion  may  be 
carried  out  by  exploding  the  mixture  of  hydrogen  and  oxygen 
over  mercury  in  a  suitable  pipette  or  the  burning  may  be  made 
to  proceed  more  slowly. 

If  a  mixture  of  hydrogen  with  an  excess  of  oxygen  or  air  is 
burned  the  resulting  water  vapor  condenses  and  only  the  excess 
of  oxygen  or  air  remains  as  gas.  From  the  equation 

2H2+O2-»2H20 
1  J.  Am.  Chem.  Soc.,  26,  503  (1903). 


FIG.  84. — Hempel's  explosion  pipette. 


FUELS 


337 


it  is  seen  that  two-thirds  of  the  volume  of  the  disappearing  gas 
is  that  of  hydrogen.  Therefore,  two-thirds  of  the  contraction 
measured  after  cooling  equals  the  volume  of  hydrogen. 

The  explosion  pipette  is  shown  in  Fig.  84.  The  confining 
liquid  should  not  be  water  since  larger  quantities  of  gases  will 
dissolve  in  it  at  the  moment  of  explosion  because  of  the  momen- 
tary increase  in  pressure.  Mercury  is  substituted  for  water  and 
the  pipette  is  so  constructed  as  to  permit  altering  at  will  the 
difference  in  level  between  the  mercury  in  the  two  bulbs.  If 
the  ordinary  single  pipette  were  used  it  would  be  impossible  to 
force  gas  into  the  pipette  because  of  the  great  density  of  mercury 
and  the  consequent  back  pressure.  Ignition  is  effected  by  con- 
necting with  the  secondary  of  an  induction  coil. 


FIG.  85. — Pipette  for  the  preparation 
of  hydrogen. 


FIG.  86. — Pipette  for  slow 
combustion. 


If  pure  hydrogen  is  mixed  with  pure  oxygen  and  burned  the 
explosion  is  too  violent  for  safety.  If  the  gas  to  be  burned  is 
rich  in  hydrogen  it  is  mixed  with  air  instead  of  with  oxygen. 
One  volume  of  hydrogen  requires  more  than  two  and  one-half 
volumes  of  air  for  complete  combustion,  allowing  a  small  excess. 
Dilution  with  air  is  not  necessary  if  the  residual  gas  is  poor  in 
combustibles.  On  the  other  hand  it  may  be  necessary  to  enrich 
the  gas,  before  burning,  by  adding  a  measured  volume  of  pure 
hydrogen.  This  is  conveniently  generated  from  zinc  and  sul- 
phuric acid  in  a  special  pipette  (Fig.  85). 

Combustion  may  also  be  effected  by  passing  the  mixture  with 
oxygen  through  a  heated  capillary  tube  or  by  exposing  the  mix- 
22 


338  QUANTITATIVE  ANALYSIS 

tore  to  a  glowing  platinum  wire  in  the  pipette  arranged  for 
slow  combustion  (Fig.  86).  In  using  this  pipette  either  of  two 
methods  of  procedure  may  be  followed :  The  hydrogen  is  placed 
in  the  pipette,  the  wire  made  to  glow  by  the  passage  of  a  current 
and  a  measured  volume  of  oxygen  led  in,  or  the  hydrogen  and 
oxygen  are  mixed  in  the  burette  and  slowly  brought  into  the 
pipette,  in  which  the  wire  is  glowing.  In  either  case  combustion 
occurs  without  explosion. 

Hydrogen  may  be  separated  from  nitrogen  and  methane  by 
absorption  in  palladium  sponge  which  has  been  superficially 
coated  with  oxide.  Absorption  readily  takes  place  at  100°  and 
the  hydrogen  may  be  later  removed  by  passing  oxygen  through 


FIG.  87. — Palladium  tube. 

the  palladium,  the  hydrogen  being  thereby  oxidized  and  palla- 
dium oxide  again  formed  on  the  surface.  A  tube  of  the  form 
shown  in  Fig.  87  is  used.  The  enlarged  part  is  filled  with  asbestos 
which  has  been  coated  with  spongy  palladium  and  the  tube  is 
connected  directly  with  the  burette  at  one  side  and  with  a 
pipette  filled  with  water  at  the  other.  Upon  passing  the  gases 
through  two  or  three  times  the  hydrogen  is  quantitatively  ab- 
sorbed, a  small  amount  being  oxidized  by  the  trace  of  palladious 
oxide,  and  a  certain  amount  is  also  burned  by  oxygen  of  air 
which  was  already  in  the  tube.  Except  for  this  small  amount 
of  oxygen,  the  shrinkage  in  volume  gives  directly  the  volume  of 
hydrogen.  The  amount  of  air  in  the  tube  must  be  known.  This 
may  be  determined  by  connecting  with  the  gas  burette  and 
measuring  the  expansion  between  two  temperatures.  One- 
fifth  of  the  total  volume  is  taken  as  the  contraction  due  to  con- 
tained oxygen. 

The  absorption  of  hydrogen  by  palladium  is  hindered  by  traces 
of  hydrochloric  acid.     On  this  account  the  ammoniacal  solution 


FUELS  339 

of  cuprous  chloride  should  be  used  for  the  absorption  of  carbon 
monoxide  if  this  is  to  be  followed  by  palladium  absorption  of 
hydrogen. 

The  explosion  pipette  gives  fairly  accurate  results  and  is  not" 
difficult  to  manipulate  but  requires  a  battery  and  an  induction 
coil.  It  is  subject  to  the  disadvantage  that  only  a  small  amount 
of  gas  may  be  used,  on  account  of  the  relatively  large  volume  of 
air  that  must  be  mixed  with  it  in  the  pipette,  so  that  the  error  in 
reading  volumes  is  relatively  large.  Gill  has  devised  a  pipette1 
which  overcomes  this  objection.  The  bulb  in  which  the  explosion 
is  to  take  place  is  large  enough  to  hold  the  entire  residue  from  100 
cc  of  gas,  together  with  the  necessary  oxygen  for  the  combustion, 
and  is  made  of  quite  heavy  glass.  Both  the  slow  combustion 
pipette  and  the  palladium  tube  permit  the  use  of  larger  quantities 
of  gas. 

Methane. — Methane  is  determined  by  combustion,  the  pro- 
cedure being  the  same  as  for  hydrogen.  In  the  analysis  of 
natural  gas  and  illuminating  gas,  as  well  as  many  other  com- 
mercial gas  mixtures,  hydrogen  and  methane  will  both  occur  in 
the  residue  after  other  gases  have  been  absorbed.  They  must 
therefore  be  burned  together  unless  hydrogen  is  to  be  absorbed 
by  palladium.  According  to  the  equation 

CH4+202-+C02+2H20 

one  volume  of  methane  with  two  volumes  of  oxygen  will  produce 
one  volume  of  carbon  dioxide,  the  rest  of  the  oxygen  disappear- 
ing as  condensed  water  vapor.  The  contraction  is  therefore 
twice  the  volume  of  the  methane.  Since  a  volume  of  carbon 
dioxide  equal  to  that  of  the  methane  is  produced  a  measurement 
of  the  former  by  absorption  in  potassium  hydroxide  will  give  a 
direct  determination  of  the  volume  of  methane.  For  the  residue 
of  hydrogen  and  methane,  therefore,  the  procedure  is  as  follows: 
An  excess  of  air  or  oxygen  is  mixed  with  the  gases  and  the  mix- 
ture exploded.  The  gases  are  cooled  and  measured  in  the 
burette,  the  contraction  being  noted.  Carbon  dioxide  is  then 
determined  by  absorption  in  potassium  hydroxide.  The  volume 
of  carbon  dioxide  is  equal  to  the  volume  of  methane.  Twice  this 

1  J.  Am.  Chem.  Soc.,  17,  771  (1895). 


340  QUANTITATIVE  ANALYSIS 

volume  is  the  contraction  due  to  the  combustion  of  methane. 
This  contraction  subtracted  from  the  total  contraction  leaves 
the  contraction  due  to  the  combustion  of  hydrogen.  Two- 
thirds  of  this  contraction  is  equal  to  the  volume  of  hydrogen. 
The  volumes  of  hydrogen  and  methane  so  determined,  multi- 
plied by  the  ratio  of  the  total  residue  to  the  volume  taken  for 
explosion,  gives  the  volumes  of  hydrogen  and  methane  in  the 
original  gas.  The  following  example  will  illustrate  the  calcula- 
tions involved: 

100  cc  of  illuminating  gas  gave,  after  all  absorbable  gases  had 
been  removed,  65.2  cc  of  residue,  this  consisting  of  hydrogen, 
methane  and  nitrogen.  15  cc  of  the  residue  was  mixed  with  air, 
the  total  volume  then  being  90.5  cc.  After  explosion  the  volume 
was  71.0  cc.  Carbon  dioxide  was  absorbed,  the  volume  of  the 
remaining  gases  being  then  66.6  cc. 

Volume  methane  =  volume  carbon  dioxide  =  71. 0—66.6  = 

4.4  cc. 
Contraction  due  to   combustion  of  methane  =  2X4.4  = 

8.8  cc. 

Total  contraction  =  90.5 -71. 0=19.5  cc. 
Contraction  due  to  combustion  of  hydrogen  =  19.5  —  8.8  = 

10.7  cc. 

2 

Volume  of  hydrogen  =  ^X  10.7  =  7.1  cc. 
o 

65  2 

Volume  of  methane  in  original  gas  =        '    X4.4  =  19.1  cc. 

lo 

(\  Pv    O 

Volume  of  hydrogen  in  original  gas  =   ~^-X 7. 1  =  30.9  cc. 

lo 

(  volume  of  sample 
—  sum  of  volumei 
of  all  other  gases. 

Since  100  cc  of  gas  was  taken  for  analysis,  the  volume  of -the  con- 
stituents will  also  be  their  percents  by  volume. 

Analysis  of  Illuminating  Gas. — For  this  exercise  the  Hempel  appara- 
tus may  be  used  or  any  of  the  modified  pipettes  or  burettes  may  be 
substituted.  The  method  of  manipulation  is  not  essentially  different 
for  the  different  forms  of  apparatus  except  in  minor  details  and  such 
variations  will  readily  suggest  themselves.  Throughout  the  analysis 
avoid  touching  the  body  of  the  burette  or  the  bulbs  of  the  pipettes  with 


Volume  of  nitrogen  in  original  gas  =  I  —  sum  of  volumes 


FUELS 


341 


the  hands,  or  breathing  upon  them.  Allow  the  sides  of  the  burette  to 
drain  thoroughly  each  time  before  reading. 

Prepare  water  for  the  gas  burette  by  allowing  the  gas  to  bubble 
through  it  for  ten  minutes.  Fill  the  burette  with  this  water,  raise  the 
levelling  tube  until  the  water  flows  out  of  the  top  of  the  burette,  then 
close  the  upper  cock.  Place  a  rubber  tube  on  a  gas  cock  and  allow 
gas  to  escape  through  it  until  all  air  is  displaced.  With  the  gas  still 
running  connect  the  tube  with  the  top  of  the  burette,  open  the  burette 
cock  and  fill  with  gas  until  the  100  cc  mark  has  been  passed.  Close 
the  upper  cock  and  detach  from  the  gas  supply.  It  is  desirable  that  ex- 
actly 100  cc  of  gas  be  taken,  measured  at  the  prevailing  pressure  of  the 
atmosphere.  In  order  to  do  this  allow  the  water  to  drain  down  the  sides 
of  the  burette  for  1  minute  then  raise  the  levelling  tube,  compressing 
the  gas  until  the  100  cc  mark  is  exactly  reached.  Now  close  the  cock 
at  the  bottom  of  the  burette  or  close  a  pinch  cock  which  is  placed  on  the 
rubber  connecting  tube.  Open  the  upper  cock  momentarily  and  close 
again.  This  permits  gas  to  escape  until  the  pressure  within  the  burette 
is  the  same  as  that  of  the  atmosphere. 

Hydrocarbon  Vapors. — Place  the  pipette  filled  with  absolute  alcohol 
on  the  stand  by  the  burette  and  connect  with  the  burette  by  a  bent 
capillary  tube,  having  previously  caused  the  alcohol  to  fill  the  lower  bulb 
and  the  capillary  up  to  a  point  near  the  top.  Force  the  gas  into  the 
pipette,  detach  the  latter  and  shake  for  1  minute.  Return  the  gas 
to  the  burette,  allow  the  water  to  drain  down  the  sides,  adjust  the 
levelling  tube  to  provide  atmospheric  pressure  and  measure.  Record 
the  difference  as  "hydrocarbon  vapors." 

Carbon  Dioxide  (and  Hydrogen  Sulphide). — Attach  the  burette  to  the 
pipette  containing  potassium  hydroxide  solution,  pass  the  gas  into  the 
pipette  and  directly  back  again.  Measure  and  record  the  percent  of 
carbon  dioxide  (including  also  hydrogen  sulphide  if  present). 

Illuminants. — Determine  illuminants  by  absorption  in  fuming  sul- 
phuric acid  or  bromine  water,  drawing  back  to  the  burette  at  once. 
Avoid  the  entrance  of  any  water  into  the  fuming  sulphuric  acid. 

Oxygen. — Absorb  oxygen  by  yellow  phosphorus,  allowing  three 
minutes,  or  by  potassium  pyrogallate,  shaking  the  pipette  for  three 
minutes.-  If  pyrogallate  is  used  in  a  pipette  containing  rolls  of  iron 
gauze  the  shaking  may  be  omitted. 

Carbon  Monoxide. — Absorb  carbon  monoxide  in  either  acid  or  basic 
solution  of  cuprous  chloride.  The  gas  should  be  shaken  with  the  cu- 
prous chloride  solution  for  three  minutes,  then  passed  into  the  pipette 
containing  potassium  hydroxide  to  absorb  vapors  of  hydrochloric  acid. 

Hydrogen,  Methane  and  Nitrogen. — Pass  all  of  the  gas  residue  into  the 
cuprous  chloride  pipette  for  storage,  pour  out  the  water  from  the 


342  QUANTITATIVE  ANALYSIS 

burette  and  replace  with  water  that  has  been  saturated  with  air.  Deter- 
mine hydrogen  and  methane  by  one  of  the  following  described  methods. 

Combustion  by  Explosion. — Return  10  to  12  cc  of  the  gas  to  the 
burette,  measure  accurately,  then  draw  air  into  the  burette  until  a 
total  volume  of  nearly  100  cc  is  obtained.  Do  not  attempt  to  obtain 
exactly  100  cc  as  there  is  danger  of  loss  of  gas  during  the  adjustment  of 
volume.  Measure,  then  transfer  the  mixture  of  air  and  gas  to  the  ex- 
plosion pipette,  allowing  water  from  the  burette  to  enter  and  fill  the 
capillary  of  the  explosion  pipette.  Close  the  rubber  connecting  tube 
(which  should  have  thick  walls  and  be  securely  wired  in  place)  with  a 
screw  clamp.  Place  the  mercury  reservoir  bulb  so  that  the  mercury 
is  at  the  same  level  as  inside  the  explosion  bulb,  then  connect  the  ter- 
minal wires  with  the  secondary  of  an  induction  coil  and  cause  a  spark  to 
pass.  A  flash  will  pass  across  the  bulb  and  mercury  will  almost  imme- 
diately begin  to  flow  into  the  bulb,  on  account  of  the  contraction  of  gas 
volume  resulting  from  the  combustion.  At  all  times  when  the  gas  is 
in  the  explosion  pipette  the  mercury  must  be  so  adjusted  in  level  that  a 
pressure  much  greater  or  less  than  that  of  the  atmosphere  is  avoided. 
Return  the  gas  to  the  burette,  allow  the  water  to  drain  down  the  sides, 
then  measure.  Absorb  the  carbon  dioxide  and  remeasure.  In  order 
to  be  sure  that  an  excess  of  oxygen  was  present  the  gas  should  be  passed 
into  the  phosphorus  or  pyrogallate  pipette.  If  no  oxygen  is  found 
the  explosion  must  be  repeated  with  another  sample  of  gas,  using  a 
larger  proportion  of  air.  Calculate  the  percents  of  hydrogen,  methane 
and  nitrogen  by  the  method  already  discussed.  Repeat  with  another 
portion  of  the  residue  in  the  cuprous  chloride  pipette. 

Slow  Combustion. — Use  the  pipette  shown  in  Fig.  86.  Measure 
about  half  of  the  residue  which  is  stored  in  the  cuprous  chloride  pipette 
and  transfer  this  to  the  combustion  pipette.  If  the  residue  is  known 
to  be  chiefly  methane  not  more  than  25  cc  should  be  used.  If  it  is 
chiefly  hydrogen  more  may  be  taken  since  hydrogen  requires  for  com- 
bustion only  half  its  own  volume  of  oxygen.  Fill  the  burette  with  pure 
oxygen,  and  measure  accurately.  Connect  the  terminals  of  the  plati- 
num wire  of  the  pipette  with  a  current  source  and  heat  the  coil  to  bright 
redness.  Pass  the  oxygen  into  the  combustion  pipette  but  not  so  rapidly 
as  to  cause  an  explosion.  When  the  combustion  is  completed  transfer 
the  entire  gas  mixture  to  the  burette  and  record  the  volume  and  con- 
traction. Determine  carbon  dioxide,  test  for  excess  of  oxygen  and 
calculate  exactly  as  in  the  case  of  explosion. 

Absorption  of  Hydrogen  by  Palladium,  Followed  by  Combustion  of 
Methane. — If  the  palladium  tube  is  to  be  used  for  absorption  of  hydro- 
gen the  solution  of  cuprous  chloride  in  ammonium  hydroxide  must  have 
been  used  for  absorption  of  carbon  monoxide.  The  entire  gas  residue 


FUELS 


343 


is  used.  Connect  the  palladium  tube  with  the  burette  on  one  side  and  a 
pipette  filled  with  water  on  the  other.  The  palladium  tube  should 
dip  into  a  beaker  of  water  which  is  kept  nearly  boiling.  Pass  the  gases 
through  the  tube  and  back,  repeating  two  or  three  times.  Replace  the 
hot  water  with  water  at  the  temperature  of  the  room  and  again  pass  the 
gas  through  the  tube  to  cool  it.  Determine  the  internal  volume  of  the 
palladium  tube  as  already  directed  and  subtract  one-fifth  of  this  volume 
from  the  total  contraction.  The  remainder  is  the  volume  of  hydrogen. 
Determine  methane  by  combustion  by  either  of  the  methods  already 
described. 


FIG.  88. — Aspirator  for  sampling  chimney  gases. 

Chimney  Gases. — Ideal  combustion  of  fuel  gases  or  of  coal 
should  yield  waste  gases  containing  only  carbon  dioxide,  water 
vapor,  and  nitrogen.  In  practice  complete  combustion  is  not 
secured  without  a  considerable  excess  of  air,  and  oxygen  is  there- 
fore found  in  the  chimney  gases.  The  presence  of  carbon 
monoxide  is  an  indication  of  imperfect  draught  and  incomplete 


344  QUANTITATIVE  ANALYSIS 

combustion  while  a  large  excess  of  oxygen  shows  that  heat  has 
been  wasted  in  raising  the  temperature  of  unused  air.  For 
control  work  the  determination  of  oxygen,  carbon  dioxide  and 
carbon  monoxide  is  sufficient  and  the  portable  form  of  apparatus 
(Fig.  82)  is  used.  It  contains  a  burette  and  three  pipettes  for 
these  determinations.  Many  modifications  of  this  apparatus 
will  be  found  illustrated  and  described  in  the  scientific  journals 
and  trade  catalogues. 

To  obtain  the  sample  for  analysis  a  porcelain  or  iron  tube  is 
inserted  into  the  stack  at  the  proper  point.  An  aspirator  is 
caused  to  draw  a  continuous  stream  of  gas  from  the  stack,  the 
sample  being  removed  by  the  burette  as  often  as  desired.  The 
determination  of  the  three  gases  is  made  as  with  the  Hempel 
apparatus.  Potassium  pyrogallate  should  be  used  fgr  the  ab- 
sorption of  oxygen  because  of  the  possible  presence  of  traces  of 
ethylene. 


CHAPTER  XIII 
OILS,  FATS  AND  WAXES 

BUBNING  OILS 

The  chemist's  examination  of  fuel  oils  usually  has  more  to  do 
with  the  determination  of  certain  physical  constants  than  with 
the  actual  analysis.  Petroleum  products  are  cheaper  than  animal 
or  vegetable  oils  and  are,  consequently,  seldom  adulterated  with 
the  latter.  Animal  and  vegetable  oils  are  rarely  used  for  burning. 
The  examination  of  the  fuel  oil,  therefore,  usually  resolves  itself 
into  a  determination  of  the  fitness  of  the  oil  for  the  purpose  for 
which  it  is  to  be  used.  The  determinations  may  include  specific 
gravity,  flash  point,  burning  point  and  fractional  distillation. 

Specific  Gravity. — The  relation  between  the  specific  gravity 
and  the  volatility  of  petroleum  fractions  is  fairly  definite,  so 
that  it  is  often  possible  to  secure  the  correct  oil  by  specifying 
only  the  specific  gravity.  This  may  be  determined  by  means 
of  a  Westphal  balance  or  a  floating  hydrometer.  The  latter  is 
most  conveniently  used  and  is  sufficiently  accurate  for  most 
purposes.  The  specific  gravity  may  be  expressed  in  relation 
to  water  or  in  degrees  Baume*.  The  system  of  Baume*  is  much 
used  in  commercial  testing.  In  this  system  two  scales  are  used, 
one  being  for  liquids  lighter  than  water,  the  other  for  liquids 
heavier  than  water.  The  first  is  applicable  to  all  petroleum 
products  and  to  most  other  oils  and  fats. 

In  the  original  Baume*  scale  for  liquids  heavier  than  water  the 
point  to  which  the  hydrometer  sinks  in  a  solution  of  sodium 
chloride,  15  percent  by  weight  and  at  15°  C.,  was  taken  as  15°. 
The  corresponding  point  for-  pure  water  was  taken  as  0°  and  all 
other  points  were  located  by  these  two.  For  liquids  lighter  than 
water  the  scale  has  the  point  10°  for  the  density  of  pure  water  at 
15°  C.  and  the  point  0°  corresponds  to  the  density  of  a  10  percent 
solution  of  sodium  chloride. 

345 


346  QUANTITATIVE  ANALYSIS 

Several  modifications  of  these  scales  have  come  into  use  and 
much  confusion  has  resulted  thereby.  As  the  system  is  at 
present  used  in  many  industrial  laboratories  the  following  for- 
mulas may  be  used  for  converting  specific  gravity  into  Baume* 
degrees  and  vice  versa. 

For  liquids  heavier  than  water: 

145 


15  5° 
where  B  =  degrees  Baume*  and  S  =  specific  gravity  at  T-^^O' 

JLO.  o 

For  liquids  lighter  than  water  : 

140 
S=130+Band 

B-f-,30. 

On  account  of  the  complexity  of  this  system  and  the  fact  that  it 
is  entirely  unnecessary  it  is  unfortunate  that  it  has  become  so 
generally  used  in  chemical  industries. 

Flash  Point.  —  The  "  flash  point"  is  the  temperature  at  which 
the  oil  gives  off  vapor  rapidly  enough  that  the  mixture  with  air 
becomes  explosive  and  will  flash  if  a  small  flame  is  brought  into 
the  mixture.  This  is  one  of  the  most  important  tests  to  be 
applied  to  burning  oils  because  it  determines  the  degree  of  safety 
attending  the  use  of  the  oil  in  enclosed  vessels,  such  as  lamps  and 
burners  of  various  kinds.  In  most  of  our  States  the  lower 
limits  of  flash  and  burning  points  are  specified  for  kerosene  by 
legal  restriction. 

The  location  of  the  flash  point  depends  to  a  great  extent  upon 
the  manner  of  confining  and  heating  the  oil.  The  mixture  of 
vapor  with  air  is  explosive  at  any  temperature  if  the  concentra- 
tion of  vapor  is  sufficiently  great.  Under  ordinary  circumstances 
the  vapor  is  evolved  so  slowly  that  it  escapes  by  diffusion  before 
an  inflammable  mixture  is  obtained  and  it  is  only  when  the  tem- 
perature is  raised  that  rapid  evolution  of  vapor  results  in  the 
production  of  a  mixture  that  will  ignite.  From  this  it  will 


OILS,  FATS  AND  WAXES  347 

readily  be  seen  that  the  flash  point  is  lowered  by  rapid  heating,  by 
confinement  of  the  vapor  by  covering  the  tester,  as  well  as  by  too 
close  contact  of  the  test  flame  with  the  surface  of  the  oil.  It  has 
therefore  become  necessary  to  regulate  by  law  not  only  the  tem- 
peratures of  the  flash  point  but  also  the  exact  form  of  the  tester 
and  the  manner  of  heating.  The  following  extract  from  the 
Indiana  law  of  March  11,  1901,  is  an  illustration  of  the  manner  in 
which  these  tests  are  governed  by  law. 

" The  test  shall  be  made  in  a  test  cup  of  metal  or  glass, 

cylindrical  in  shape,  two  and  one-quarter  inches  in  diameter  and  four 
inches  deep  (both  measurements  being  made  inside  the  cup)  and  this 
cup  shall  be  filled  to  within  one -quarter  of  an  inch  of  the  brim  with  the 
oil  or  other  substance  to  be  tested.  The  cup  shall  be  placed  in  a  water 
bath  sufficiently  large  to  leave  a  clear  space  of  one  inch  under  the  cup 
and  three-eighths  of  an  inch  around  it,  and  in  such  a  manner  as  to 
project  about  one-quarter  of  an  inch  above  the  water  bath.  The  space 
between  the  cup  and  the  water  bath  shall  be  filled  with  water  of  medium 
temperature  and  shall  be  heated  by  an  alcohol  lamp,  with  its  flames  so 
graduated  that  the  rise  in  temperature,  from  60  degrees  Fahrenheit  to 
the  highest  test  temperature,  shall  not  be  less  than  two  degrees  per 
minute  and  shall,  in  no  case,  exceed  four  degrees  per  minute.  A 
Fahrenheit  thermometer  shall  be  suspended  in  such  a  manner  that 
the  upper  surface  of  its  bulb  shall  be,  as  near  as  practicable,  one-quarter 
of  an  inch  below  the  surface  of  the  oil  undergoing  the  test.  As  soon  as 
the  temperature  reaches  the  point  of  ninety-eight  degrees  Fahrenheit, 
the  lamp  shall  be  removed  from  under  the  water  bath,  and  the  oil  shall 
then  be  allowed  to  rise  to  the  temperature  of  one  hundred  degrees 
Fahrenheit  by  the  residual  heat  of  the  water,  and  at  that  point  the  first 
test  for  flash  shall  be  made  as  follows :  A  taper  (hereinafter  described) 
shall  be  lighted  and  the  surface  of  the  oil  shall  be  touched  with  the  flame 
of  the  taper  (and  it  shall  be  lawful  to  apply  this  flame  either  to  the  center 
of  the  oil  surface  or  to  any  or  all  parts  of  it)  but  the  taper  itself  shall 
not  be  plunged  into  the  oil.  If  no  flash  takes  place  at  the  temperature 
of  one  hundred  degrees  Fahrenheit,  the  lamp  shall  be  placed  under  the 
water  bath,  and  the  temperature  raised  to  one  hundred  and  three  degrees 
Fahrenheit,  when  the  lamp  shall  be  again  withdrawn  and  the  oil  allowed 
to  rise  to  one  hundred  and  five  degrees  by  the  residual  heat  of  the  water, 
when  the  test  shall  be  made  by  again  applying  the  flame  of  the  taper  as 
hereinbefore  specified;  if  no  flash  occurs  the  test  shall  be  repeated  as 
often  as  the  oil  gains  five  degrees  in  temperature,  three  degrees  with 
the  lamp  under  the  water  bath,  and  two  with  the  lamp  removed.  These 


348  QUANTITATIVE  ANALYSIS 

tests  shall  be  repeated  until  a  flash  is  obtained.  The  one  making  the 
test  shall  further  test  the  oil  by  applying  the  taper  at  every  two  degrees 
rise  without  removing  the  lamp  or  stirring;  but  if  a  flash  is  obtained  by 
this  means,  by  a  less  rise  in  temperature  than  five  degrees  herein  re- 
quired, he  shall  at  once  remove  the  lamp,  stir  the  oil,  and  immediately 
apply  the  flame.  The  taper  used  for  testing  may  be  of  any  wood 
giving  a  clear  flame,  and  it  shall  be  made  as  slender  as  possible,  and 
with  a  tip  no  more  than  one-sixteenth  of  an  inch  in  thickness.  No 
taper  or  match  with  sulphur  on  it  shall  be  used,  unless  the  sulphur  is 
first  removed  before  lighting.  When  a  taper  is  first  lighted,  it  shall 
be  applied  to  the  oil  immediately  (that  is  to  say,  before  an  ash  or  coal 
has  had  time  to  form  on  the  end  of  the  taper  beyond  the  end  of  the  flame) 
and  the  flame  shall  be  made  to  touch  the  oil,  but  the  taper  itself  shall 
not  be  brought  in  contact  with  the  oil;  provided,  that  if  the  taper  be  so 
brought  in  contact  with  the  oil,  but  not  held  there  longer  than  for  the 
space  of  one  second,  and  the  oil  flashes,  the  test  shall  not  thereby  be 
vitiated,  but  the  Supervisor  of  Oil  Inspection  shall  immediately  remove 
the  lamp,  and  again  test  the  oil  by  the  flame  without  allowing  the  body 
of  the  taper  to  touch  the  oil.  No  oil  or  other  substance,  which,  by  the 
test  herein  described,  flashes  at  any  temperature  below  one  hundred 
and  twenty  degrees  Fahrenheit,  shall  be  allowed  to  be  sold,  offered  for 
sale,  or  consumed  for  illuminating  purposes  in  this  State.  And  it  shall 
be  lawful  to  sell  for  illuminating  purposes  any  oil  or  oils  herein  described, 
to  be  consumed  within  this  State,  which  shall  bear  a  flash  test  of  one 
hundred  and  twenty  degrees  Fahrenheit,  as  shown  by  said  apparatus." 

The  Indiana  law  is  not  specific  in  the  matter  of  covering  the 
tester  and  the  inference  is  that  the  open  tester  is  permitted. 

Burning  Point — The  burning  point  ("fire  test")  is  the  tem- 
perature at  which  vapor  is  evolved  with  sufficient  rapidity  to 
sustain  a  continuous  flame.  It  is  determined  by  removing  the 
cover,  if  one  was  used  during  the  flash  test,  and  continuing  the 
heating  after  the  flash  point  has  been  passed,  applying  the  test 
flame  until  a  temperature  is  reached  where  continuous  flame 
results.  The  thermometer  bulb  is  immersed  in  the  oil  and  the 
temperature  is  always  noted  just  before  the  application  of  the 
test  flame,  which  should  be  as  small  as  possible. 

Examination  of  Kerosene. — Determine  the  specific  gravity  at  15°  C. 
with  a  hydrometer  float  or  a  Westphal  balance,  reporting  in  the  usual 
units  and  also  in  degrees  Baume*,  using  the  formula  given  on  page  346 
for  calculation  of  degrees  Baume". 


OILS,  FATS  AND  WAXES  349 

Determine  the  flash  and  burning  points,  using,  preferably,  the  tester 
specified  by  the  law  of  the  state  in  which  the  oil  was  sold  and  following  in 
detail  the  directions  furnished  with  the  instrument.  If  no  such  tester 
is  available  construct  one  as  follows :  Upon  a  small  sand  bath  plaee 
a  3-inch  porcelain  dish,  pressing  the  dish  into  the  sand  until  the  latter 
is  within  1/4  inch  of  the  top  of  the  dish.  Fill  to  the  same  height  with 
the  oil  to  be  tested  and  suspend  in  the  middle  of  it  a  thermometer. 
Cover  the  dish  with  a  watch  glass  having  a  perforation  for  the  ther- 
mometer and  a  notch  at  the  side  for  the  application  of  the  test  flame. 
Heat  the  oil  so  that  the  temperature  shall  rise  at  the  rate  of  about  2° 
per  minute.  When  the  temperature  has  reached  85°  F.  begin  testing 
and  test  for  each  two  degrees  rise  in  temperature  by  inserting  a  small 
flame  (a  gas  flame  1/4  inch  long)  and  immediately  withdrawing  it. 
The  experiment  should  be  performed  where  the  light  of  the  room  is  not 
strong  and  in  a  place  free  from  air  currents.  The  flash  point  is  reached 
when  a  flash  passes  entirely  across  the  dish.  Remove  the  cover  and 
continue  the  heating  and  testing  until  a  permanent  flame  is  sustained. 
This  temperature  is  the  "fire  point"  or  "burning  point." 

The  method  used  with  the  form  of  apparatus  just  described  will  not 
give  the  same  flash  point  as  that  obtained  by  another  form  of  apparatus 
and  cannot  be  used  as  a  legal  check  where  another  form  of  tester  is 
specified  by  law.  It  is  here  described  because  it  will  afford  practice 
in  the  determination  when  no  other  tester  is  available. 

Elliott  Tester. — This  instrument  is  also  known  as  the  New  York 
Board  of  Health  tester.  It  may  be  used  either  open  or  closed  for 
the  flash  test  but  the  closed  test  is  preferable  unless  otherwise 
specified. 

The  outer  cup  is  filled  with  water  to  the  mark  placed  on  the  inner 
surface.  If  no  mark  is  found,  entirely  fill  the  cup  with  water  then  push 
the  oil  cup  in  as  far  as  it  will  go,  the  excess  water  overflowing.  Remove 
the  oil  cup  and  take  out  10  cc  more  of  water.  This  allows  room  for 
expansion  during  heating. 

Flash  Test. — Place  the  tester  in  a  room  which  is  free  from  air  currents 
and  in  which  the  light  is  not  bright.  Insert  the  oil  cup  and  carefully 
fill  with  the  kerosene  sample  to  within  5  mm  of  the  shoulder,  but  allowing 
no  oil  to  splash  on  the  shoulder.  Replace  the  glass  cover,  in  which  the 
thermometer  is  fixed  at  such  height  as  to  bring  the  bulb  just  beneath 
the  surface  of  the  oil.  Heat  the  water  in  the  bath  without  stirring, 
fast  enough  to  cause  the  thermometer  in  the  oil  to  indicate  a  rise  of  2° 
to  3°  F.  per  minute.  When  the  temperature  reaches  85°  F.  begin  the 
tests.  The  test  flame  should  be  a  gas  jet,  not  over  5  mm  long.  In 


350  QUANTITATIVE  ANALYSIS 

making  a  test  the  flame  is  inserted  at  the  notch  in  the  cover  so  that  it 
passes  well  into  the  cup  but  without,  at  any  time,  touching  the  surface 
of  the  oil.  Withdraw  the  flame  immediately.  Repeat  the  test  at 
every  2°  rise  in  temperature,  always  reading  the  thermometer  immediately 
before  applying  the  flame.  When  a  flash  passes  across  the  cup  the 
flash  point  is  reached. 

Fire  Test. — Remove  the  cover  and  suspend  the  thermometer  in 
the  same  position  that  was  used  during  the  flash  test.  Continue  the 
heating  at  a  rate  not  exceeding  10°  per  minute,  making  the  tests  as 
before.  When  the  vapor  finally  burns  with  a  continuous  flame  the 
burning  point  is  reached  and  the  temperature  indicated  by  the  ther- 
mometer just  before  the  final  test  is  called  the  fire  test. 

Fractional  Distillation. — Any  fraction  of  petroleum  now  ap- 
pearing in  commerce  includes  many  different  chemical  com- 
pounds and  can  itself  be  separated  by  fractional  distillation  into 
other  fractions  having  boiling  points  within  still  more  narrow 
limits.  The  determination  of  the  amount  distilling  between 
certain  specific  limiting  temperatures  yields  information  regard- 
ing the  composition  of  the  mixture.  The  results  have  little 
significance,  however,  unless  the  distillation  is  conducted  in  a 
standard  apparatus  and  by  a  standard  method. 

LUBRICATING  OILS 

For  purposes  of  lubrication  either  mineral,  animal  or  vege- 
table oils  or  mixtures  of  these  are  used.  Such  an  oil  should  have 
the  proper  viscosity  for  the  purpose,  should  be  free  from  acidity, 
should  produce  the  minimum  of  gumming  under  continued  use, 
and,  if  to  be  used  as  a  lubricant  for  cylinders  of  internal  com- 
bustion engines,  it  must  be  capable  of  undergoing  distillation 
without  the  deposition  of  more  than  a  very  small  percent  of  free 
carbon.  This  is  analogous  to  the  "fixed  carbon"  of  coal.  In 
many  cases  specifications  provide  against  the  presence  of  more 
than  small  amounts  of  animal  or  vegetable  oils  or  even  against 
any  quantity,  because  of  the  gumming  action  that  occurs  by 
oxidation  and  because  of  the  development  of  acids  through  par- 
tial hydrolysis  of  the  oil. 

Viscosity. — Viscosity  is  usually  expressed  either  as  a  specific 
property  with  the  viscosity  of  water  considered  as  unity,  or  in 
terms  of  an  arbitrary  scale  of  one  of  the  standard  instruments. 


OILS,  FATS  AND  WAXES 


351 


The  exact  determination  of  viscosity  is  a  difficult  process.  For 
commercial  purposes  an  approximate  determination  is  all  that 
is  necessary.  The  various  instruments  that  are  used  for  the 
determination  of  viscosity  of  oils  do  not  give  the  same  results 
but  when  the  arbitrary  scale  of  a  given  instrument  is  used,  com- 


FIG.  89. — Engler's  viscosimeter. 


parative  results  are  obtained  for  different  oils.  The  Engler 
viscosimeter1  is  illustrated  in  Fig.  89.  The  principle  used  in 
this  and  many  other  viscosimeters  is  that  of  measuring  the  time 
required  for  a  given  quantity  of  oil  to  flow  through  a  standard 
orifice. 

1  Z.  angew.  Chem.,  (1892)  725;  J.  Soc.  Chem.  Ind.,  12,  291  (1893). 


352 


QUANTITATIVE  ANALYSIS 


Determination. — If  the  oil  is  not  perfectly  free  from  suspended  solids, 
filter  through  muslin.  If  the  oil  is  not  dry,  decant  after  long  standing. 
Pour  the  oil  into  the  inner  cup  until  the  points  marking  the  required 
level  are  reached.  Fill  the  outer  cup  with  water  to  the  mark  on  the 
inside  and  heat  by  the  ring  burner  until  both  water  and  oil  are  at  the 
desired  temperature  (25°  unless  otherwise  specified).  A 
wooden  plug  closes  the  gold-lined  orifice  in  the  bottom 
of  the  oil  cup.  When  this  is  lifted  note  the  time  on  a 
stop  watch  and  allow  the  oil  to  flow  out  until  200  cc  is 
measured  in  the  graduated  flask,  noting  the  time  when 
the  graduation  is  reached.  The  viscosity  is  the  number 
of  seconds  required  for  200  cc  of  oil  to  flow  out.  The 
instrument  is  standardized  by  measuring  the"  time 
necessary  for  200  cc  of  water  to  flow  at  20°.  This 
should  be  50  to  52  seconds.  The  relative  viscosity  is  the 
ratio  of  the  time  required  for  the  oil  to  that  for  water  at 
the  same  temperature. 

Specific  Gravity.— Determine  as  with  burning  oils 
unless  the  viscosity  is  too  high  to  permit  the  use  of  either 
of  these  methods.  In  the  latter  case  a  picnometer  is  to 
be  used  or  the  specific  gravity  is  determined  at  higher 
temperatures.  The  special  hydrometer  designed  by 
Sommer1  may  also  be  used  for  the  determination  of  the 
specific  gravity  of  highly  viscous  oils.  This  is  illustrated 
in  Fig.  90.  The  brass  cup  has  a  capacity  of  exactly  10  cc. 
It  is  filled  with  the  oil  at  20°,  the  cap  is  screwed  on  and 
the  cup  is  then  suspended  from  the  hydrometer  float, 
which  is  placed  in  pure  water  at  20°.  The  specific 
gravity  is  read  on  the  stem  of  the  float,  at  the  position 
of  the  meniscus. 

Acidity. — Shake  a  small  amount  of  oil  in  a  test-tube 
with  warm  water  and  test  the  water  with  litmus.     If 
F  J  Q  'r,9  °'~    acidity  is  shown  a  weighed  sample  of  oil  is  shaken  with 
drometer     for    alcohol  and  the  acids  titrated  with  a  standard  alcoholic 
asphalt      and    solution  of  a   base   which  is  preferably  tenth-normal 


viscous  oils. 


potassium  hydroxide. 


Separation  of  Saponifiable  from  Mineral  Oils. — The  method 
of  separation  depends  upon  the  difference  in  chemical  nature 
between  mineral  oils  and  those  of  animal  or  vegetable  origin. 
The  former  are  mostly  hydrocarbons  while  the  latter  are  esters 

1  J.  Ind.  Eng.  Chem.,  2,  181  (1910). 


OILS,  FATS  AND  WAXES  353 

derived  from  glycerine  and  small  quantities  of  other  higher  alco- 
hols with  fatty  acids.  The  esters  are  saponifiable  by  bases  and 
the  resulting  soaps  are  soluble  in  water  while  the  unsaponified 
mineral  oils  easily  dissolve  in  petroleum  ether. 

Determination. — Weigh  a  100  cc  Erlenmeyer  flask,  add  about  10 
gm  of  the  oil  and  weigh  again.  Add  50  cc  of  an  approximately  half- 
normal  solution  of  potassium  hydroxide  in  alcohol,  place  in  the  neck 
of  the  flask  a  funnel  having  a  stem  not  more  than  5  cm  long  and  warm 
on  the  steam  bath  for  30  minutes.  Remove  the  funnel  and  evaporate, 
frequently  blowing  out  the  vapor,  until  the  odor  of  alcohol  disappears. 
The  evaporation  of  alcohol  may  be  hastened  by  inserting  a  glass  tube 
in  the  flask  so  that  the  end  is  four  or  five  centimeters  above  the  liquid, 
attaching  a  pump  and  drawing  air  through  the  flask.  The  tube  must 
be  slanted  downward  outside  the  flask  in  order  to  prevent  condensed 
alcohol  from  returning  to  the  flask.  Cool,  add  50  cc  of  petroleum 
ether,  stir  thoroughly  with  the  soap  and  rinse  into  a  separatory  funnel 
with  petroleum  ether,  disregarding  any  soap  that  may  adhere  to  the 
flask.  Add  to  the  ethereal  solution  in  the  separatory  funnel  an  equal 
volume  of  water,  shake  and  allow  to  separate  completely.  The  water 
will  dissolve  the  soap  that  was  produced  from  animal  or  vegetable 
oils  while  the  petroleum  ether  containing  the  mineral  oil  will  form 
the  upper  layer.  Separate  and  discard  the  water  solution  and  then 
rinse  the  ethereal  solution  into  the  flask  in  which  saponification  was 
accomplished,  having  previously  washed  and  dried  the  flask.  Evapo- 
rate the  petroleum  ether  by  placing  the  flask  in  a  steam  bath  from 
which  the  flame  has  been  removed.  The  evaporation  may  be  hastened 
by  the  same  device  as  was  used  in  evaporating  alcohol  from  the  soap. 
After  all  ethereal  odor  has  disappeared  the  flask  is  cooled  and  weighed. 
This  gives  directly  the  percent  of  mineral  oil  in  the  sample,  and  this 
percent  subtracted  from  100  gives  the  percent  of  saponifiable  oil.  The 
method  gives  somewhat  high  results  for  saponifiable  oils  because  some 
loss  of  mineral  oil  occurs  during  the  extraction  of  the  soap. 

Chill  Test. — The  chill  test  is  the  determination  of  the  tempera- 
ture at  which  turbidity  appears  because  of  the  formation  of 
crystals.  This  is  the  temperature  at  which  the  oil  would  tend  to 
clog  oil  holes  in  bearings.  It  has  little  significance  except  where 
saponifiable  oils  are  present  because  mineral  oils  do  not  crystallize 
upon  cooling. 

Determination. — A  4-ounce  bottle  having  a  wide  mouth  is  half 
filled  with  the  oil  and  a  thermometer  placed  in  it.  The  bottle  is  placed 

23 


354  QUANTITATIVE  ANALYSIS 

in  a  freezing  mixture  and  stirred  continuously  with  the  thermometer. 
When  the  liquid  ceases  to  be  perfectly  clear  the  temperature  is  noted 
as  the  "chill  test." 

Cold  Test.  —  This  is  the  determination  of  the  temperature  at 
which  the  oil  ceases  to  flow  freely  and  at  which  it  will  therefore 
fail  to  be  delivered  to  bearings  from  the  oil  cup.  From  a  con- 
sideration of  the  composite  nature  of  all  oils  it  will  be  seen  that 
both  chill  and  cold  tests  can  give  results  which  are  only  approxi- 
mately constant.  They  are  of  service,  however,  in  forming  a 
basis  for  judging  the  fitness  of  oils  for  use  within  known  tempera- 
ture ranges. 

Determination.  —  The  bottle  containing  the  oil  that  was  used  in  the 
chill  test  is  placed  in  the  freezing  mixture  and  cooled  until  the  oil  becomes 
solid.  It  is  then  removed  and  allowed  to  warm  by  contact  With  the  air, 
being  stirred  with  the  thermometer  meanwhile.  At  intervals  of  two 
degrees  rise  in  temperature  the  bottle  is  inverted.  When  the  oil  has 
become  sufficiently  fluid  to  flow  from  one  end  of  the  bottle  to  the  other 
the  temperature  is  noted  as  the  "cold  test." 

EDIBLE  FATS  AND  OILS 

For  an  interesting  discussion  of  the  fat  and  oil  industries 
reference  may  be  made  to  an  address  by  Lewkowitsch.  l 

Composition.  —  The  chief  constituents  of  animal  and  vegetable 
oils  are  esters  derived  from  fatty  acids  and  the  triatomic  alcohol, 
glycerine.  Of  the  former  the  most  important  are  palmitic,  stearic 
and  oleic  acids,  the  first  two  being  saturated  acids,  the  last  an 
unsaturated  acid.  The  glycerides  of  these  acids  are  respectively 
known  as  palmitin,  stearin  and  olein  and  they  have  the  following 
composition  : 


Palmitin  Stearin  Olein 

In  addition  to  these  are  esters  of  higher  alcohols  other  than 
glycerine  and  of  other  saturated  and  unsaturated  fatty  acids, 
also  in  certain  cases  small  amounts  of  free  higher  alcohols.  The 
chief  differences  in  properties  of  different  oils  are  caused  by  varia- 
tions in  the  proportions  of  the  constituent  esters.  Vegetable 
oils  contain  much  palmitin  while  stearin  predominates  in  animal 
1  Bull.  soc.  chim.,  [41  6,  1  (1909);  Am.  Chem.  J.,  43,  428  (1910). 


OILS,  FATS  AND  WAXES  355 

oils.  The  more  liquid  oils  contain  more  olein  and  esters  of  acids 
having  smaller  molecular  weights. 

The  true  waxes  differ  chemically  from  the  oils  and  fats  in  that 
they  are  not  glycerides  but  are  esters  of  mono-  or  diatomic 
alcohols  with  the  higher  fatty  acids.  These  alcohols  are  either 
aliphatic  or  aromatic.  Some  examples  of  such  esters  are  as 
follows:  Cetyl  palmitate,  derived  from  palmitic  acid  and  cetyl 
alcohol,  CieHssOH;  this  is  the  chief  constituent  of  spermaceti. 
Ceryl  palmitate,  the  chief  constituent  of  opium  wax,  is  derived 
from  palmitic  acid  and  ceryl  alcohol,  C27H550H.  Myricyl 
palmitate  occurs  in  beeswax.  It  is  an  ester  of  palmitic  acid  and 
myricyl  alcohol,  C3oH6iOH.  Ceryl  cerotate  is  the  chief  constitu- 
ent of  Chinese  wax.  It  is  an  ester  of  cerotic  acid,  C25H5iCOOH, 
and  ceryl  alcohol.  The  most  important  aromatic  alcohols  occur- 
ring in  waxes  are  the  isomeric  alcohols  cholesterol  and  phytos- 
terol,  C26H43OH.  These  are  found  as  esters  of  palmitic,  stearic 
and  oleic  acids. 

Notwithstanding  the  differences  in  composition  the  task  of 
separating  and  determining  the  percent  of  different  oils  in  a 
mixture  is  a  difficult  and  often  impossible  one,  because  of  the 
fact  that  the  same  general  compounds  constitute  the  greater 
proportion  of  all  fats  and  oils.  The  chemist  must  usually  be 
satisfied  if  he  can  recognize  single  oils  or,  with  the  nature  of  a 
single  oil  known,  determine  the  approximate  extent  and  nature 
of  adulteration.  The  differences  in  molecular  weight  and  degree 
of  saturation,  the  presence  and  percent  of  free  alcohols  or  acids 
and  the  occasional  occurrence  of  traces  of  unusual  substances, 
characteristic  of  certain  oils,  constitute  the  bases  of  the  tests  used 
in  the  effort  to  identify  an  oil.  The  examination  becomes  there- 
fore not  an  analysis,  in  the  usual  sense,  but  a  series  of  tests  applied 
in  order  to  gain  information  regarding  the  identity  of  a  pure  oil 
and,  so  far  as  is  possible,  the  composition  of  a  mixture.  Certain 
physical  and  chemical  " constants"  are  determined  and  compared 
with  the  constants  obtained  from  oils  of  known  purity.  The 
chief  obstacle  to  the  use  of  such  figures  lies  in  the  fact  that,  for  a 
given  kind  of  oil  they  are  actually  variable  within  certain  limits. 
These  limits  may  be  very  narrow,  but  since  they  do  include  a 
certain  range  it  sometimes  happens  that  the  ranges  for  two  or 
more  oils  overlap.  Thus  olive  oil  from  Italy  is  not  chemically 


356  QUANTITATIVE  ANALYSIS 

identical  with  olive  oil  from  California.  The  soil,  climate, 
variety  of  plant  and  method  of  expressing  from  the  olive  have 
their  influence  upon  the  properties  of  the  various  glycerides 
and  other  substances  present  in  the  oil.  It  is  only  when  the 
ranges  of  variation  do  not  overlap  that  it  is  easy  to  determine 
the  identity  of  a  single  oil,  although  it  often  happens  that  while 
overlapping  occurs  with  a  single  constant  it  does  not  occur  with 
others. 

The  significance  of  the  various  constants  and  their  methods  of 
determination  will  be  described. 

Specific  Gravity. — In  a  general  way  the  specific  gravity  of  oils 
increases  with  the  percent  of  (a)  glycerides  of  unsaturated  acids, 
(b)  glycerides  of  soluble  acids  and  (c)  free  fatty  acids.  Old 
oils  also  usually  have  higher  specific  gravities  than  the  normal, 
on  account  of  oxidation.  The  specific  gravity  of  the  waxes 
and  of  solid  fats  is  usually  higher  than  of  liquid  oils.  These 
rules  do  not  hold  in  all  cases  and  the  determination  of  specific 
gravity,  like  that  of  the  other  constants  of  oils,  is  made  for  com- 
paring with  recorded  data  for  the  purpose  of  identification  more 
often 'than  for  throwing  light  upon  the  chemical  constitution 
of  oils  of  known  purity. 

Unfortunately  there  has  been  a  great  lack  of  uniformity  in 
selecting  conditions  and  modes  of  expression  for  specific  gravities 
of  oils  as  they  are  recorded  in  the  literature.  Temperatures  of 
15.5°,  20°,  25°,  40°,  60°,  100°  and  others  are  commonly  used. 
In  favor  of  the  higher  temperatures  it  may  be  said  that  the  fats 
and  waxes  are  all  liquid  at  these  temperatures  so  that  determina- 
tions may  readily  be  made.  The  specific  gravity  has  been  vari- 
ously expressed  as  the  weight  of  oil  at  t°  divided  by  the  weight 
of  the  same  volume  of  water  at  either  t°,  0°,  4°  or  15.5°.  These 
differences  make  the  compilation  of  comparison  tables  difficult. 
However  it  has  been  found1  that  a  fair  degree  of  approximation 
may  be  made  in  correcting  the  specific  gravity  to  another  tem- 
perature by  using  the  coefficient  0.0007  as  the  change  for  each 
centigrade  degree.  This  is  the  average  value  for  a  considerable 
number  of  oils  between  temperatures  of  15.5°  and  98°.  Of  course 
this  does  not  remedy  the  lack  of  uniformity  of  expression,  noted 
above. 

i  Wright:  J.  Soc.  Chem.  Ind.,  26,  513  (1907). 


OILS,  FATS  AND  WAXES  357 

For  the  determination  use  a  picnometer,  a  Westphal  balance  or  an 
accurately  calibrated  hydrometer.  If  a  Westphal  balance  is  used  the 
plummet  should  be  accurately  calibrated  at  the  temperature  at  which 
the  balance  is  to  be  used.  The  thermometer  in  the  plummet  should  bn 
compared  with  a  standard  thermometer.  The  picnometer  method  is 
recommended. 

20° 

Determination  at  o^. — Use  a  25  cc  specific  gravity  bottle  (picno- 
meter) .  Clean  with  chromic  acid,  followed  by  distilled  water,  then  rinse 
with  alcohol  and  dry  in  an  oven  at  100°.  Cool  in  the  balance  case  (in 
which  the  air  should  be  at  a  temperature  not  above  20°)  and  weigh. 
Fill  with  distilled  water  which  has  been  recently  boiled  to  expel  dissolved 
gases  and  cooled  to  a  few  degrees  below  20°.  Insert  the  stopper  and 
nearly  immerse  the  stoppered  bottle  in  a  distilled  water  bath  which  is 
kept  at  exactly  20°.  After  30  minutes  take  off  the  drop  of  water  from 
the  tip  of  the  stopper,  remove  the  bottle  and  wipe  perfectly  dry  with  a 
clean  towel  but  without  warming  the  bottle  to  above  20°.  Place  in  the 
balance  case  and  weigh  after  15  minutes.  Calculate  the  weight  of 
contained  water. 

Empty  and  dry  the  bottle  inside  and  out,  then  fill  with  oil  and  ma- 
nipulate as  before,  calculating  the  weight  of  contained  oil.  This  weight 
divided  by  the  weight  of  contained  water  gives  the  specific  gravity  of 

20° 
the  oil  at  ™o' 

If  the  specific  gravity  has  been  determined  at  any  other  temperature 
or  if  it  is  desired  to  calculate  the  specific  gravity  at  any  temperature 
from  the  determination  at  20°,  these  changes  may  be  made  with  a  fair 
degree  of  accuracy  by  the  use  of  the  following  formula: 

G   =  G'  +  0.0007  (T'-T),  where 

G   =  specific  gravity  at  temperature  T, 

G'  =  specific  gravity  at  temperature  T'. 

20°  20° 

Determination  at  -^. — Multiply  the  specific  gravity  at  ^  by  the 

density  of  water  at  20°,  as   shown  in  the  table  on  page  181.     The 

20° 
product  is  the  specific  gravity  of  the  oil  at  -TO~" 

Determination  at  the  Temperature  of  Boiling  Water. — Fill  a  25  cc 
picnometer,  dried  and  weighed  as  above  described,  with  freshly  boiled 
hot  water.  Nearly  immerse  in  a  bath  of  briskly  boiling  water  and  leave 
for  30  minutes,  replacing  evaporated  water  with  boiling  distilled  water. 
Insert  the  stopper,  previously  heated  to  100°,  remove  the  picnometer 


358  QUANTITATIVE  ANALYSIS 

from  the  bath,  wipe  dry,  cool  to  room  temperature  and  weigh.  Cal- 
culate the  weight  of  contained  water. 

Fill  the  flask,  dried  at  100°,  with  the  dry,  hot,  freshly  filtered  fat  or 
oil,  which  must  be  entirely  free  from  air  bubbles.  Keep  in  the  boiling 
water  bath  for  30  minutes  then  insert  the  stopper,  which  has  been  heated 
to  100°  wipe  dry,  cool  to  room  temperature  and  weigh.  Calculate  the 
weight  of  contained  oil  and  from  this  and  the  weight  of  water  contained 
at  boiling  temperature  calculate  the  specific  gravity  of  the  oil  at  the 
temperature  of  boiling  water. 

This  determination  is  necessarily  less  accurate  than  the  one  at  20°, 
on  account  of  the  difficulty  involved  in  keeping  the  bath  at  any  constant 
temperature.  Superheating  may  easily  occur  with  distilled  water  and 
less  pure  water  will  have  a  boiling  point  above  100°.  Variation  in 
barometric  pressure  will  also  change  the  temperature  of  the  bath  so 
that  it  becomes  necessary  to  carry  out  both  parts  of  the  experiment 
at  the  same  atmospheric  pressure.  However  the  determination  is 
sanctioned  and  has  been  made  official  by  the  Association  of  Official 
Agricultural  Chemists.1 

The  specific  gravity  at  any  temperature  other  than  20°  may  be 
determined  by  the  method  outlined  for  this  temperature  or  it  may  be 
calculated  from  the  determination  at  this  temperature,  using  the  formula 
given  above.  It  should  be  understood  that  the  figure  desired  for 
purposes  of  identification  is  the  specific  gravity  at  the  temperature  for 
which  data  may  be  found  in  the  literature. 

Index  of  Refraction. — The  measurement  of  index  of  refraction 
is  a  valuable  addition  to  the  list  of  tests  for  oils.  While  not  in 
all  cases  characteristic  it  will  frequently  serve  to  distinguish 
between  certain  possibilities  when  other  tests  are  not  conclusive. 
The  refractive  index  increases  with  increasing  molecular  weight 
of  the  combined  acids  and  with  increasing  unsaturation.  If 
free  fatty  acids  are  present  in  an  oil  the  refractive  index  will  be 
lower  than  the  normal  value  for  the  oil.  In  consequence  of  the 
latter  fact  one  may  expect  to  find  abnormally  low  indices  for 
old  or  rancid  fats  or  oils. 

The  selection  of  standard  temperatures  for  the  determination 
is  highly  desirable  in  order  to  make  comparison  data  useful. 
Temperatures  of  20°  for  oils  and  40°  or  60°  for  fats  and  waxes 
are  suitable  in  most  cases.  For  calculating  the  index  of  refrac- 
tion at  any  temperature  from  experimental  results  at  another 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Pt.  II,  p.  299. 


OILS,  FATS  AND  WAXES 

the  formula  of  Tolman  and  Munson1  may  be  used: 
R    =  R'  +  0.000365(T'-T),  where 
R    =  index  of  refraction  at  temperature  T, 
R'  =  index  of  refraction  at  temperature  T'. 


359 


FIG.  91. — Abb6's  refractometer. 

The  coefficient  0.000365  is  the  average  change  of  refractive 
index  for  1°  for  a  large  number  of  common  oils. 

The  index  of  refraction  is  determined  by  the  use  of  any  of  the 
standard  instruments,  such  as  the  Abbe,  Pulfrich,  Zeiss  butyro- 
refractometer  or  the  immersion  refractometer.2  Of  those  named 

1  J.  Am.  Chem.  Soc.,  24,  754  (1902). 

2  For  a  discussion  of  the  theories  of  refraction  and  of  the  various  types  of 
refractometers,  see  Shook:  Met.  Chem.  Eng.,  12,  572  and  630  (1914)  and 
13,  19  (1915). 


360 


QUANTITATIVE  ANALYSIS 


the  Abbe*  refractometer  is  probably  the  most  generally  useful 
instrument  for  the  laboratory  because  it  may  be  used  with  either 
solids  or  liquids  covering  a  wide  range  of  refractive  indices  and 
because  it  does  not  require  the  use  of  monochromatic  light. 
This  instrument  is  shown  in  Fig.  91.  A  layer  of  the  oil  is  enclosed 
between  two  prisms  in  such  a  manner 
that  light  rays  enter  it  at  an  angle  differ- 
ent from  the  normal,  refraction  resulting 
(Fig.  92).  The  instrument  measures  the 
angle  of  total  reflection  of  the  ray  emerg- 
ing from  the  oil,  the  field  being  a  divided 
light  and  dark  one.  Dispersion  is  cor- 
rected by  a  "  compensator "  consisting 
of  two  similar  Amici  prisms,  of  direct 
vision  for  the  D-line  and  rotated  simul- 
taneously, though  in  opposite  directions, 
around  the  axis  of  the  telescope  by  means 
of  the  screw  head.  In  this  process  of 
rotation  the  dispersion  of  the  compen- 
sator passes  through  every  value  from 
zero  (when  the  refracting  edges  of  the 
two  prisms  are  parallel  and  on  different 
sides  of  the  optical  axis)  to  double  the 
amount  of  dispersion  of  a  single  Amici 
prism  (the  refracting  edges  being  parallel 
and  on  the  same  side  of  the  optical  axis). 
The  dispersion  produced  by  the  oil  in 
the  refractometer  may  thus  be  annulled 
by  rotating  the  screw  head  of  the  com- 
pensator until  the  latter  produces  a  dis- 
persion equal  to  that  of  the  oil  but  in 
the  opposite  direction.  The  border  line 
between  light  and  dark  fields  then  becomes  sharp  and  distinct, 
even  when  white  light  is  used  for  illumination  of  the  refrac- 
tometer prisms. 

The  scale  is  graduated  to  read  directly  the  index  of  refraction. 
The  prisms  are  enclosed  in  such  a  manner  that  water  at 
any  desired  temperature  may  be  circulated  about  them.  The 
heating  arrangement  for  the  water  is  a  special  feature  of  the  Zeiss 


FIG.  92. — Path  of   rays   in 
the  Abbe  refractometer. 


OILS,  FATS  AND  WAXES 


361 


instruments.  This  is  shown  in  Fig.  93.  Water  is  caused  to 
pass  through  the  heating  spiral  and  the  refractometer  under  a 
constant  pressure,  the  temperature  being  controlled  by  regulating 
the  size  of  the  burner  flame  and  the  rate  of  flow  through  the 
clamp  C.  Since  the  pressure  of  water  in  the  labo- 
ratory mains  is  not  constant  the  pressure  is  made 
independent  by  fixing  the  upper  and  lower  levels 
by  means  of  the  overflow  tubes  in  the  vessels  A 
and£. 

The  Zeiss  "  butyro-ref ractometer "  is  an  instru- 
ment which  uses  the  same  arrangement  of  prisms 
as  that  of  the  Abbe  instrument.  It  is  made  espe- 
cially for  use  in  the  examination  of  butter  and  has 
a  purely  arbitrary  scale. 
Readings  of  the  butyro-refrac- 
tometer  can  be  converted 
into  indices  of  refraction  by 
use  of  a  table  furnished  with 
the  instrument.  The  chief 
disadvantage  of  this  instru- 
ment is  the  absence  of  the 
compensator.  The  prisms  are 
achromatized  for  pure  butter 
and  give  no  dispersion  of 
white  light  when  this  fat  is 
used.  For  all  other  liquids 
the  line  of  division  between  the  light  and  dark  fields  is  indistinct 
and  consists  of  a  prismatic  series  of  colors  unless  monochromatic 
light  is  used. 

Determination  by  Means  of  the  Abbe  Refractometer. — Set  up  the 

refractometer  in  front  of  a  window  or  a  source  of  sodium  light.  Con- 
nect the  heating  apparatus  as  shown  in  the  figure  and  adjust  the  flow 
of  water  and  the  height  of  the  flame  until  the  desired  temperature  (20° 
for  oils,  40°  or  higher  for  fats  and  waxes)  is  attained.  Open  the  prism 
so  that  the  lower  half  is  in  a  horizontal  position  and  place  two  or  three 
drops  of  oil  or  melted  fat  upon  it,  using  a  glass  rod  or  pipette  but 
avoiding  scratching  the  prisms.  Quickly  close  and  lock  the  prisms, 
allow  time  for  the  temperature  to  become  constant  then  adjust  the 
compensator  until  the  line  of  division  of  the  field  is  sharply  defined  and 


FIG.  93. — Zeiss'   apparatus  for  heating 
refractometer  prisms. 


362  QUANTITATIVE  ANALYSIS 

bring  this  line  to  the  cross  hairs.     Read  the  index  of  refraction  upon 
the  scale. 

Clean  the  prisms  by  applying  a  mixture  of  equal  volumes  of  alcohol 
and  ether,  using  a  tuft  of  absorbent  cotton.  (Ordinary  cotton  may  con- 
tain grit.) 

Melting  Points  of  Fats. — From  the  fact  that  fats  are  not  single, 
pure  compounds  it  will  be  seen  that  they  cannot  have  definite 
and  sharp  melting  points  and  the  observation  will,  therefore, 
be  a  somewhat  arbitrary  one.  The  official  method1  follows. 

Determination. — Prepare  an  alcohol- water  mixture  of  graduated 
density  as  follows:  Boil,  separately,  water  and  95  percent  alcohol  for 
10  minutes  to  remove  dissolved  gases.  While  still  hot  pour  the  water 
into  an  8-inch  test-tube  until  it  is  almost  half  full.  Nearly  fill  the  tube 
with  the  hot  alcohol,  pouring  down  the  side  of  the  inclined  tube  to  avoid 
too  much  mixing.  If  the  alcohol  be  added  after  the  water  has  cooled 
the  mixture  will  contain  so  many  air  bubbles  as  to  be  unfit  for  use. 

Prepare  discs  of  fat  as  follows:  Allow  the  melted  and  filtered  fat  to 
fall  a  distance  of  15  to  20  cm  from  a  dropping  tube  upon  a  piece  of  ice 
or  upon  the*surface  of  cold  mercury.  The  discs  thus  formed  should  be 
1  to  1.5  cm  in  diameter  and  should  weigh  about  200  mg.  Since  a 
recently  melted  and  solidified  fat  does  not  possess  its  normal  melting 
point  the  discs  should  stand  for  2  to  3  hours  before  testing. 

Place  the  test-tube  containing  the  alcohol-water  mixture  in  a  tall 
beaker  containing  ice  water,  until  cold.  Drop  the  disc  of  fat  into  the 
tube  and  it  will  at  once  sink  to  a  point  where  the  density  of  the  mixture 
is  exactly  equal  to  its  own.  Lower  an  accurate  thermometer,  which 
can  be  read  to  0.1°,  into  the  test  tube  until  the  bulb  is  just  above  the 
disc,  stirring  very  gently  with  the  thermometer.  Slowly  heat  the  water 
in  the  beaker,  stirring  constantly  by  means  of  an  air  blast  or  some  other 
device. 

When  the  temperature  of  the  alcohol-water  mixture  has  risen  to 
about  6°  below  the  melting  point  of  the  fat  the  disc  will  begin  to  shrivel 
and  roll  into  an  irregular  mass.  Now  lower  the  thermometer  until  the 
fat  particle  is  even  with  the  center  of  the  bulb.  Rotate  the  thermometer 
gently  and  regulate  the  temperature  so  that  about  10  minutes  is  required 
for  the  last  increment  of  2°.  As  soon  as  the  fat  becomes  spherical  read 
the  thermometer.  This  serves  as  a  preliminary  observation  of  melting 
point.  Remove  the  tube  from  the  bath  and  place  in  the  latter  a  second 
tube  of  alcohol-water  mixture.  The  test-tube  is  of  sufficiently  low 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Pt.  II,  p.  301. 


OILS,  FATS  AND  WAXES  363 

temperature  to  cool  the  bath  to  the  desired  point,  ice  water  having  been 
used  for  cooling.  Add  another  disc  of  fat  and  regulate  the  temperature 
so  as  to  reach  a  maximum  of  1.5°  above  the  melting  point  as  already 
determined.  Run  still  another  determination,  which  should  agree 
closely  with  the  second.  The  disc  should  not  be  allowed  to  touch  the 
sides  of  the  tube  in  any  determination. 

Melting  Point  of  Fatty  Acids  ("Titer  Test"). — A  preliminary  saponi- 
fication  of  the  fat  and  separation  and  washing  of  the  resulting  fatty 
acids  is  necessary  for  this  determination.  The  author  considers  the  time 
consumed  in  the  entire  experiment  to  be  out  of  proportion  to  the  value 
of  the  results,  in  most  cases.  If  the  determination  is  required  it  may  be 
found  described  in  the  official  methods.1 

Iodine  Absorption  Number. — The  iodine  absorption  number 
is  the  percent  of  halogen,  expressed  as  iodine,  absorbed  by  the 
fat  or  oil  when  subjected  to  the  action  of  a  halogen  solution 
under  specified  conditions.  The  absorption  takes  place  because 
of  the  presence  in  the  oil  of  glycerides  of  unsaturated  acids  which 
contain  double  or  triple  bonded  carbon  atoms. 

This  action  is  analogous  to  the  addition  of  oxygen,  forming 
saturated  oxygen  compounds  which  are  often  hard  and  resinous 
in  their  nature.  Such  absorption  of  oxygen  from  the  air  is  known 
as  "drying/'  although  the  term  is  here  misapplied  since  no  real 
drying  occurs.  The  determination  of  halogen  absorption  number 
is,  in  a  general  way,  a  measure  of  " drying"  properties  and 
serves  for  the  distinction  between  the  broad,  general  classes  of 
"drying,"  "semi-drying"  and  "non-drying"  oils. 

Of  the  unsaturated  acids  whose  glycerides  commonly  occur  in 
fats  and  oils  the  following  important  members  may  be  mentioned: 

Oleic  Acid,  Ci8H34O2.  The  structure  of  this  acid  is  sufficiently 
indicated  by  the  formula  CH3(CH2)7CH=CH(CH2)7COOH. 
Olein,  the  triglyceride  of  this  acid,  occurs  to  some  extent  in  all 
oils  and  fats.  Olein  is  liquid  at  ordinary  temperatures  and  its 
presence  in  oils  is  responsible,  in  many  cases,  for  their  liquid 
character.  Either  oleic  acid  or  olein  will  absorb  two  atoms  of 
bromine  or  one  molecule  ,of  iodine  monochloride  or  mono- 
bromide,  the  double  bonded  carbon  atoms  thus  becoming  satu- 
rated. Similarly  either  oleic  acid  or  olein  might  be  expected 
to  absorb  one  atom  of  oxygen  and  to  give  "drying"  properties 

i  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Pt.  II,  p.  302. 


364  QUANTITATIVE  ANALYSIS 

to  a  fat  or  oil  but  this  action  does  not  take  place  readily  and 
most  of  the  oils  of  pronounced  drying  properties  are  found  to 
contain  considerable  quantities  of  simple  or  mixed  glycerides  of 
linolic  or  linolenic  acids. 

Linolic  Acid,  Ci8H32O2,  contains  two  pairs  of  doubly  linked 
carbon  atoms: 


This  acid  or  its  glyceride,  linolin,  will  absorb  four  atoms  of 
halogen  or  one  molecule  of  oxygen.     It  gives  marked  drying 
properties  to  oils,  linolin  being  abundant  in  linseed,  soy  bean  and 
poppy  seed  oils. 
Linolenic  Acid,  CigHsoC^,  probably  to  be  represented^ 


CH3.CH2.CH  =  CH.CH2.CH  =  CH.CH2.CH  =  CH.  (CH2)  7COOH. 

This  acid  possesses  three  sets  of  double  bonds  and  will  absorb 
six  halogen  atoms  or  three  oxygen  atoms.  It  occurs  as  simple 
or  mixed  glycerides  in  linseed  oil  and,  together  with  linolic  acid, 
plays  the  most  important  part  in  the  hardening  or  "  drying" 
of  this  oil  when  it  is  exposed  to  the  air.  An  isomer,  isolinolenic 
acid,  also  occurs  as  a  constituent  of  the  glycerides  of  drying  oils. 
Clupanodonic  Acid,  CisH^gC^,  has  the  following  structure: 

CH3.CH2.CH  =  CH.CH2.CH  =  CH.CH2.CH  =  CH.CH2.CH 

II 
HOOC(CH2)4.CH. 

Having  four  pairs  of  doubly  linked  carbon  atoms  it  is  able  to 
absorb  eight  halogen  or  hydrogen  atoms  or  four  oxygen  atoms. 
This  acid  will  be  mentioned  in  connection  with  the  detection  of 
fish  oils. 

Ricinoleic  Acid,  Ci8H34O3,  is  hydroxyoleic  acid  and,  like  oleic 
acid  itself,  contains  only  one  pair  of  doubly  linked  carbon  atoms. 
It  will  not  readily  absorb  oxygen  from  the  air  and  it  does  not 
impart  drying  properties  to  an  oil.  It  is,  however,  an  important 
constituent  of  castor  oil  and  will  be  mentioned  later  in  the 
discussion  of  acetyl  value. 


OILS,  FATS  AND  WAXES  365 

The  five  acids  named  above  serve  to  illustrate  the  principle  that 
only  those  unsaturated  acids  which  contain  more  than  one  pair 
of  doubly  bonded  carbon  atoms  are  important  from  the  stand- 
point of  drying.  Also  an  interesting,  although  perhaps  un- 
expected fact  is  that  trebly  linked  carbon  atoms  do  not,  under 
ordinary  conditions,  absorb  halogens  or  oxygen  to  the  point  of 
complete  saturation,  only  two  atoms  of  halogen  or  one  of  oxygen 
adding  to  each  such  pair.  Thus,  the  acids  of  the  tariric  series, 
CnH2n-4O2,  are  isomeric  with  those  of  the  linolic  series.  Tariric 
acid,  Ci8H3202,  is  isomeric  with  linolic  acid.  Its  unsaturated 
state  is  shown  by  the  formula,  CH3(CH2)10C=C(CH2)4COOH. 
This  and  other  acids  of  the  series,  or  their  glycerides,  absorb 
two  halogen  atoms  quite  readily  but  are  not  oxidized  upon  ex- 
posure to  air.  They  are  therefore  quite  unimportant  as  con- 
stituents of  the  drying  oils.  After  the  treble  linking  has  been  half 
saturated  by  the  absorption  of  halogens  the  remaining  double 
bond  becomes  saturated  but  very  slowly  and  this  property  is 
partly  responsible  for  the  fact  that,  to  some  extent,  the  iodine 
absorption  number  is  a  function  of  the  time  allowed  for  the  re- 
action and  of  the  nature  of  the  halogen  solution. 

Many  of  the  methods  for  determining  iodine  absorption  num- 
ber have  been  open  to  the  objection  that  they  permit  more  or 
less  substitution  in  saturated  compounds  as  well  as  addition  to 
unsaturated  compounds.  Hubl's1  method,  formerly  much  used, 
is  especially  faulty  in  this  respect.  Hubl's  solution  is  made  by 
dissolving  26  gm  of  iodine  in  500  cc  of  alcohol  and  30  gm  of 
mercuric  chloride  in  500  cc  of  alcohol,  the  two  solutions  being 
then  mixed.  The  resulting  solution  probably  contains2  some 
mercuric  chloriodide  and  iodine  monochloride,  the  reaction 
being  expressed  as  follows: 

HgCl2+l2->HgICl+ICl. 

The  latter  is  the  active  constituent  of  the  solution  but  its  con- 
centration is  relatively  small,  which  accounts  for  the  fact  that 
much  time  is  required  for  the  absorption.  The  oil  is  dissolved 
in  chloroform  and  allowed  to  stand  with  a  measured  volume  of 

1  Dingl.  polyt.  J.,  263,  281  (1884);  J.  Soc.  Chem.  Ind.,  3,  641  (1884). 

2  Ephriam:  Z.  angew.  Chem.  (1895),  254. 


366  QUANTITATIVE  ANALYSIS 

the  solution  for  three  hours,  after  which  the  excess  of  iodine  is 
titrated.  Substitution  takes  place  to  a  considerable  extent 
and  the  amount  of  iodine  absorbed  varies  with  the  time  allowed. 
When  hydrogen  in  a  saturated  ester  is  substituted  by  iodine, 
hydriodic  acid  is  also  formed.  In  the  case  of  palmitin: 


By  determining  the  amount  of  hydriodic  acid  so  formed  the 
amount  of  substitution  may  be  determined  but  it  is  better  to 
use  one  of  the  solutions  suggested  by  Hanus  and  Wijs  because 
these  cause  very  little  substitution. 

Hanus'1  solution  is  made  by  dissolving  iodine  in  glacial  acetic 
acid  and  adding  an  equivalent  weight  of  bromine.  The  active 
constituent  is  iodine  monobromide,  IBr.  The  oil  is  dissolved  in 
chloroform. 

Wijs'2  solution  contains  iodine  monochloride,  IC1,  and  is  made 
by  adding  an  equivalent  amount  of  chlorine  to  a  solution  of  iodine 
in  glacial  acetic  acid.  Either  chloroform  or  carbon  tetrachloride 
is  used  as  the  solvent  for  the  oil. 

Both  Hanus7  and  Wijs7  solutions  are  more  active  than  that  of 
Hiibl,  the  absorption  being  completed  in  thirty  minutes.  The 
solutions  are  also  more  stable  and  need  not  be  so  frequently 
restandardized.  The  amount  of  substitution  taking  place  is 
also  much  less  and  is  practically  zero  with  many  oils.  The  solu- 
tion of  Hanus  is  more  conveniently  made  than  that  of  Wijs  and 
the  method  of  Hanus  will  therefore  be  described.3 

Determination.  —  Prepare  an  iodine  monobromide  solution  as  follows: 
Equivalent  quantities  of  bromine  and  iodine  are  dissolved  in  glacial 
acetic  acid  in  such  a  ratio  as  to  make  a  solution  somewhat  more  than 
fifth-normal,  referred  to  total  halogen.  The  glacial  acetic  acid  is  first 
tested  to  insure  absence  of  reducing  substances.  A  drop  of  sulphuric 
acid  and  two  or  three  drops  of  tenth-normal  potassium  dichromate  solu- 
tion are  added  to  10  cc  of  the  acetic  acid  and  the  mixture  is  warmed. 
The  yellow  color  should  persist  without  the  appearance  of  green  chromium 
salts. 

1  Z.  nahr.  Genussm.,  4,  913  (1901). 

2  Ber.,  31,  750  (1898). 

8  See  a  comparison  of  methods  by  Tolman  and  Munson:  J.  Am.  Chem. 
Soc.,  26,  244  (1903);  26,  826  (1904). 


OILS,  FATS  AND  WAXES  367 

Dissolve  13.6  gm  of  powdered  iodine  in  825  cc  of  glacial  acetic  acid, 
warming  the  flask  if  necessary.  Cool,  decant  to  insure  that  no  particles 
of  iodine  remain  undissolved,  and  mix.  Measure  from  a  burette  25  cc  of 
the  solution  into  a  250  cc  Erlenmeyer  flask,  add  10  cc  of  15  percent 
sodium  iodide  solution  and  100  cc  of  water  and  mix.  Titrate  at  once 
with  tenth-normal  sodium  thiosulphate  solution. 

From  a  small  burette  measure  3  cc  of  bromine  into  200  cc  of  glacial 
acetic  acid.  Mix  and  titrate  5  cc  of  the  solution  against  sodium  thiosul- 
phate solution,  adding  sodium  iodide  and  water  as  in  the  iodine  titration. 
Calculate  the  volume  of  tenth-normal  thiosulphate  solution  that  would 
be  equivalent  to  800  cc  of  iodine  solution,  then  calculate  the  volume 
of  bromine  solution  that  would  be  equivalent  to  this  volume  of  thiosul- 
phate solution.  Add  this  quantity  of  bromine  solution  to  the  iodine  in 
a  glass  stoppered  bottle  and  mix  well. 

Prepare  starch  solution,  also  prepare  and  standardize  a  tenth-normal 
solution  of  sodium  thiosulphate  according  to  the  directions  given  on 
page  263. 

Half  fill  a  20  cc  weighing  bottle  with  oil,  place  in  it  a  piece  of  glass 
rod  and  weigh  without  the  stopper.  Carefully  pour  about  0.25  gm  of 
the  oil  into  a  500  cc  bottle  having  a  ground  glass  stopper,  using  the  glass 
rod  to  assist  in  the  transference.  Reweigh  and  prepare  two  more 
samples  in  the  same  manner. 

Dissolve  the  weighed  sample  of  oil  in  10  cc  of  chloroform  then  add 
25  cc  of  iodine  monobromide  solution,  measuring  from  a  pipette.  Stop- 
per the  bottle,  mix  and  allow  to  stand  for  thirty  minutes,  shaking 
occasionally.  The  bottle  should  not  be  left  in  strong  light. 

At  the  time  that  the  iodine  monobromide  solution  is  measured  into 
the  oil  solution,  measure  the  same  amount  of  solution  into  two  bottles, 
containing  the  chloroform  but  no  oil.  •  Treat  these  in  exactly  the  same 
manner  as  the  solution  containing  oil. 

At  the  end  of  the  absorption  period  add  10  cc  of  the  sodium  iodide 
solution  which  was  used  in  standardizing  the  sodium  thiosulphate  solu- 
tion. Add  100  cc  of  water,  washing  down  any  iodine  that  may  be  on 
the  stopper.  Titrate  the  unabsorbed  iodine  with  standard  sodium 
thiosulphate,  shaking  constantly.  When  only  a  faint  yellow  remains 
add  1  cc  of  starch  solution  and  finish  the  titration.  At  the  last  the  bottle 
should  be  closed  and  shaken  until  all  iodine  remaining  in  the  chloroform 
has  been  extracted  by  the  potassium  iodide.  The  temperature  should 
be  kept  as  nearly  constant  as  possible  throughout  the  experiment. 

From  the  volume  of  sodium  thiosulphate  required  for  the  iodine  solu- 
tion alone  subtract  that  required  for  the  oil  and  iodine  solutions.  The 
remainder  is  the  volume  corresponding  to  the  absorbed  iodine.  Calcu- 
late the  percent  of  iodine  absorbed. 


368  QUANTITATIVE  ANALYSIS 

Acid  Value. — Fresh  oils  sometimes  contain  small  amounts  of 
free  fatty  acids  produced  during  the  process  of  extraction. 
Rancid  fats  and  oils  contain  free  acids  as  products  of  hydroly- 
sis of  the  glycerides  composing  them.  The  acid  value  is  defined  as 
the  number  of  milligrams  of  potassium  hydroxide  required  to 
neutralize  the  free  fatty  acids  in  1  gm  of  oil  or  fat.  Acidity  is  also 
sometimes  expressed  in  terms  of  oleic  acid  as  percent,  or  as  "acid 
degree,"  which  is  cubic  centimeters  of  normal  base  equivalent  to  the 
free  acids  in  100  gm  of  oil  or  fat.  The  determination  of  acid 
value  is  made  for  the  purpose  of  determining  the  condition  of  the 
oil  and  its  fitness  for  a  given  use,  rather  than  for  the  purpose  of 
identifying  it,  since  the  acid  value  is  a  variable  within  rather  wide 
limits  for  any  oil. 

Determination. — Weigh  20  gm  of  oil  or  fat  into  a  200  cc  flask  and  add 
50  cc  of  95  percent  alcohol  which  has  been  made  neutral  to  phenolph- 
thalein  by  a  dilute  solution  of  so.dium  hydroxide.  Heat  to  the  boiling- 
point  in  a  steam  bath  and  agitate  thoroughly.  Titrate  with  a  tenth-nor- 
mal solution  of  sodium  or  potassium  hydroxide  using  phenolphthalein. 
Shake  vigorously  during  the  titration  and  add  the  standard  solution 
until  the  pink  color  persists. 

Saponification  (Kottstorfer)  Number. — The  saponification 
number1  is  the  number  of  milligrams  of  potassium  hydroxide 
required  to  saponify  1  gm  of  oil  or  fat.  Different  oils  show  differ- 
ent saponification  numbers  because  of  variation  in  the  molecular 
weight  of  the  esters  contained  in  them,  those  of  relatively  low 
average  molecular  weights  requiring  more  base  for  the  saponifica- 
tion of  a  given  weight  of  oil  than  those  of  relatively  higher  mo- 
lecular weights.  The  variation  is,  however,  not  as  great  as  is  the 
case  with  iodine  absorption  numbers  and  the  saponification  num- 
ber is  consequently  not  as  valuable  for  use  in  identifying  oils  as  is 
the  iodine  number.  The  comparatively  small  variation  in  saponi- 
fication number  is  due  to  the  relatively  small  variation  in  the 
average  molecular  weight  of  the  esters  entering  into  the  compo- 
sition of  the  oils.  Of  the  whole  list  of  the  more  common  oils 
nearly  all  are  chiefly  composed  of  stearin,  palmitin  and  olein,  the 
molecular  weights  of  these  being  890,  806  and  884,  respectively 
and  their  equivalent  weights  are  one-third  of  these  numbers. 
The  saponification  numbers  of  the  pure  esters  would  be  as  follows : 

1  Z.  anal.  Chem.,  18,  199  and  431  (1878). 


OILS,  FATS  AND  WAXES  369 

rn  r£» 

for   stearin   ^95  X  1000=189,    for    palmitin   ^9  X  1000  =  208, 

rs* 

and  for  olein  ^rr  X  1000  =  190.    The  greatest  possible  variation  in 


the  proportions  of  these  three  esters  could  make  a  difference  of  but 
19  in  the  saponification  numbers.  The  occurrence  of  appreciable 
quantities  of  esters  of  lower  acids  in  certain  oils  causes  a  much 
greater  deviation  from  the  numbers  given  above.  For  example, 
butter  fat  is  chiefly  composed  of  the  following  glycerides  in  the 
approximate  proportions  indicated:  butyrin,  C3H6(C4H7O2)3,  7.0 
percent;  caproin,  CsHXCeHnC^s,  caprylin,  CsEWCsHuC^s  and 
caprin,  CsHsCCioHigC^s,  2.3  percent;  olein  37.7  percent;  palmitin, 
stearin  and  glycerides  of  small  quantities  of  other  acids  53.0  per- 
cent. The  calculated  saponification  number  of  butyrin  is  554,  of 
caproin  434,  of  caprylin  356  and  of  caprin  302.  The  presence  of 
these  esters  of  small  molecular  weight  raises  the  saponification 
number  to  about  227,  a  number  which  serves  to  distinguish  butter 
fat  from  a  large  number  of  other  fats,  particularly  from  oleomar- 
garine which  has  a  saponification  number  of  about  195. 

On  the  other  hand  the  saponification  numbers  of  true  waxes  are, 
on  the  whole,  considerably  lower  than  those  of  oils  or  fats.  As 
noted  on  page  355,  the  characteristic  difference  between  oils  and 
fats,  on  the  one  hand,  and  waxes  on  the  other  is  that  the  latter 
consist  of  esters  of  higher  fatty  acids  with  monohydric  and  di- 
hydric  alcohols  instead  of  with  the  trihydric  alcohol,  glycerol. 
These  esters  have  higher  equivalent  weights  than  those  of  the 
glycerides  and  the  saponification  numbers  are  correspondingly 
lower. 

Thus,  spermaceti  consists  chiefly  of  cetin  (cetyl  palmitate), 
CieHasO.CO.CisHsi,  mixed  with  smaller  proportions  of  other 
esters  and  possibly  of  free  alcohols.  Cetyl  palmitate  is  an  ester 

of  palmitic  acid  and  the  monohydric  cetyl  alcohol.     Its  theoret- 

fft 

ical  saponification  number  is  117  (  =  |oQXlOOO)  and  this  gives 

spermaceti  the  actual  saponification  number  of  122  to  129,  which 
at  once  serves  to  characterize  it  as  a  true  wax. 

Beeswax  may  be  noticed  as  another  example.  This  wax  is 
largely  composed  of  a  mixture  of  cerotic  acid,  C25H5iCOOH,  and 
myricyl  palmitate,  C3oH6iO.CO.Ci5H3i.  The  saponification 


24 


370  QUANTITATIVE  ANALYSIS 

r  £» 

number  of  free  cerotic  acid  is  141  (  =  ^r=r  X  1000)    and    that   of 


» 

myricyl  palmitate  is  83   (  =  ^^  XI  000).     It    might     therefore 

be  expected  that  beeswax  would  have  an  abnormally  low  saponi- 
fication  number  and  this  also  on  account  of  the  presence  of  about 
10  to  15  percent  of  unsaponifiable  hydrocarbons.  The  actual 
saponification  number  is  found  to  be  about.  94. 

Water  solutions  of  potassium  hydroxide  act  upon  oils  very 
slowly  because  of  the  small  solubility  of  most  oils  in  water. 
Hot  alcohol  dissolves  oils  more  readily  and  alcoholic  solutions 
of  potassium  hydroxide  are  therefore  used  for  the  saponification. 
Commercial  alcohol  contains  aldehydes  which  are  changed  by 
potassium  hydroxide  into  resinous  bodies,  a  dark  red  solution 
being  produced  and  the  basic  concentration  being  diminished. 
The  alcohol  should  therefore  be  purified  by  first  heating  with  a 
stick  of  potassium  hydroxide  in  a  flask  fitted  with  a  reflux  con- 
denser and  placed  on  a  water  or  steam  bath,  then  distilling. 

Insoluble  Acids  (Hehner  Value)  and  Soluble  Acids.  —  The 
determination  of  the  saponification  number  may  be  conveniently 
combined  with  the  determination  of  soluble  acids  and  insoluble 
acids.  Among  the  most  important  of  the  acids  of  smaller 
molecular  weight  than  oleic  acid  combined  as  glycerides  are 
butyric,  caproic,  caprylic  and  capric  acids,  discussed  above. 
These  acids  are  soluble  in  water,  the  solubility  decreasing  as  the 
molecular  weight  increases,  so  that,  while  butyric  acid  is  infi- 
nitely soluble,  capric  acid  dissolves  to  the  extent  of  1  part  in  1000 
parts  of  boiling  water.  The  next  acid  in  the  series,  lauric  acid, 
is  almost  insoluble  while  the  next  member,  myristic  acid,  is 
practically  insoluble.  An  approximate  separation  of  the  lower 
acids  from  the  higher  ones  may  be  accomplished  by  saponifying 
the  oil,  decomposing  the  resulting  soap  with  sulphuric  acid  and 
washing  the  fatty  acids  with  water.  The  percent  of  insoluble 
acids  is  called  the  Hehner  value.1 

An  inspection  of  the  formula  for  a  typical  triglyceride,  as  that 
of  palmitin,  C3H5(OCi6H3iO)3,  shows  that  the  acid  residue 
comprises  the  greater  part  of  the  compound.  Also  since  the 
variation  in  the  molecular  weights  of  the  three  acids,  palmitic, 

1  Z.  anal.  Chem.,  16,  145  (1877). 


OILS,  FATS  AND  WAXES  371 

stearic  and  oleic,  which  make  the  greater  part  of  the  acids  of 
most  oils  and  fats,  is  small  as  compared  with  the  molecular 
weights  themselves,  it  is  not  to  be  expected  that  there  would  be  a 
large  variation  in  either  the  Hehner  value  or  the  percent  of  soluble 
acids.  The  former  has  an  average  value  of  about  95  and  the 
latter  of  considerably  less  than  1.  Therefore  these  numbers  are 
without  any  great  significance  in  most  cases  and  their  determina- 
tion will  give  little  assistance  in  the  task  of  identifying  most 
oils.  A  few  exceptions- to  this  statement  should  be  noticed. 

Butter  has  already  been  mentioned  as  containing  unusually 
large  quantities  of  butyric,  caproic,  caprylic  and  capric  acids. 
Consequently  its  Hehner  value  falls  to  88-90  and  its  percent  of 
soluble  acids  rises  to  about  5.  Other  notable  exceptions  are 
cocoanut,  palm  nut,  croton  and  porpoise  oils.  Practically,  it  is 
in  these  cases  only  that  the  determination  of  soluble  and  insoluble 
acids  will  be  of  any  great  use. 

Determination. — Prepare  a  tenth-normal  solution  of  sodium  or  potas- 
sium hydroxide  in  boiled  and  cooled  water,  using  phenolphthalein  in 
standardizing.  Purify  two  liters  of  alcohol  by  heating  on  a  steam  bath 
for  thirty  minutes  with  about  10  gm  of  sodium  hydroxide,  using  a  reflux 
condenser.  Distill  and  make  1000  cc  of  a  solution  of  40  gm  of  potassium 
hydroxide  in  the  alcohol.  The  potassium  hydroxide  should  be  as  nearly 
free  from  carbonate  as  is  possible.  Allow  the  solution  to  stand  until 
the  small  amount  of  potassium  carbonate  that  is  always  present  has 
settled  out,  then  decant  into  another  bottle.  The  concentration  does 
not  remain  constant  for  long  and  the  solution  need  not  be  standardized 
until  it  is  used  for  saponifying  the  oil. 

Prepare  also  a  half-normal  solution  of  hydrochloric  acid  in  water. 

Saponification  Number. — Select  two  ordinary  flasks  of  250  cc  capacity 
having,  if  possible,  necks  of  slightly  larger  diameter  at  the  top  than  at 
the  bottom,  though  this  feature  is  not  essential.  Clean  with  alcohol. 
Weigh  into  each  flask  about  5  gm  of  oil  or  fat,  using  a  small  bottle  and 
glass  rod  as  in  the  determination  of  iodine  number.  Add  to  each  flask 
50  cc  of  the  alcoholic  solution  of  potassium  hydroxide  from  a  calibrated 
pipette  or  burette,  place  in  the  neck  of  the  flask  a  funnel  having  a  short 
stem  and  warm  on  the  water  bath  until  the  alcohol  boils,  though  it 
should  not  be  evaporated  more  than  is  necessary.  The  oil  is  usually 
saponified  in  about  thirty  minutes.  A  homogeneous  solution  must  be 
produced,  so  that  no  separation  will  occur  when  boiling  is  interrupted. 
Measure  50  cc  of  the  alcohol  solution  of  potassium  hydroxide  into  each 


372  QUANTITATIVE  ANALYSIS 

of  two  other  flasks,  for  standardization.  While  saponification  of  the  oils 
is  proceeding  titrate  these  solutions  with  the  half-normal  acid,  using 
phenolphthalein.  Cool  the  flasks  in  which  the  oil  was  saponified,  add  a 
drop  of  phenolphthalein  and  titrate  the  excess  of  base  with  half-normal 
acid,  deduct  from  the  volume  used  for  50  cc  of  the  base  in  the  standard- 
ization and  calculate  the  saponification  number.  Preserve  the  neutral 
solution  for  the  determination  of  soluble  and  insoluble  acids. 

Soluble  Acids. — Evaporate  the  alcohol  by  placing  the  flasks  upon  a 
water  bath  and  drawing  air  through  them  as  explained  on  page  355. 
When  the  odor  of  alcohol  has  entirely  disappeared  add  enough  standard 
acid  to  make  the  total  amount,  including  that  used  in  titrating  the  excess 
of  base,  1  cc  more  than  the  volume  that  is  equivalent  to  the  50  cc  of 
potassium  hydroxide  used  in  saponifying  the  oil.  It  is  necessary  to  be 
very  careful  about  the  removal  of  all  alcohol  at  this  point.  The  acids 
that  are  classed  as  insoluble  are  considerably  more  soluble  in  alcohol. 
Consequently  any  alcohol  that  might  be  left  with  the  soap  would  cause 
an  error  in  the  separation  of  soluble  from  insoluble  acids. 

Connect  a  reflux  condenser  and  warm  on  the  water  bath  until  the 
insoluble  fatty  acids  have  melted  and  separated  from  the  water  solution. 
Add  hot  water  to  bring  the  liquid  within  about  2  cm  of  the  top  of  the 
neck  of  each  flask,  again  allow  the  insoluble  acids  to  separate,  then  cool 
in  ice  water.  Carefully  detach  the  cake  of  insoluble  acids  and  pour  the 
cold  solution  through  a  filter  into  a  flask  of  1000  cc  capacity.  Replace 
the  cake  of  acids  in  the  flask,  fill  with  hot  water,  separate  as  before 
and  filter.  Repeat  the  treatment  once  more  and  do  not  discard  the 
insoluble  acids. 

In  some  cases  the  insoluble  acids  will  not  solidify,  even  at  0°,  on 
account  of  the  preponderance  of  oleic  acid,  a  liquid  acid.  In  such 
cases  the  entire  liquid  is  poured  into  an  already  wet  filter  and  the  flask 
and  the  insoluble  acids  on  the  filter  are  washed  with  cold  water. 

The  combined  filtrates  now  contain  the  excess  of  half-normal  acid 
(1  cc)  and  the  soluble  acids  of  the  oil,  besides  potassium  chloride, 
glycerine  and  other  alcohols,  etc.  Titrate  the  acids  with  tenth-normal 
base  in  presence  of  phenolphthalein.  From  the  volume  of  standard  base 
used  deduct  the  volume  equivalent  to  1  cc  of  the  standard  acid  and  cal- 
culate the  percent  of  soluble  acids  as  butyric  acid.  The  arbitrary 
assumption  that  butyric  acid  is  the  only  soluble  acid  present  is  merely 
a  convenience. 

Insoluble  Acids  (Hehner  Value}. — Allow  the  cake  of  insoluble  acids  to 
dry  on  the  filter  paper  for  twelve  hours  at  the  temperature  of  the  room, 
then  transfer  to  a  small  weighed  dish.  Wash  thoroughly  with  warm 
alcohol  the  paper  and  the  flask  in  which  saponification  was  accomplished, 
allowing  the  solution  to  run  into  the  dish.  Evaporate  the  alcohol  ou  the 


OILS,  FATS  AND  WAXES  373 

steam  bath  and  dry  to  constant  weight  at  100°.  From  2  to  5  hours  dry- 
ing will  usually  be  required. 

If  the  insoluble  acids  did  not  solidify  the  filter  is  pierced  and  the 
acids  are  allowed  to  run  into  a  weighed  dish.  The  flask  and  paper  are 
washed  thoroughly  with  hot  alcohol,  this  running  into  the  dish.  Evapo- 
rate and  dry  as  with  solid  acids. 

Calculate  the  percent  of  insoluble  acids. 

Reichert  Number  and  Reichert-Meissl  Number. — There  is  no 
sharp  line  of  division  between  the  fatty  acids  volatile  with  steam 
and  those  not  volatile  and  it  is  not  possible  to  effect  more  than  a 
very  approximate  separation  by  a  method  of  distillation  unless 
this  is  continued  for  a  very  long  time.  On  the  other  hand 
fairly  constant  proportions  of  acids  may  be  distilled  if  the  method 
is  rigidly  standardized.  In  this  way  figures  may  be  obtained 
that  have  a  value  in  identifying  certain  oils  and  fats.  The 
determination  is  made  chiefly  in  the  examination  of  butter  and 
its  substitutes.  Pure  butter  contains  volatile  acids  to  the  extent 
of  nearly  10  percent  of  the  total  fatty  acids. 

The  saturated  acids  that  have  been  classed  as  " soluble"  (to 
and  including  capric  acid)  are  the  only  ones  of  the  series  that  may 
be  distilled  without  decomposition.  They  are  therefore  known 
as  " volatile"  acids  while  the  higher  acids  (above  lauric)  decom- 
pose when  distilled  and  are  therefore  called  "  non-volatile." 
Lauric  acid  distills  with  steam  but  is  slightly  decomposed.  Al- 
though the  volatile  acids  boil  at  temperatures  higher  than  100° 
they  can  be  distilled  with  steam.  The  boiling  points  of  the  more 
commonly  occurring  " volatile"  acids  are  as  follows: 


Acid 

Boiling  point,  degrees 

Butyric 

162  3 

Caproic  

200 

Caprvlic 

236 

Capric  

270 

The  method  proposed  by  Reichert1  and  the  modifications  of 
this  method  by  Meissl2  have  been  extensively  adopted.  It 
should  be  understood  that  neither  method  gives  the  correct 
percent  of  volatile  acids  but  simply  the  proportion  that  will  be 

1  Z.  anal.  Chem.,  18,  68  (1879). 

2  Dingl.  polyt.  J.f  233,  229  (1879);  Chem.  Zentr.,  10,  586  (1879). 


374 


QUANTITATIVE  ANALYSIS 


distilled  under  certain  stated  conditions.  The  Reichert  Number 
is  the  number  of  cubic  centimeters  of  tenth-normal  base  required  to 
titrate  the  adds  obtained  from  2.5  gm  of  oil  or  fat  by  Reichert's 
distillation  process.  The  Reichert-Meissl  number  is  the  same 
as  the  Reichert  number  except  that  5  gm  of  oil  or  fat  is  used.  The 
Reichert-Meissl  number  is  not  exactly  double  the  Reichert 
number. 

The  Reichert-Meissl  number  of  most  oils,  fats  and  waxes  is 
less  than  1  and  the  determination  will  be  of  little  service  in 
identifying  these  oils.  The  following  oils  are  exceptional  in 
this  respect. 


Oil  or  fat 

Reichert-Meissl  number 

Butter  fat  

28 

Cocoanut  

7 

Croton        

13 

Mocaya 

7 

Palmnut  

5 

Porpoise  

47 

Determination. — Prepare  the  following  reagents : 

(a)  Sodium  hydroxide  solution  in  water,  50  percent  by  weight. 

(b)  Alcohol,    95    percent,    redistilled    from    sodium    or    potassium 
hydroxide. 

(c)  Sulphuric  acid,  1  part  concentrated  acid  in  5  parts  water. 

(d)  Potassium  hydroxide,  approximately  tenth-normal,  standardized 
against  standard   acid,   using  phenolphthalein  as  indicator. 

If  the  sample  is  either  real  or  imitation  butter  it  will  contain  water 
and  curd.  Melt  and  keep  at  60°  until  the  fat  has  separated  and,  if 
necessary,  filter  the  fat  through  a  dry  paper  placed  in  a  hot-water 
funnel.  If  the  sample  is  an  oil  it  may  usually  be  weighed  without 
treatment. 

Ordinary  flasks  of  200  cc  capacity,  are  cleaned  and  dried.  The  oil 
or  melted  fat  is  dropped  in  from  a  weighed  bottle  until  5  gm,  measured 
to  within  one  drop,  is  obtained.  The  oil  must  not  be  left  on  the  neck  of 
the  flask.  Record  the  exact  weight.  Add  10  cc  of  alcohol  and  2  cc  of  50 
percent  sodium  hydroxide  solution,  connect  with  a  reflux  condenser  and 
heat  upon  the  steam  bath  until  the  oil  is  saponified.  Remove  the  con- 
denser and  evaporate  the  alcohol  as  in  the  determination  of  soluble  acids. 
Add  135  cc  of  recently  boiled  water  and  warm  on  the  water  bath  until 
solution  is  complete,  then  cool.  Add  two  or  three  pieces  of  pumice 
stone  or  about  1  gm  of  crushed  porcelain  to  prevent  bumping,  then  add 
5  cc  of  the  diluted  sulphuric  acid.  Again  attach  the  reflux  condenser 


OILS,  FATS  AND  WAXES 


375 


FIG.  94. — Kjeldahl's  distilling  tube. 


and  heat  on  the  steam  bath  until  the  acids  form  a  clear  layer.  Connect 
the  flask  with  a  Kjeldahl  or  Hopkins  distilling  tube  and  a  condenser 
and  distill  over  a  flame  at  such  a  rate  that  110  cc  shall  be  obtained  in 
approximately  thirty  minutes.  The 
distillate  is  received  in  a  flask 
which  is  graduated  to  contain  110 
cc.  Mix  the  distillate,  and  filter 
through  a  dry  filter  to  remove 
traces  of  insoluble  acids  carried 
over  by  the  steam,  receiving  the 
filtrate  in  a  flask  graduated  to  con- 
tain 100  cc.  Titrate  100  cc  of  the 
filtrate  with  standard  potassium 
hydroxide.  Make  the  proper  cor- 
rection for  the  fact  that  only  100  cc 
of  the  distillate  was  used,  also  cor- 
rect the  number  of  cubic  centimeters 
of  standard  potassium  hydroxide 
used,  in  case  this  solution  was  not 
exactly  tenth-normal  or  in  case  the 
sample  weight  was  not  exactly  5 
gm.  The  result  is  the  Reichert-Meissl  number. 

Polenske  Value.1 — One  of  the  very  important  constituents 
of  some  butter  substitutes  is  cocoanut  oil,  a  pure  white  vegetable 
fat  having  a  pleasant  taste  and  a  consistency  which  is  about  the 
same  as  that  of  butter.  Its  Reichert-Meissl  number  is  somewhat 
lower  than  that  of  butter,  as  is  shown  in  the  table  on  page  374. 
Salkowski2  noticed  that  the  volatile  acids  obtained  from  cocoanut 
oil  in  the  Reichert-Meissl  distillation  contained  much  larger 
quantities  of  acids  insoluble  at  15°  than  do  the  volatile  acids 
from  butter.  Butyric  acid  comprises  from  60  percent  to  70 
percent  of  the  volatile  acids  from  butter  and  this  acid  is  soluble 
in  water  in  all  proportions.  The  volatile  acids  from  cocoanut 
oil  contain  larger  quantities  of  caproic,  caprylic,  capric  and  lauric 
acids,3  these  being  almost  insoluble  at  15°.  The  Polenske 
value  (called  by  its  author  the  "new  butter  value")  is  the  num- 

*Z.  Nahr.  Genussm.,  7,  273  (1904);  J.  Soc.  Chem.  Ind.,  23,  387  (1904). 

2  Z.  anal.  Chem.,  26,  581  (1887). 

3  Elsdon:  Analyst,  38,  8   (1913).     Percents  of  acids  here  reported   are 
caproic  2,  caprylic  9,  capric  10,  lauric  45,  myristic  2,  palmitic  7,  stearic  5, 
and  oleic  2. 


376  QUANTITATIVE  ANALYSIS 

ber  of  cubic  centimeters  o]  decinormal  base  required  to  titrate  the 
insoluble  acids  obtained  in  the  Reichert-Meissl  distillation. 

The  Polenske  value  for  pure  butter  varies  from  1.5  to  3.0, 
while  that  for  cocoanut  oil  varies  from  16  to  18. 

It  is  necessary  to  avoid  the  use  of  alcohol  in  the  saponification 
of  the  fat  and  therefore  the  determination  of  Reichert-Meissl 
number  must  be  modified  if  the  two  determinations  are  to  be 
combined.  Polenske's  modification  is  essentially  as  follows: 

Determination. — Saponify  5  gm  of  the  fat  by  heating  in  a  300-cc  round 
flask,  using  a  reflux  condenser.  For  the  saponification  use  20  gm  of  glyc- 
erol  and  2  cc  of  a  50  percent  solution  of  sodium  hydroxide  in  water. 
When  saponification  is  complete  dissolve  the  soap  in  135  cc  of  recently 
boiled  water  and  add  25  cc  of  dilute  sulphuric  acid  (50  cc  of  concentrated 
acid  in  1000  cc  of  solution)  and  a  small  amount  of  crushed  porcelain 
or  pumice.  Connect  with  a  condenser  by  means  of  a  Kjeldahl  or  Hop- 
kins distilling  tube  and  distill  into  a  flask  which  is  graduated  at  100  cc 
and  110  cc;  the  distillation  should  proceed  at  such  a  rate  that  110  cc 
passes  over  in  about  20  minutes.  When  the  distillate  reaches  the  1 10  cc 
mark  on  the  flask  replace  the  latter  by  a  25  cc  cylinder  and  stop  the 
distillation.  Immerse  the  flask  in  water  at  15°  and  allow  to  remain  for 
15  minutes.  The  level  of  the  water  must  be  above  the  110  cc  mark  on 
the  flask.  Mix  the  contents  of  the  flask  and  pass  through  a  dry,  8  cm 
filter  and,  if  desired,  determine  the  Reichert-Meissl  number,  using  100  cc 
of  the  filtrate.  Rinse  the  110-cc  flask  but  without  removing  any  of  the 
insoluble  acids  adhering  to  it.  Wash  the  filter  three  times  with  15  cc 
of  water,  this  water  having  previously  been  used  for  washing  the  con- 
denser, cylinder  and  flask.  Dissolve  the  insoluble  acids  from  the  con- 
denser, cylinder  and  filter,  using  three  successive  portions  of  neutral 
90  percent  alcohol  and  allowing  the  solution  to  run  into  the  110  cc  flask. 
Titrate  the  alcoholic  solution  with  decinormal  potassium  hydroxide 
solution,  using  phenolphthalein,  and  calculate  the  Polenske  value. 

Acetyl  Value. — Compounds  containing  a  hydroxyl  group  will 
readily  combine  with  acetic  anhydride,  acetic  acid  and  an  acetyl 
compound  being  produced.  This  takes  place  with  an  oil  con- 
taining free  higher  alcohols  or  hydroxy-acids,  the  latter  either  in 
the  form  of  esters  or  of  free  acids.  The  general  reaction  may  be 
thus  shown: 

RCHOHCOOH + (CH3CO)  20-^RCHOCH3COCOOH + 

CH3COOH, 

ROH  +  (CH8CO)  20->ROCH3CO + CH8COOH. 


OILS,  FATS  AND  WAXES  377 

For  example  lanopalmic  acid  forms  acetolanopalmic  acid: 

C15H30OHCOOH  +  (CH3CO)  2O^Ci5H3oOCH3COCOOH  + 

CHsCOOH. 

After  washing  out  the  excess  of  acetic  anhydride  the  amount 
absorbed  may  be  determined  by  saponifying  the  oil  with  an 
alcohol  solution  of  potassium  hydroxide,  evaporating  the  alcohol, 
adding  standard  sulphuric  or  hydrochloric  acid  to  liberate  the 
acetic  and  fatty  acids  and  either  distilling  the  acetic  acid  or 
washing  out  with  water,  then  titrating.  The  reactions  illustrated 
by  the  case  of  aceto-lanopalmitin  are 


3Ci5H3oOHCOOK+3CH3COOK, 


2CH3COOK+H2SO4^CH3COOH+K2S04. 

Effect  of  Soluble  or  Volatile  Acids.  —  It  should  be  noticed  that 
whether  the  distillation  or  the  filtration  process  is  employed, 
the  standard  base  required  to  finally  titrate  the  acid  will  include 
that  equivalent  to  acids  other  than  acetic.  That  is,  the  distilla- 
tion process  will  yield  a  distillate  of  acetic  acid  and  volatile  or- 
ganic acids  while  the  filtration  process  will  yield  a  filtrate  con- 
taining acetic  acid  and  soluble  organic  acids.  The  close  relation 
between  soluble  acids  and  volatile  acids  has  already  been  dis- 
cussed (page  373).  To  correct  for  the  presence  of  these  acids  in 
the  solution  containing  the  acetic  acid  one  may  either  subtract 
the  volume  of  base  used  in  the  determination  of  soluble  (or  vola- 
tile) acids,  or  a  different  method  may  be  used.  As  a  rule  this 
correction  will  be  small  but  with  oils  showing  a  high  soluble- 
acid  number  or  Reichert-Meissl  number,  failure  to  apply  the  cor- 
rection may  result  in  serious  error. 

Benedikt  and  Ulzer1  proposed  first  saponifying  the  oil  and  then 
liberating  the  fatty  acids  by  the  addition  of  sulphuric  acid.  After 
washing  the  fatty  acids  they  are  acetylated  and  the  excess  of 
acetic  anhydride  removed.  The  acids  are  then  titrated  in  cold 
alcoholic  solution,  under  which  circumstances  the  carboxyl 
alone  reacts  with  the  base.  The  acetylated  soap  is  then  heated 

1  Monatsh.,  8,  41  (1887). 


378  QUANTITATIVE  ANALYSIS 

with  alcoholic  potassium  hydroxide  when  the  acetyl  radical  is 
saponified.  A  titration  of  the  excess  of  base  gives  the  acetyl 
value.  This  method  avoids  the  interference  of  soluble  fatty 
acids  but,  as  was  shown  by  Lewkowitsch1  it  is  subject  to  another 
error  in  the  fact  that  acetic  anhydride  also  reacts,  to  a  small 
extent,  with  non-hydroxylated  fatty  acids  forming  acetic  acid 
and  fatty  acid  anhydride: 

2C15H3iCOOH+(CH3CO)20-^(Ci5H3iCO)20+2CH3COOH. 

Palmitic  acid  Acetic  anhydride       Palmitic  anhydride  Acetic  acid 

In  cold  alcoholic  solution  these  anhydrides  are  not  at  once  saponi- 
fied, part  remaining  until  the  treatment  with  hot  potassium 
hydroxide  solution,  being  then  saponified  and  giving  rise  to  a 
positive  error  in  the  calculation  of  acetyl  value. 

The  most  desirable  method  is  to  acetylate  the  oil/ wash  free 
from  acetic  acid,  saponify,  liberate  the  fatty  acids  and  acetic 
acid  from  the  soap  and  then  either  distill  or  filter,  titrating  the 
acids  of  the  distillate  or  filtrate  and  making  the  proper  correction 
for  volatile  or  soluble  acids. 

The  " acetyl  value"  is  defined  to  be  the  number  of  milligrams  of 
potassium  hydroxide  required  to  combine  with  the  acetic  acid  lib- 
erated from  1  gm  of  acetylated  fat  or  oil.  Certain  oils  are  charac- 
terized by  unusually  high  acetyl  values.  Castor  oil  is  the  most 
noteworthy  of  these,  having  a  value  of  about  150.  Another  class 
of  oils  having  high  acetyl  values  is  composed  of  " blown"  or 
''oxidized"  oils.  By  blowing  air  through  oils  at  somewhat  ele- 
vated temperatures  (70°  to  115°)  the  viscosity  and  specific 
gravity  are  considerably  increased  and  they  become  suitable  for 
use  as  lubricating  oils.  The  chemical  changes  that  take  place 
are  not  thoroughly  understood  but  oxidation  is  known  to  occur. 
This  is  partly  due  to  combination  with  unsaturated  acids  (evi- 
denced by  a  diminished  iodine  absorption  number)  and  partly 
to  the  formation  of  hydroxyl  radicals  from  hydrogen.  The 
latter  change  results  in  an  increased  acetyl  value  and  this  may 
even  reach  a  number  as  great  as  that  for  castor  oil. 

The  large  variation  in  acebyl  values  recorded  in  the  table  on 
page  379,  adapted  from  a  similar  table  by  Lewkowitsch,2  will 

1  Proc.  Chem.  Soc.,  6,  72  (1890). 

2  J.  Soc.  Chem.  Ind.,  16,  503  (1897). 


OILS,  FATS  AND  WAXES 


379 


indicate  the  value  of  this  determination  for  the  identification  of 
certain  oils  and  fats.  In  other  cases  the  determination  will 
have  little  value. 


Oil  or  fat 

Acetyl  value  (average) 

Butter  fat  

0.0 

Castor 

149  5 

Colza  

16.6 

Cotton  seed  

21.5 

Croton  

19.9 

Fish         

41.0 

Linseed 

6  9 

Maize  

8.2 

Olive.              

13.5 

Shark  liver  

17.8 

Abnormal  Variation  in  Acetyl  Values.  —  Certain  abnormalities 
in  acetyl  values  should  be  noticed  and  due  allowance  made  in 
specific  cases. 

Since  acetic  anhydride  is  absorbed  by  the  hydroxyl  radical 
it  might  be  expected  that  free  acids,  free  alcohols  or  partially 
hydrolyzed  glycerides  or  other  esters  would  show  such  absorption 
and  that  their  occurrence  in  oils  or  fats  would  cause  these  to  ex- 
hibit unusually  high  acetyl  values.  This  is  found  to  be  the  case 
and,  since  the  three  classes  of  substances  named  above  are  the 
direct  products  of  hydrolysis,  it  follows  that  rancid  oils  or  fats 
will  not  give  normal  acetyl  values.  For  example,  hydrolysis 
of  stearin  will  yield  free  stearic  acid,  together  with  distearin, 
monostearin  or  glycerol,  according  to  the  degree  of  hydrolysis: 


Each  of  these  reactions  produces  a  hydroxylated  compound, 
which  is  capable  of  combining  with  acetic  anhydride.  The  acetyl 
values  of  these  substances  are  as  follows: 


Compound  ' 

Acetyl  value 

Distearin  
Monostearin  .... 
Glycerol  

84.2 
253.9 
772.0 

380  QUANTITATIVE  ANALYSIS 

The  free  acids  will  also  combine  with  acetic  anhydride  to  a 
varying  degree  and  this  property  will  still  further  increase  the 
acetyl  value  of  rancid  materials. 

The  measurement  of  add  value  is  a  convenient  method  for 
determining  this  property.  In  case  high  acid  values  have  been 
obtained  the  proper  correction  should  be  made  in  the  acetyl 
value  as  the  latter  has  been  determined  experimentally.  In 
other  words  it  is  only  in  fresh  oils  that  acetyl  values  can  be  used 
with  certainty  for  identification  purposes. 

The  most  important  application  of  this  determination  is  in 
the  identification  of  castor  oil.  This  oil  is  nearly  pure  ricinolein, 
a  glyceride  of  ricinoleic  acid.  The  latter  is  hydroxylated  oleic 
acid, 

CH3(CH2)5CH.OH.CH2.CH  =  CH(CH2)7COQH, 

and  the  glyceride,  ricinolein,  has  a  theoretical  acetjd  value  of 
159.1.  Its  abundance  in  castor  oil  gives  the  latter  an  actual 
acetyl  value  of  about  148,  a  value  which  is  far  above  that  of 
any  other  natural  oil,  only  blown  oils  approaching  it  in  this 
respect. 

Lastly  may  be  mentioned  the  occurrence  of  certain  quantities 
of  free  alcohols,  especially  in  the  waxes  which  have,  on  this  ac- 
count, appreciable  acetyl  values.  Cholesterol,  C27H460H,  in  fats, 
oils  and  waxes  of  animal  origin,  and  its  isomers,  the  phytosterols, 
in  vegetable  oils,  etc.,  are  the  most  important  of  such  alcohols. 

The  method  here  described  for  the  determination  of  acetyl 
value  is  essentially  that  of  Lewkowitsch  and  adopted  as  a  pro- 
visional method  by  the  Association  of  Official  Agricultural 
Chemists.1  It  involves  the  acetylation  of  the  oil  before  saponi- 
fication  and  includes  the  soluble  or  volatile  fatty  acids,  if  calcu- 
lated according  to  the  "  official "  method.  Much  confusion  would 
be  avoided  if  the  true  acetyl  value  were  recorded  instead  of  this 
acetyl-soluble  acid  value.  In  the  following  exercises  the  true 
acetyl  value  will  be  calculated. 

Determination. — Place  about  20  gm,  approximately  weighed,  of 
oil  or  fat  in  a  100  cc  flask,  add  an  equal  volume  of  acetic  anhydride, 
insert  a  short-stemmed  funnel  and  boil  gently  for  two  hours.  Cool  and 
pour  into  500  cc  of  water  contained  in  a  beaker.  Pass  a  current  of  car- 

1  U.  S.  Dept.  of  Agr.,  Chem.  Bull.  107,  142. 


OILS,  FATS  AND  WAXES  381 

bon  dioxide  into  the  beaker  through  a  fine  orifice  of  a  glass  tube  and  boil 
for  30  minutes.  At  the  end  of  this  time  siphon  out  the  water  layer  and 
repeat  the  treatment  with  water  and  boiling  until  the  water  is  no  longer 
acid,  as  shown  by  a  litmus  test.  Separate  the  acetylated  oil  in  a  separa- 
tory  funnel,  filter  in  a  drying  oven  and  dry. 

Weigh  accurately  2  to  4  gm  of  the  acetylated  oil  into  a  flask  and 
saponify  according  to  the  method  used  in  determining  the  saponification 
number,  measuring  the  alcohol  solution  of  potassium  hydroxide  accu- 
rately and  running  blank  determinations  for  standardization.  Evap- 
orate the  alcohol  and  dissolve  the  soap  in  water.  Add  standard  sul- 
phuric acid  in  a  quantity  exactly  equivalent  to  the  potassium  hydroxide 
added,  warm  to  melt  the  fatty  acids  and  filter  through  a  wet  paper. 
Wash  with  boiling  water  until  the  washings  are  no  longer  acid,  testing 
with  litmus  paper  by  barely  touching  a  corner  to  the  bottom  of  the 
funnel.  The  combined  filtrate  and  washings  are  titrated  with  tenth- 
normal  base.  Subtract  the  volume  of  base  already  found  to  be  equiva- 
lent to  soluble  acids  and  calculate  the  true  acetyl  value  according  to 
the  definition  of  this  number. 

Maumene  Number  and  Specific  Temperature  Reaction. — All 
oils  and  fats  react  with  concentrated  sulphuric  acid,  heat  being 
evolved.  The  reactions  are  complex  and  cannot  be  expressed 
by  a  simple  equation  but  oxidation  occurs  to  a  considerable 
degree.  The  heat  evolution  varies  with  different  oils  and  is,  to 
some  extent,  characteristic.  The  Maumene  number1  is  the 
number  of  centigrade  degrees  rise  in  temperature  caused  by  mixing 
10  cc  of  concentrated  sulphuric  acid  with  50  gm  of  oil.  A  small 
variation  in  the  proportion  of  water  in  the  acid  causes  a  con- 
siderable variation  in  the  heat  evolved  and  to  this  extent  the 
figures  recorded  by  different  investigators  are  not  comparable 
because  "  concentrated  sulphuric  acid,"  as  obtained  commer- 
cially, is  not  a  substance  with  any  definite  percent  of  water. 

In  order  to  eliminate  the  errors  due  to  variation  in  water  a 
determination  may  be  made,  using  the  same  amount  of  acid  but 
substituting  50  gm  of  water  for  the  oil.  The  ratio 

Rise  in  temperature  with  oil 

Rise  in  temperature  with  water 

is  known  as  the  "specific  temperature  reaction."2     That  this 

1  Compt.  rend.,  36,  572  (1852). 

2  Thomson  and  Ballantyne:  J,  Soc.  Chera.  Ind.,  10,  233  (1891). 


382 


QUANTITATIVE  ANALYSIS 


number  is  not  subject  to  variation  as  is  the  Maumene"  number  is 
shown  by  the  following  table  in  which  the  specific  temperature 
reaction  is  multiplied  by  100. 


Kind  of 
oil 

Sulphuric  acid  of  95.4 
percent 

Sulphuric  acid  of  96.8 
percent 

Sulphuric  acid  of  99 
percent 

Maumene1 
No. 

Sp.  temp, 
reaction 

Maumene' 
No. 

Sp.  temp, 
reaction 

Maumen6 
No. 

Sp.  temp, 
reaction 

Olive  
Rape  

36.5 
49 
34 
104.5 

95 
127 

88 
270 

39.4 

95 

44.8 
58 

96 
124 

Castor  
Linseed.  .  .  . 

37 

89 

125.2 

269 

Water.  . 

38.6 

100 

41.4 

100 

46.5                 100 

Determination.— Place  a  beaker,  about  5X1.5  inches,  inside  one  that 
is  about  6X3  inches  and  pack  the  open  space  between  *  with  wool, 
asbestos  or  cotton.  Cover  the  beakers  with  a  piece  of  cardboard 
through  which  passes  a  thermometer.  Weigh  into  the  inner  beaker  50 
gm  of  oil.  Bring  concentrated  sulphuric  acid  to  the  same  temperature  as 
that  of  the  oil  and  then  add  under  a  hood,  10  cc  of  this  acid,  stirring 
thoroughly  with  the  thermometer.  When  the  acid  is  all  in,  place  the 
thermometer  in  the  center  of  the  oil-acid  mixture  and  note  the  highest 
point  attained  by  the  mercury.  The  total  rise  in  temperature  is  the 
Maumen6  number. 

Determine  also  the  specific  temperature  reaction  as  follows:  Clean 
the  inner  beaker  and  introduce  50  cc  of  water.  Add  10  cc  of  acid  as 
before  and  note  the  rise  in  temperature.  The  Maumen6  number  divided 
by  this  rise  is  the  specific  temperature  reaction. 

The  drying  oils  often  develop  so  much  heat  that  active  foaming 
results.  Such  oils  should  be  first  diluted  with  petroleum  oils  or  olive 
oil  and  the  proper  correction  made  in  the  temperature  rise. 

Qualitative  Reactions. — If  simple  and  reliable  qualitative 
tests  were  known  for  all  of  the  oils,  it  is  not  likely  that  the  work 
outlined  in  the  preceding  pages  would  often  be  carried  out.  It 
has  already  been  explained  that  comparatively  few  such  tests 
are  known  because  of  the  similarity  in  the  composition  of  the 
various  animal  and  vegetable  oils.  Aside  from  the  mere  varia- 
tion in  the  proportion  of  the  various  glycerides,  free  alcohols  and 
free  acids,  there  are  certain  constituents  of  certain  oils  that  will 
give  color  reactions  which  are  characteristic.  A  few  of  those 
that  are  reliable  will  be  described.  In  most  cases  these  tests 


OILS,  FATS  AND  WAXES  383 

should  accompany  the  determination  of  the  analytical  constants, 
rather  than  be  substituted  for  them. 

Resin  Oil. — Polarize  the  oil  in  a  200-mm  tube.  If  the  oil  is  too  dark 
in  color  for  this  purpose  it  may  be  diluted  with  petroleum  ether  and  the 
proper  correction  made  in  the  reading.  Resin  oil  has  a  polarization  in 
a  200-mm  tube  of  from  +30°  to  +40°  on  the  sugar  scale  (Schmidt  and 
Haensch)  while  other  oils  read  between  +1°  and  —1°. 

Cotton  Seed  Oil:  Halphen  Test.1 — Mix  carbon  disulphide  containing 
about  1  percent  of  sulphur  in  solution,  with  an  equal  volume  of  amyl 
alcohol.  Mix  equal  volumes  of  this  reagent  and  the  oil  and  heat  in  a 
bath  of  boiling,  saturated  solution  of  sodium  chloride  for  1  to  2  hours. 
In  the  presence  of  as  little  as  1  percent  of  cotton  seed  oil  a  character- 
istic red  color  is  produced.  Lard  and  lard  oil  from  animals  fed  on 
cotton  seed  meal  will  give  a  faint  reaction  for  cotton  seed  oil.  The 
unknown  constituent  which  gives  the  color  apparently  is  assimilated 
by  the  animal  without  change. 

A  negative  result  does  not  prove  the  absence  of  cotton  seed  oil  because 
heating  the  oil  for  10  minutes  at  250°  renders  it  incapable  of  giving  the 
color. 

Arachis  (Peanut)  Oil. — Modified  Renard2  Test. — This  test  is 
based  upon  the  fact  that  about  5  percent  of  "  crude  arachidic 
acid"  may  be  isolated  from  arachis  oil,  whereas  stearic  acid  is  the 
highest  acid  that  occurs  in  any  considerable  quantity  in  most 
oils  and  fats.  " Crude  arachidic  acid'*'  is  a  mixture  of  true 
arachidic  acid,  C2oH4oO2,  and  lignoceric  acid,  C24H48O2. 

The  oil  is  first  saponified  and  the  excess  of  base  is  neutralized 
with  acetic  acid.  Lead  acetate  is  then  added  and  the  lead 
soaps  of  the  higher  acids  are  separated  from  those  of  the  lower 
acids  by  washing  with  ether,  in  which  lead  soaps  of  the  soluble 
acids  dissolve.  The  insoluble  soap  is  decomposed  by  hydro- 
chloric acid  and  the  resulting  palmitic,  stearic,  arachidic,  ligno- 
ceric and  traces  of  other  higher  fatty  acids  are  extracted  with 
ether,  which  is  then  evaporated.  Finally  the  acids  are  dissolved 
in  90  percent  alcohol  and  the  solution  is  cooled  to  15°.  Stearic 
and  palmitic  acids  remain  in  solution  and  the  higher  acids 
crystallize,  leaving  a  saturated  solution  containing  0.00025  gm 
of  " crude  arachidic  acid"  in  each  cubic  centimeter  of  alcohol. 

1  J.  pharm.  Chim.,  [61,  6,  390  (1897). 

2  Z.  anal,  chem.,  12,  231  (1871);  Compt.  rend.,  73,  1330  (1871). 


384  QUANTITATIVE  ANALYSIS 

By  applying  this  solubility  correction  the  approximate  weight 
of  arachis  oil  in  a  mixture  can  be  calculated.  The  test  is  made 
as  follows: 

Weigh  20  gm  of  the  oil  into  a  250  cc  Erlenmeyer  flask.  Saponify  with 
a  solution  of  potassium  hydroxide  in  alcohol  as  directed  in  the  discussion 
of  the  determination  of  saponification  number.  Add  a  drop  of  phenol- 
phthalein  and  exactly  neutralize  with  5  percent  acetic  acid  and  wash 
into  a  500-cc  flask  containing  a  boiling  mixture  of  100  cc  of  water  and 
120  cc  of  a  20  percent  lead  acetate  solution.  Boil  for  a  minute  and  then 
cool  the  precipitated  lead  soap  by  immersing  the  flask  in  water,  occa- 
sionally giving  it  a  whirl  to  cause  the  soap  to  stick  to  the  sides  of  the  flask. 
After  the  flask  has  cooled,  the  water  and  excess  of  lead  acetate  can  be 
poured  off  and  the  soap  washed  with  cold  water  and  with  90  percent 
(by  volume)  alcohol.  Add  200  cc  of  ether,  cork  and  allow  to  stand 
for  some  time  until  the  soap  is  disintegrated;  heat  on  the^  water  bath, 
using  a  reflux  condenser,  and  boil  for  about  five  minutes.  In  the  oils 
most  of  the  soap  will  be  dissolved,  while  in  lards  which  contain  much 
stearin,  part  will  be  left  undissolved.  Cool  the  ether  solution  of  soap 
to  from  15°  to  17°  and  let  stand  until  all  the  insoluble  soaps  have  crys- 
tallized out  (about  twelve  hours). 

Filter  upon  a  Biichner  funnel  or  a  folded  filter  and  thoroughly  wash 
the  insoluble  lead  soaps  with  ether,  then  wash  them  into  a  separatory 
funnel  by  means  of  a  jet  of  ether  from  a  wash  bottle,  alternating  at  the 
end  of  the  operation,  if  a  little  of  the  soap  sticks  to  the  paper,  with  hy- 
drochloric acid,  10  percent  solution.  Add  sufficient  of  this  dilute  hydro- 
chloric acid  so  that  the  total  volume  of  the  acid  layer  amounts  to  about 
200  cc  and  enough  ether  to  make  the  volume  of  the  ether  layer  150  to 
200  cc  and  shake  vigorously  for  several  minutes.  Allow  the  layers  to 
separate,  run  off  the  acid  layer  and  wash  the  ether  once  with  100  cc  of 
dilute  hydrochloric  acid  and  then  with  several  portions  of  water  until 
the  washings  are  no  longer  acid  to  methyl  orange.  If  a  few  undecom- 
posed  lumps  of  lead  soap  remain  (indicated  by  solid  particles  remaining 
after  the  third  washing  with  water)  break  these  up  by  running  off  almost 
all  of  the  water  layer  and  then 'add  a  little  concentrated  hydrochloric 
acid,  shake  and  then  continue  the  washing  with  water  as  before. 

Distill  the  ether  from  the  solution  of  higher  fatty  acids  and  dry  the 
latter  in  the  flask  by  adding  a  little  absolute  alcohol  and  evaporating 
on  the  steam  bath.  Dissolve  the  dry  fatty  acids  by  warming  with  100  cc 
of  90  percent  alcohol  (volume)  and  cool  slowly  to  15°,  shaking  gently 
to  aid  crystallization.  Allow  to  stand  at  15°  for  30  minutes.  If  arachis 
oil  has  been  present  arachidic  acid  will  separate  from  the  solution  as 
crystals.  Filter,  wash  the  precipitate  twice  with  90  percent  alcohol 


OILS,  FATS  AND  WAXES  385 

and  then  with  10  cc  portions  of  70  percent  alcohol,  care  being  taken  to 
keep  the  acid  crystals  and  the  washing  alcohol  at  definite  temperature 
in  order  to  be  able  to  apply  the  solubility  corrections  given  below. 

Dissolve  the  arachidic  acid  upon  the  filter  with  boiling  absolute 
alcohol,  evaporate  to  dry  ness- in  a  weighed  dish  and  weigh.  Add  to  the 
weight  found  0.0025  gm  for  each  10  cc  of  90  percent  alcohol  used  in  the 
crystallization  and  washing.  20  times  the  corrected  weight  of  acid  will 
be  the  approximate  weight  of  arachis  oil  in  the  sample  used.  The 
crystals  should  be  tested  qualitatively  by  determining  the  melting 
point,  which  should  be  71°  to  72°.  The  crystals  should  also  be  examined 
under  the  microscope.  As  little  as  5  percent  of  peanut  oil  may  be 
detected  by  this  method. 

Sesame  Oil:  Baudouin  Test.1 — Dissolve  0.1  gm  of  finely  powdered 
sugar  in  10  cc  of  hydrochloric  acid  (sp.  gr.  1.20),  add  20  cc  of  the  oil 
to  be  tested,  shake  thoroughly  for  a  minute,  and  allow  to  stand.  The 
aqueous  solution  separates  almost  at  once.  In  the  presence  of  even 
a  very  small  admixture  of  sesame  oil  this  is  colored  crimson.  Some 
olive  oils  give  a  slight  pink  coloration  with  this  reagent,  but  they  are  not 
hard  to  distinguish  if  comparative  tests  with  sesame  oil  are  made. 

The  color  was  thought  by  Villa vecchia  to  be  due  to  a  reaction 
of  a  constituent  of  sesame  oil  with  furfurol,  the  latter  being 
produced  by  the  interaction  of  sugar  with  hydrochloric  acid. 
Furfurol  was  accordingly  substituted  for  sugar  and  hydrochloric 
acid  and  the  method  somewhat  modified  as  follows: 

Sesame  Oil:  Villavecchia  Test.2 — Add  2  gm  of  furfurol  to  100  cc 
of  alcohol  (95  percent)  and  mix  thoroughly  0.1  cc  of  this  solution,  10  cc 
of  hydrochloric  acid  (sp.  gr.  1.20)  and  10  cc  of  oil  by  shaking  them 
together  in  a  test-tube.  The  same  color  is  developed  as  when  sugar  is 
used,  as  in  the  Baudouin  test.  Villavecchia  explained  this  reaction  on 
the  basis  that  furfurol  is  formed  by  the  action  of  levulose  and  hydro- 
chloric acid  and  he  therefore  substituted  furfurol  for  sucrose.  As  fur- 
furol gives  a  violet  tint  with  hydrochloric  acid  it  is  necessary  to  use  the 
very  dilute  solution  specified  in  this  method. 

Fish  and  Marine  Animal  Oils  in  Mixtures  with  Vegetable 
Oils. — Practically  all  of  these  oils  have  very  considerable 
" drying"  properties,  as  shown  by  their  iodine  absorption 
numbers.  They  are  characterized  by  the  presence  of  glycerides 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Pt.  II,  314. 

2  J.  Soc.  Chem.  Ind.,  12,  67  (1893);  13,  69  (1894). 

25 


386  QUANTITATIVE  ANALYSIS 

containing  the  unsaturated  clupanodonic  acid,  whose  formula 
and  properties  are  mentioned  on  page  364.  The  peculiar 
" fishy"  odor  of  these  oils  is  probably  due  to  the  presence  of 
this  acid. 

Absorption  of  bromine  by  unsaturated  acids  or  their  glycerides 
produces  bromides  of  limited  solubility  and  high  melting  point. 
Octobromstearin,  obtained  from  clupanodonin,  melts  at  a  higher 
temperature  (above  200°)  and  has  a  lower  solubility  than  hexa- 
bromstearin,  obtained  by  brominating  linolenin,  and  this  also 
differs  in  a  similar  manner  from  tetrabromstearin,  obtained  from 
linolin.  Therefore  the  separation  of  Octobromstearin  from 
brominated  fish  and  blubber  oils  provides  a  means  for  detecting 
marine  animal  oils  in  the  presence  of  vegetable  oils.  The  test  is 

performed  as  follows: 

« 

Dissolve  in  a  test-tube  about  6  gm  of  the  oil  in  12  cc  of  a  mixture  of 
equal  parts  of  chloroform  and  glacial  acetic  acid.  Add  bromine,  drop 
by  drop,  until  a  slight  excess  is  indicated  by  the  color,  keeping  the 
solution  at  about  20°.  Allow  to  stand  for  15  minutes  or  more  and 
then  place  the  test-tube  in  boiling  water.  If  only  vegetable  oils  are 
present  the  solution  will  become  perfectly  clear,  while  fish  oils  will 
remain  cloudy  or  contain  a  precipitate  of  insoluble  bromides. 

Color  Reactions. — A  large  number  of  qualitative  tests,  based 
upon  certain  color  reactions,  have  been  proposed  and  considerably 
used  in  the  past  for  the  detection  of  various  oils.  Color  reactions 
produced  by  adding  concentrated  nitric  or  sulphuric  acids 
may  be  mentioned.  Almost  without  exception  these  have  been 
found  to  be  unreliable  and  they  will  not  be  described  here  in 
detail. 

The  "elaidin  test"  is  worthy  of  brief  mention.  This  is  based 
upon  the  conversion  of  liquid  olein  into  its  solid  isomer,  elaidin, 
by  the  action  of  nitrous  acid,  the  change  being  from  oleic  acid 
of  olein  to  elaidic  acid  of  elaidin: 

CH3(CH2)7CH  CH3(CH2)7CH 

II  -  II 

HC(CH2)7COOH  HOOC(CH2)7CH 

Linolin,  linolenin  or  clupanodonin  are  not  thus  affected  and  the 
test  serves  to  distinguish  between  liquid  non-drying  oils  and 


OILS,  FATS  AND  WAXES  387 

drying  oils.  It  has  been  used  to  a  considerable  extent  in  testing 
the  purity  of  olive  oil  but  must  be  performed  under  strictly 
standardized  conditions  if  it  is  to  have  even  a  qualitative  value. 
Examination  of  an  Oil  or  Fat  Whose  Identity  is  Unknown.— 
For  the  purpose  of  identifying  an  oil  or  fat  of  unknown  character 
the  complete  chemical  and  physical  examination  is  made  unless 
its  identity  can  be  determined  unmistakably  by  a  qualitative 
test.  With  all  of  the  constants  determined  a  comparison  is  then 
made  with  all  available  data  contained  in  published  analyses  of 
oils  and  fats  and  a  reasonable  agreement  with  such  data  will 
generally  fix  the  identity  of  the  unknown  oil.  In  making  com- 
parisons the  most  important  figure  is  the  iodine  number  because 
this  will  serve  to  classify  the  oil  at  once  as  a  drying,  semi-drying, 
or  non-drying  oil,  the  approximate  ranges  for  these  somewhat 
arbitrary  divisions  being  as  follows: 

Oils  Iodine  Number 


Drvinsr 

200  and  higher  to  120 

Semi-drying   

120  to  95 

Non-drying.            

95  to  70  and  lower 

The  choice  will,  by  this  means,  be  narrowed  down  to  a  limited 
list  of  oils  or  fats.  The  remaining  constants  are  then  considered, 
one  by  one,  and  each  comparison  will  narrow  the  choice  still 
further.  At  the  last  all  available  qualitative  tests  are  made  in 
order  to  confirm  the  results  of  comparative  tests  or  to  aid  in 
making  a  final  decision.  It  sometimes  happens  that  the  figures 
obtained  in  the  examination  of  the  unknown  oil  will  not  all 
correspond,  even  to  a  reasonable  degree,  with  the  recorded  data 
for  any  of  the  common  oils.  This  may  be  the  result  of  (1) 
errors  in  the  determinations,  (2)  adulteration,  or  (3)  a  real  abnor- 
mality of  the  oil  which  is  being  examined.  The  first  case  should 
be  at  once  excluded  by  repeating  the  determination  of  constants 
in  which  lack  of  agreement  is  observed.  A  careful  inspection 
of  all  data  may  serve  to  indicate  certain  oils  which,  by  addition 
to  one  that  most  nearly  resembles  the  oil  under  examination, 
would  change  the  " constants"  in  the  manner  observed.  The 
matter  of  commercial  values  should  also  be  considered  in  this. 


388 


QUANTITATIVE  ANALYSIS 


OIL  CONSTANTS 

The  following  figures  represent  average  values  as  determined  on  many  samples  of  the  oils 
indicated,  and  by  many  different  chemists.  Exact  agreement  should  not  be  expected. 
Specific  gravity  and  index  of  refraction  are  calculated  for  the  temperatures  chosen,  from 
results  reported  for  various  temperatures. 


Oil 

Sp.  gr. 
at  20° 

Ind.  refr. 
at  20° 

Iodine 

Sapon. 

R.M. 

Acety 

Maumene 

Sp. 
temp. 

Hemp  seed  
Henbane  
Linseed                   .  .  . 

0.919 
0.936 
0  929 

1.477 
1  482 

148 
138 
186 

192 
171 
193 

o 

4 

97 
127 

331 

Poppy  seed  
Soy  bean  

0.922 
0  922 

1.475 
1.480 
1  476 

138 
140 
127 

195 
193 
193 

0 
0 

88 
90 
72 

220 
167 

Tune 

0  936 

1  503 

161 

193 

Walnut 

0  922 

1  478 

145 

195 

102 

0  985 

1  552 

105 

178 

Cotton  seed  
Croton  
Grape  seed  

0.920 
0.947 
0.932 

1.472 
1.478 
1.473 

109 
103 
96 

194 
212 
185 

0.8 
13 
0  5 

13 

26 

82 
•     63 

170 

Maize  

0.920 

1.475 

119 

190 

5 

7 

83 

180 

Rape 

0  912 

1  472 

98 

175 

0  5 

15 

60 

135 

0  920 

1  474 

105 

191 

1   2 

6 

65 

155 

0  915 

1  470 

95 

191 

1  0 

13 

53 

110 

0  916 

1  471 

90 

193 

0  5 

9 

47 

116 

Castor      

0  961 

1  478 

86 

184 

1  5 

148 

46 

84 

Hazel  nut  
Olive  
Pistachio  

0.913 
0.914 
0.915 

1.469 
1.469 
1.471 

86 
83 
90 

192 
190 
191 

1.0 
0.6 

3 

11 

36 
43 

91 

Quince  
Cod  liver  
Herring  

0.919 
0.921 
0.927 

1.471 
1.481 

113 
168 
133 

182 
180 
177 

0.5 
0.6 

5 

115 

258 

Menhaden  
Neat's  foot  
Salmon  

0.927 
0.912 

1.480 
1.467 
1  479 

156 

73 
161 

191 
195 
183 

1  0 
1.5 

22 

126 
52 

306 

95 

Seal 

0  925 

1  475 

147 

192 

16 

92 

278 

Sheep's  foot  

0.914 

74 

195 

50 

Whale 

0  92*> 

1  476 

134 

190 

92 

157 

Caoao  butter  
Cocoa  nut  fat  
Japan  wax  
Myrtle  wax 

at  60° 
0.929 
0.897 
0.744 
0  964 

at  60° 
1.450 
1.441 
1.450 
1  444 

36 
9 
10 
3 

194 
253 
227 
209 

0.5 

7 

3 

11 
28 

Palm 

0  890 

1  451 

54 

199 

1   2 

18 

25 

59 

Palm  nut  
Beef  tallow  
Butter  

0.921 
0.916 
0  902 

1.443 
1.451 
1  440 

15 

42 
38 

246 
196 
227 

5 
0.7 
27 

5 
5 
5 

Chicken  

0  893 

65 

193 

Goose 

0  896 

1  452 

65 

193 

0  2 

Hare       

1  452 

102 

201 

Horse  

0  888 

1  455 

78 

196 

1  8 

50 

105 

Lard  

0.905 

1  454 

55 

196 

0.4 

3 

27 

Mutton  tallow 

0  914 

1  451 

40 

193 

Beeswax     

0  938 

1  450 

g 

94 

0  4 

15 

Carnaiiba  wax  
Spermaceti  
Sperm  oil  
Wool  wax  

0.964 
0.901 
0.851 
0.913 

1.463 
1.440 
1.465  (20°) 
1.473 

13 
4 
86 
23 

87 
129 
135 
100 

1.3 

55 
3 
5 
23 

51 

97 

OILS,  FATS  AND  WAXES  389 

connection,  since  any  commercial  material  is  adulterated,  if  at 
all,  by  a  cheaper  material. 

After  the  decision  as  to  the  identity  of  the  oil  has  been  made 
or  has  been  limited  to  two  or  three  possible  oils,  consult  a  good 
reference  book  for  a  complete  discussion  of  these  oils  and  make 
any  additional  tests  that  may  be  there  suggested.  For  this 
purpose  are  to  be  recommended  Lewkowitsch's  Chemical  Analysis 
of  Oils,  Fats  and  Waxes,  5th  edition,  volume  2,  and  Allen's 
Commercial  Organic  Analysis,  volume  2,  part  1. 

The  figures  in  the  table  on  page  388  are  given  for  the  purpose 
of  comparison.  They  are  gathered  from  various  published 
analyses  of  the  more  common  oils  and  fats.  Exact  agreement 
should  not  be  expected.  For  more  extensive  tables  consult 
Lewkowitsch:  Chemical  Analysis  of  Oils,  Fats  and  Waxes,  and 
the  technical  journals. 

Hardened  Oils. — Under  any  circumstances  the  analytical 
investigation  of  oils  and  fats  offers  difficulties  that  are  often 
serious.  The  problems  of  the  analyst  are  now  increased  many 
fold  by  the  large  development  of  the  industry  of  hydrogenation 
of  liquid  oils. 

It  has  been  seen  that  the  most  important  difference  between  oils 
and  fats  lies  in  the  larger  proportion  of  olein  in  the  former  and  of 
stearin  and  palmitin  in  the  latter.  Olein  differs  from  stearin 
only  in  that  it  contains  one  unsaturated  double  bond  in  each 
oleic  acid  residue;  the  problem  of  saturating  this  group  by  the 
insertion  of  hydrogen,  thus  forming  stearin,  is  one  that  has  oc- 
cupied the  attention  of  chemists  for  many  years.  At  the  present 
time  the  hydrogenation  of  the  cheaper  liquid  oils  (e.g.,  cottonseed, 
corn  and  peanut)  to  form  edible  fats  is  an  industry  that  has  at- 
tained large  proportions.  While  this  process  changes  liquid 
oils  to  solid  fats,  it  will  also  make  a  corresponding  change  in  any 
analytical  constants  or  tests  that  depend  upon  the  degree  of 
unsaturation,  as  well  as  in  the  physical  properties  of  the  oil. 
Linolin,  linolenin  and  clupanodonin  will  be  changed  to  stearin. 
Consequently  the  halogen  absorption  number,  drying  properties, 
specific  gravity,  refractive  index  and  temperature  reactions  will 
be  materially  altered,  as  will  also  the  odor  and  the  general 
appearance  and  consistency.  It  has  been  stated  that  fish  oils 
probably  owe  their  characteristic  odor  to  glycerides  containing 


390  QUANTITATIVE  ANALYSIS 

clupanodonic  acid,  while  the  somewhat  similar  odor  of  linseed 
oil  is  due  to  gycerides  of  linolenic  acid.  It  is  interesting  to  note 
that  these  odors  are  entirely  lost  through  hydrogenation  and  that 
the  oils  are  no  longer  recognizable  by  any  ordinary  tests.  Many 
individual  tests  for  other  oils,  such  as  the  Halphen  reaction  for 
cottonseed  oil  and  the  Renard  test  for  sesame  oil,  fail  in  the 
hydrogenated  product. 

From  one  standpoint  it  might  appear  that  the  determination 
of  what  oils  originally  formed  the  raw  materials  for  the  "hard- 
ended"  product  is  not  a  necessary  one  for  the  analyst  to  solve, 
since  the  properties  of  the  finished  product  are,  after  all,  the  ones 
that  have  the  chief  practical  interest  for  us.  Yet  it  may  some- 
times happen  that  the  identity  of  the  original  oil,  or  the  proof 
that  a  hydrogenating  process  has  been  employed  may  have  a 
legal  or  other  significance  and  the  development  of  a  series  of 
suitable  tests  is  very  desirable. 

Analytical  chemistry  has  made  little  progress  in  this  direction. 
The  application  of  delicate  tests  for  metals  (nickel,  palladium, 
etc.)  that  are  used  as  catalyzers  in  the  hardening  process,  may 
sometimes  serve  to  show  that  the  material  is  a  hardened  oil, 
rather  than  a  natural  fat.  Other  than  this  one  can  say  very 
little.  But  this  knowledge  of  the  nature  of  the  changes  caused 
by  hydrogenation  should  serve  to  make  the  analyst  more  cautious 
than  he  might  otherwise  be  when  interpreting  the  results  of  his 
tests  of  oils  or  fats  of  unknown  origin. 


CHAPTER  XIV 
WATER 

The  chemical  examination  of  water  may  be  made  to  determine 
its  fitness  for  drinking  or  for  industrial  uses,  such  as  steam  pro- 
duction, laundering,  textile  industries,  etc.  It  is  not  necessary 
that  a  complete  analysis  should  be  made  for  all  of  these  purposes 
because  not  all  substances  occurring  in  water  are  equally  impor- 
tant in  the  different  applications  of  the  water.  Natural  waters 
often  contain  substances  that  are  objectionable  if  they  are  to 
be  used  industrially  and  these  substances  are,  for  the  most  part, 
inorganic  salts  and,  occasionally,  acids.  Most  of  such  inorganic 
materials  are  without  appreciable  effect  upon  the  human  system 
and  the  examination  for  potability  is  rather  directed  toward  the 
detection  of  pollution  by  sewage.  On  this  account  it  becomes 
necessary  to  treat  the  subject  of  water  analysis  in  two  distinct 
divisions. 

Industrial  Analysis. — By  far  the  largest  industrial  consumption 
of  water  is  for  the  production  of  steam  and  for  this  reason  the 
chemist  is  more  often  called  upon  for  the  analysis  of  water  to 
determine  its  fitness  for  steaming  than  for  any  other  industrial 
purpose.  Pure  water,  however  desirable  it  may  be  for  use  in 
the  steam  boiler,  is  not  a  natural  product.  Water  from  streams 
and  other  surface  origins  contains  mineral  and  organic  substances 
derived  from  the  surface  soil  as  well  as  inorganic  compounds 
derived  from  springs  which  feed  the  stream.  Water  from  wells 
contains  whatever  mineral  matter  is  common  to  the  region 
through  which  it  has  flowed.  Even  rain  water  contains  organic 
matter  and  ammonia  and  may  develop  organic  acids  when  stand- 
ing. Some  of  the  compounds  contained  in  water  are  com- 
paratively unobjectionable  because  their  action  is  slight.  It  is 
to  be  remembered,  however,  that  in  steam  boilers  the  tempera- 
ture is  higher  than  100°  because  of  the  increased  pressure.  At 
a  pressure  of  100  pounds  per  square  inch  the  boiling-point  of 

391 


392  QUANTITATIVE  ANALYSIS 

water  is  164°  and  at  200  pounds  per  square  inch  the  boiling-point 
is  194°.  At  these  temperatures  the  chemical  activity  of  many 
dissolved  substances  is  very  much  augmented. 

According  to  their  effects  upon  boiler  steel  the  constituents  of 
natural  waters  may  be  classified  as  corrosives,  incrustants  and 
foam  producers. 

Corrosives. — Any  soluble  compound  that  can  dissolve  iron  at 
high  temperatures  will  give  rise  to  pitting  of  the  boiler,  especially 
when  the  steel  is  not  of  uniform  composition.  Corrosives  com- 
monly occurring  in  water  are  chlorides,  nitrates,  and  sulphates, 
particularly  of  the  alkaline  earth  metals,  and  free  carbonic  acid. 
Free  inorganic  acids  are  of  rare  occurrence  and  absolutely  unfit 
a  water  for  steaming  without  preliminary  treatment.  A  small 
amount  of  acid  will  cause  corrosion  for  an  indefinite  period 
because  of  the  ready  hydrolysis  of  iron  salts.  A  cycle  of  re- 
actions takes  place  as, follows: 

Fe+2HCl->FeCl2+H2, 

6FeCl2+3O-+4FeCl3+Fe2O3, 

FeCls+3H20-+Fe(OH)8+3HCl. 

A  metal  chloride  which  is  easily  hydrolyzed  will  also  produce 
continuous  corrosion: 

MgCl2+2H20-^Mg(OH)2+2HCl, 
Fe+2HCl-+FeCl2+E2,  etc. 

Nitrates  are  equally  injurious,  although  they  seldom  occur  in 
more  than  small  concentration.  Sulphates  are  somewhat  less 
corrosive  and  free  carbonic  acid  still  less  so. 

Incrustants. — Any  substance  that  can  be  precipitated  by  heat- 
ing or  evaporation  of  water  is,  in  a  sense,  an  incrustant.  The 
steam  boiler  as  a  power  producer  is  also  a  machine  for  continuous 
concentration  of  water  solutions,  since  'fresh,  impure  water  is 
continually  added  and  only  vapor  is  removed.  Strictly  speaking 
only  those  substances  which  adhere  to  the  boiler  plate  when  they 
are  precipitated  are  classed  as  incrustants  because  only  these  are 
particularly  objectionable.  These  are  carbonates  of  calcium  and 
magnesium  and  calcium  sulphate.  In  presence  of  considerable 
quantities  of  these  materials  certain  other  compounds,  such  as 


WATER  393 

silicic  acid,  iron  oxide  and  aluminium  oxide,  may  be  included  with 
the  scale  and  then  become  incrustants. 

Calcium  and  magnesium  carbonates  are  not  dissolved  as  such 
in  water  but  are  present  as  bicarbonates,  having  been  dissolved 
from  the  mineral  carbonates  by  carbonic  acid. 

CaCO3+H2C03-»Ca(HC03)2, 
MgC03+H2CO3^Mg(HCO3)2. 

When  the  water  is  heated  reactions  which  are  the  reverse  of 
these  take  place  and  the  normal  carbonates  are  precipitated: 

Ca(HCO3)2-+CaC03+H2O+CO2, 
Mg(HC03)2^MgC03+H20+C02. 

These  carbonates  adhere  to  the  boiler  plate,  the  greatest  amount 
of  precipitation  occurring  over  the  heating  surface.  The  scale 
thus  formed,  although  comparatively  loose,  hinders  the  trans- 
mission of  heat  from  the  steel  to  the  water  and  causes  local  super- 
heating. The  result  is  a  loss  of  efficiency  and  injury  to  the 
boiler.  Although  these  substances  occur  in  the  water  as 
bicarbonates,  they  are  arbitrarily  calculated  as  normal  carbonates 
because  the  latter  are  precipitated  when  the  water  is  heated. 

Calcium  sulphate  precipitates  only  when  continued  evapora- 
tion of  the  water  concentrates  it  to  the  point  of  saturation.  Pre- 
cipitation then  causes  the  formation  of  a  scale  that  is  much  more 
serious  in  its  effects  than  the  scale  of  carbonates,  because  it  is 
compact  and  adheres  firmly  to  the  boiler.  While  carbonate 
scale  can  be  largely  removed  by  occasionally  blowing  off  the  water, 
calcium  sulphate  can  be  loosened  only  by  the  use  of  hammer 
and  chisel.  On  this  account  calcium  sulphate  is  one  of  the  most 
objectionable  incrustants  of  all  compounds  found  in  natural 
waters. 

Foam  Producers. — Carbonates  of  sodium  and  potassium  in- 
crease the  surface  tension  of  water  to  such  an  extent  that  the 
result  is  foaming  or  " priming"  as  steam  is  taken  from  the  boiler. 
Some  of  the  alkali  waters  of  the  West  contain  large  quantities 
of  these  salts. 

Expression  of  Results. — The  systems  used  in  the  calculation 
of  results  of  water  analysis  were  discussed  in  connection  with  the 
determination  of  hardness  of  water  (page  231).  It  is  convenient 


394  QUANTITATIVE  ANALYSIS 

to  work  with  1000  cc  of  water  or  simple  fractions  of  this  quantity 
and  to  express  results  as  milligrams  of  dissolved  substance  per  liter 
oj  water.  These  figures  may  be  changed  to  grains  per  gallon  by 
multiplying  by  the  factor  0.0583 +. 

The  analysis  of  the  water  solution  will  be  made  by  means  of 
methods  which  give  metals  and  acid  radicals  as  the  result  of 
separate  determinations.  It  was  at  one  time  customary  to  calcu- 
late these  as  basic  and  acid  anhydrides,  as  is  still  done  in  the 
analysis  of  minerals.  There  arises  the  same  difficulty  that  is 
experienced  in  mineral  analysis,  viz.:  that  salts  of  hydracids 
cannot  be  expressed  as  oxides.  A  much  better  rule  is  to  calculate 
all  constituents  as  positive  or  negative  radicals. 

Hypothetical  Compounds. — There  is  still  current  among 
industrial  chemists  and  engineers  a  custom  of  making  a  second 
calculation  of  compounds  supposed  to  exist  in  the-  water.  Most 
natural  waters  are  highly  dilute  solutions  of  mineral  matter. 
In  such  a  solution  most  of  the  compounds  are  highly  ionized  and 
all  possible  combinations  of  radicals  as  compounds  are  present 
to  some  extent,  no  matter  what  compounds  were  originally 
dissolved  by  the  water.  It  is  evident,  therefore,  that  any  list 
of  compounds  calculated  from  the  results  of  the  analysis  will  be 
entirely  fanciful,  so  far  as  the  actual  condition  of  the  solution  is 
concerned.  The  basis  of  such  a  calculation  was  formerly  the 
supposed  affinity  possessed  by  the  different  radicals  for  each 
other.  If  the  radicals  commonly  occurring  in  water  are  arranged 
in  order  of  decreasing  base  and  acid  character  the  following  series 
will  be  obtained: 


Positive  Radicals 

Negative  Radicals 

Potassium 
Sodium 
Calcium 
Magnesium 

Chloride 
Nitrate 
Sulphate 
Carbonate 

Based  upon  the  assumption  that  the  combination  of  these 
radicals  will  follow  from  their  relative  affinities  the  mathematical 
procedure  would  be  to  calculate  the  maximum  amount  of  potas- 
sium chloride  that  could  be  formed,  taking  the  excess  of  either 
potassium  or  chlorine  as  combined  with  the  next  radical  of  oppo- 


WATER  395 

site  sign  (either  sodium  or  the  nitrate  radical)  and  so  on,  down 
the  list.  If  the  analysis  has  been  accurately  carried  out  and  if 
all  substances  present  have  been  determined  the  positive  and 
negative  radicals  should  be  found  in  equivalent  quantities,  with 
a  very  slight  excess  of  either  magnesium  or  of  the  carbonate 
radical  after  the  calculation  is  finished.  This  excess  is  the 
result  of  cumulative  errors  in  the  determination  of  the  various 
radicals  existing  in  the  water  and  also  of  the  occasional  omission 
of  small  quantities  of  radicals  other  than  those  above  named. 
Silicic  acid,  iron  and  aluminium  are  not  included  in  the  calcula- 
tion of  hypothetical  compounds  because  the  colloidal  nature  of 
their  hydroxides  causes  nearly  complete,  though  indefinite,  hy- 
drolysis of  any  salts  that  might  originally  have  been  present. 
They  are  therefore,  according  to  custom,  reported  as  oxides  and 
this  conventional  method  is  sometimes  responsible  for  the  appear- 
ance of  a  slight  excess  of  negative  radicals  in  the  final  report. 
If  ammonium  salts  are  present  in  any  considerable  quantity, 
as  in  sewage  effluents  or  factory  wastes,  a  failure  to  deter- 
mine this  radical  also  will  result  in  an  apparent  excess  of  negative 
radicals. 

The  customary  method  of  calculating  hypothetical  com- 
pounds is  not  based  upon  scientific  principles,  as  has  already  been 
shown.  There  is  a  certain  justification  for  such  a  calculation, 
on  account  of  the  fact  that  when  a  water  is  heated  and  evapo- 
rated there  will  be  produced  the  least  soluble  compounds  of 
all  that  might  be  formed  from  the  various  radicals  present. 
Through  a  certain  coincidence,  this  would  leave  the  radicals  com- 
bined in  about  the  same  manner  as  is  indicated  by  the  conven- 
tional calculation.  Heating  in  the  boiler  will  produce  the  maxi- 
mum possible  quantities  of  normal  carbonates  of  calcium  and 
magnesium.  Which  of  these  carbonates  is  least  soluble  at  high 
temperatures  is  not  definitely  known  because  of  difficulties  en- 
countered in  the  determination  of  solubility.  It  is  assumed, 
however,  that  if  the  radical  of  carbonic  acid  is  not  present  in 
quantity  sufficient  to  form  carbonates  with  all  of  the  calcium  and 
magnesium,  calcium,  rather  than  magnesium,  will  ultimately 
remain  to  form  the  sulphate  as  the  water  is  evaporated.  Calcium 
sulphate  is  certainly  next  to  the  carbonates  of  calcium  and  mag- 
nesium with  respect  to  its  insolubility  and  it  will  precipitate 


396 


QUANTITATIVE  ANALYSIS 


when  evaporation  within  the  boiler  concentrates  it  to  the  point  of 
saturation.  After  these  three  compounds  have  been  formed, 
the  method  of  combining  the  remaining  radicals  is  quite  imma- 
terial because  they  will  not  precipitate  in  any  form,  on  account 
of  the  large  solubility  of  salts  of  the  alkali  metals. 

In  order  to  emphasize  the  real  basis  for  any  calculation  of 
compounds  we  shall  reverse  the  order  of  radicals  given  on  page 
394  and  calculate  combinations  in  the  following  order: 


Positive  Radicals 

Negative  Radicals 

Magnesium 
Calcium 
Sodium 
Potassium 

Carbonate 
Sulphate 
Nitrate 
Chloride 

This  will  give  precisely  the  same  result  as  the  calculations  from 
the  original  order,  unless  there  is  found  to  be  an  excess  of  either 
positive  or  negative  radicals.  In  this  case  the  excess  will  be 
found  to  be  either  potassium  or  the  chloride  radical  instead  of 
magnesium  or  the  carbonate  radical,  but  the  excess  should  be 
small  enough  to  be  insignificant  in  either  case. 

On  account  of  the  fact  that  sodium  and  potassium  have  very 
little  significance  in  most  boiler  waters  (the  negative  radicals  and 
the  metals  that  take  part  in  scale  formation  being  of  chief 
importance)  and  because  they  occur  in  relatively  small  amounts 
in  most  waters,  the  determination  of  potassium  is  frequently 
omitted  and  calculations  are  based  upon  the  assumption  that 
sodium  is  the  only  alkali  metal  present. 

The  conventional  method  of  calculating  hypothetical  com- 
pounds is  illustrated  in  the  example  given  below. 

The  analysis  of  a  ground  water  gave  the  following  results: 

Milligrams  per  liter 


Silica. 

5  01 

Oxides  of  iron  and  aluminium  
Sodium  

3.53 
6.02 

Potassium  

5  26 

Calcium     

75  41 

Magnesium 

24  19 

Chloride  radical 

4  52 

Nitrate  radical 

0  34 

Sulphate  radical  

•      32.91 

Carbonate  radical  

160.21 

WATER  397 


Following  is  the  calculation  of  compounds: 
30 


24.19+59.65  =  83.84  =  MgCO3. 

160.21  -  59.65  =  100.56  =  (CO3)"  remaining; 

~^  X  100.56  =  67.20  =  Ca<>  (CO3)"  remaining; 

100.56+67.20=  167.76  =  CaCO3. 
75.41  -67.  20  =  8.21  =  Ca  remaining; 

||^|x8.21  =  19.69  =  (S04)"oCa  remaining; 

ZO.Oo 

8.21  +  19.69  =  27.90  =  CaSO4. 

32.91  -  19.69  =  13.22  =  (SO4)"  remaining; 


X6>02  =  12.56  =  (S04)"oNa; 

6.02+  12.56  =  18.58  =  Na2SO4. 

13.22  -  12.56  =  0.66  =  (SO4)"  remaining; 

on  -JA 

||^  X  0.66  =  0.53  =  K=c=(S04)"  remaining; 

0.66  +  0.53  =  1.19  =  K2SO4. 

5.26  -0.53=  4.73  =  K  remaining; 


0.34+0.21  =  0.55  =  KNO3. 

4.73  -0.21=  4.52  =  K  remaining; 

2g^X4.52  =  4.10  =  Cl'-K  remaining; 

4.52+4.10  =  8.62  =  KCl. 

4.52—  4.  10  =  0.42  =  01'  remaining.     This  excess  of  chlorine 
represents  experimental  errors  as  is  explained  above. 

Since  "milligrams  per  liter"  multiplied  by  0.0583  gives  "grains 
per  gallon"  the  complete  statement  of  compounds  is  as  follows. 


398 


QUANTITATIVE  ANALYSIS 


Formula  for  compounds 

Milligrams  per  liter 

Grains  per  gallon 

SiO2 

5  01 

0  292 

Fe2O3+Al2O3  
KC1  

3.53 

8.62 

0.216 
0.502 

KNO3                            .    .  . 

0  55 

0  032 

K2SO4 

1  19 

0  069 

Na2SO4  
CaSO4                       

18.58 
27.90 

1.082 
1.626 

CaCO3     .  . 

167  76 

9  775 

MgCO3  

83.84 

4.885 

The  use  of  the  conversion  factors  as  in  the  above  illustration 
fits  only  the  analysis  that  was  used  as  the  example.  In  the  case  of 
other  waters  certain  factors  might  be  the  reciprocals  of  those  used 
above  or  they  might  involve  different  pairs  of  equivalent  weights. 
All  possible  combinations  should  be  calculated  as  a  preliminary 
exercise  and  the  factors  with  their  logarithms  recorded  with  the 
list  of  factors  in  the  note  book.  It  should  be  noted  that  for 
water  analysis  the  conversion  factors  are  not  multiplied  by  100, 
since  percents  are  not  to  be  calculated. 

Problem 

73.  Calculate  the  conversion  factors  used  above  into  the  corresponding 
mixed  numbers  by  carrying  out  the  divisions  indicated.  Also  calculate 
the  following  additional  factors  and  their  logarithms  and  record  all  of 
these  in  the  note  book.  The  first  factor  is  given  as  an  example. 


Found 

Required 

Factor 

Log. 

Found 

Required 

Factor 

Log. 

Mg 

C03 
SO4 

2.467 

0.3922 

C03 

Ca 
Mg 

NO3 

Na 

Cl 

K 

Ca 

CO3 

SO4 

Ca 

SO4 

Mg 

NO3 

Na 

Cl 

K 

'    Na 

CO3 

NO3 

Ca 

SO4 

Mg 

NO3 

Na 

Cl 

K 

K 

CO3 

Cl 

Ca 

SO4 

NO3 

Mg 

Na 

• 

Cl 

K 

WATER  399 

Industrial  Analysis  of  Water. — Measure  accurately  in  a  calibrated 
flask  enough  water  to  give,  upon  evaporation,  0.5  gm  to  1  gm  of  residue. 
Add  2  cc  of  concentrated  hydrochloric  acid  and  evaporate  in  a  platinum 
or  porcelain  dish  on  the  steam  bath.  Heat  the  residue  at  a  temperature 
below  redness  until  organic  matter  is  removed. 

Silicious  Matter. — Add  1  cc  of  concentrated  hydrochloric  acid  to 
the  residue  and  then  add  about  20  cc  of  hot  water.  Warm  and  stir 
until  all  soluble  matter  has  dissolved  then  filter  on  an  extracted  filter 
paper.  Wash  well  with  hot  water  until  the  combined  filtrate  and  wash- 
ings amount  to  about  75  cc.  Fold  the  filter  paper  and  burn  in  a  weighed 
platinum  crucible.  The  residue  is  reported  as  "silicious"  if  it  is 
white  and  does  not  weigh  more  than  5  mg,  otherwise  it  may  contain 
appreciable  amounts  of  metal  oxides  or  silicates.  If  the  weight  is 
greater  than  5  mg,  add  to  the  residue  a  drop  of  sulphuric  acid  and  then 
volatilize  the  silica  by  warming  with  1  cc  (more  if  necessary)  of  hydro- 
fluoric acid.  Ignite  the  residue  and  weigh.  Report  the  loss  as  silica. 
Dissolve  any  remaining  residue  in  concentrated  hydrochloric  acid  and 
add  to  the  main  solution. 

Oxides  of  Iron  and  Aluminium. — Drop  into  the  solution  a  very 
small  bit  of  litmus  paper  and  carefully  add  dilute  ammonium  hydrox- 
ide until  the  solution  is  slightly  basic.  Boil  gently  to  flocculate  the 
hydroxides  of  iron  and  aluminum  and  to  remove  any  unnecessary  excess 
of  ammonium  hydroxide.  Filter  and  wash  with  hot  water  until  free 
from  chlorides.  Ignite  and  weigh  and  report  as  oxides  of  iron  and 
aluminium. 

Usually  iron  is  not  present  in  quantity  sufficiently  large  to  make  its 
separate  determination  important.  If  this  is  desired  the  method  given 
on  page  288  may  be  used  and  the  equivalent  amount  of  ferric  oxide  sub- 
tracted from  the  combined  oxides,  the  remainder  being  aluminium 
oxide. 

Calcium.— Add  1  cc  of  dilute  ammonium  hydroxide  to  the  filtrate  and 
washings  from  the  iron  and  aluminium  hydroxides  and  then  precipitate 
and  determine  the  calcium  with  the  precautions  mentioned  in  the  discus- 
sion on  page  81.  Report  as  calcium. 

//  potassium  is  to  be  determined,  add  to  the  solution  2  cc  of  concen- 
trated sulphuric  acid  and  evaporate  in  a  weighed  platinum  dish  to 
dryness.  Heat  the  residue  carefully  to  evaporate  excess  of  sulphuric 
acid  and  then  more  strongly  to  expel  ammonium  salts,  finally  heating 
for  a  short  time  until  the  dish  is  dull  red.  If  sulphuric  acid  fumes  do 
not  appear  upon  heating,  excess  is  not  present  and  more  should  be  added 
before  the  stronger  heating.  Weigh  and  record  the  weight  of  sulphates 
of  sodium,  potassium  and  magnesium.  Dissolve  the  combined 


400  QUANTITATIVE  ANALYSIS 

sulphates  and  dilute  the  solution  to  250  cc  in  a  calibrated  volumetric 
flask. 

Magnesium. — Fill  a  dry  100-cc  volumetric  flask  with  the  solution  of 
sulphates  and  rinse  into  a  Pyrex  beaker.  Determine  magnesium  as 
directed  on  page  112.  Notice  that  only  0.4  of  the  original  sample  was 
used  for  this  determination. 

Potassium. — Use  100  cc  of  the  sulphate  solution  that  was  obtained 
just  before  the  determination  of  magnesium.  Evaporate  in  a  platinum 
dish  over  the  steam  bath  adding  the  necessary  quantity  of  chlorplatinic 
acid  before  crystallization  of  salts  begins.  Determine  potassium  by 
the  Lindo-Gladding  method,  page  102.  Here  also  0.4  of  the  original 
quantity  of  sample  was  used. 

Sodium. — Calculate  the  weight  of  potassium  sulphate  equivalent  to 
the  potassium  chlorplatinate  found  and  also  the  weight  of  magnesium 
sulphate  equivalent  to  magnesium  pyrophosphate  found.  Multiply 
the  sum  of  these  weights  by  2.5  and  subtract  the  product  so  obtained 
from  the  total  weight  of  combined  sulphates  already,  found.  The 
remainder  is  sodium  sulphate.  From  this  calculate  sodium. 

//  potassium  is  not  to  be  determined  the  weighed  residue  of  sulphates 
is  dissolved  and  the  solution  is  diluted  to  about  100  cc  in  a  250  cc  beaker 
of  Pyrex  or  similar  resistance  glass.  Magnesium  is  then  determined  in 
the  entire  solution.  From  the  weight  of  magnesium  pyrophosphate 
found  calculate  the  weight  of  magnesium  sulphate  equivalent  to  it  and 
subtract  this  from  the  weight  of  combined  sulphates,  already  found. 
The  remainder  is  assumed  as  the  weight  of  sodium  sulphate,  from  which 
sodium  is  calculated. 

Sulphates. — Use  100  cc  of  water  unless  a  qualitative  test  shows  the 
presence  of  only  a  small  concentration  of  sulphates,  in  which  case  500  cc 
or  more  should  be  evaporated  to  about  100  cc.  Add  0.5  cc  of  concen- 
trated hydrochloric  acid  and  precipitate  by  barium  chloride,  carrying 
out  the  precipitation  and  treatment  of  the  precipitate  as  directed  on 
page  95.  Calculate  milligrams  per  liter  of  the  sulphate  radical. 

Chlorides. — Make  500  cc  of  a  standard  solution  of  pure  sodium 
chloride,  1  cc  of  which  contains  0.001  gm  of  chlorine.  Make  1500  cc 
of  a  solution  of  silver  nitrate  such  that  1  cc  is  calculated  to  be  equivalent 
to  about  0.000505  gm  of  chlorine  and  standardize  against  the  sodium 
chloride  solution  as  follows :  Measure  25  cc  of  the  standard  sodium  chlo- 
ride solution  into  a  4-inch  porcelain  casserole  or  into  a  beaker  placed  on  a 
white  background.  Add  1  cc  of  a  5  percent  solution  of  potassium 
chromate  (from  which  chlorides  have  been  precipitated  by  the  addition 
of  a  slight  excess  of  silver  nitrate)  and  then  titrate  with  silver  nitrate 
solution  until  the  first  permanent  red  tint  of  silver  chromate  appears. 
This  is  the  end  point  of  the  reaction  between  silver  nitrate  and  sodium 


WATER  401 

chloride.  Calculate  the  dilution  necessary  to  make  1  cc  of  the  silver 
nitrate  solution  equivalent  to  0.0005  gm  of  chlorine  and  dilute  1000  cc 
to  the  required  volume  by  adding  distilled  water  from  a  burette. 

Determine  chlorine  in  the  water  by  titrating  100  cc  or  more  of  water 
by  standard  silver  nitrate,  using  1  cc  of  potassium  chromate  as  the 
indicator.  In  case  chlorides  are  present  in  very  small  concentration  it 
may  be  necessary  to  use  more  than  100  cc  and  to  evaporate  to  this 
volume.  The  end  point  of  the  titration  is  much  more  easily  observed  if 
a  rather  heavy  precipitate  of  silver  chloride  is  present  at  the  end.  If, 
therefore,  the  concentration  of  chlorine  in  the  water  is  small  it  is  ad- 
vantageous to  add  first  a  measured  volume  (10  to  25  cc)  of  standard 
sodium  chloride  solution,  deducting  this  volume  from  the  total  volume 
of  silver  nitrate  required.  Calculate  milligrams  per  liter  of  the  chloride 
radica  . 

Nitrates. — Use  one  of  the  methods  described  on  pages  427  and  428  for 
the  sanitary  examination  of  water.  Calculate  milligrams  per  liter  of  the 
nitrate  radical.  The  concentration  of  nitrates  in  most  waters  is  too 
small  to  be  of  consequence  in  steaming  but  this  is  not  always  the  case 
and  the  determination  should  not  be  omitted. 

Carbonates. — Titrate  100  cc  of  the  water  by  fiftieth-normal  hydro- 
chloric acid,  using  methyl  orange  as  indicator.  More  or  less  than  100  cc 
of  water  may  be  necessary  if  the  concentration  of  carbonates  is  very 
small  or  very  large.  Enough  should  be  used  to  require  25  to  45  cc  of 
standard  acid.  Calculate  milligrams  per  liter  of  the  carbonate  radical 
(COs)".  This  is  based  upon  the  customary  conventional  assumption 
of  normal  carbonates  instead  of  bicarbonates. 

The  determination  of  carbonates  by  titration  with  standard  acid  is 
the  determination  of  alkalinity  as  described  on  page  231  except  that 
hardness  is  arbitrarily  calculated  as  calcium  carbonate  while  in  this 
connection  the  carbonate  radical  is  calculated. 

Calculate  milligrams  per  liter  of  the  hypothetical  compounds  using 
the  method  illustrated  on  page  394.  Calculate  the  corresponding  grains 
per  U.  S.  gallon. 

Treatment. — If  the  chief  incrustants  of  a  water  are  bicarbonates 
of  calcium  and  magnesium  these  may  be  largely  precipitated  by 
heating  the  feed  water  by  means  of  the  exhaust  steam.  This 
kind  of  treatment  is  limited  in  its  application  on  account  of  the 
short  time  allowed  for  settling.  Other  incrustants  than  those 
mentioned  are  not  removed  by  this  process. 

Calcium  hydroxide  will  precipitate  bicarbonates  and  this  is 


20 


402  QUANTITATIVE  ANALYSIS 

the  cheapest  agent  available  for  this  purpose.  It  also  possesses 
the  advantage  of  leaving  no  by-products  in  the  water: 

Ca(HC03)2+Ca(OH)2->2CaC03+2H20. 
Mg(HCO3)2+Ca(OH)2^CaCO3+MgC03+2H20. 

Sodium  carbonate  will  also  precipitate  bicarbonates  as  well  as 
other  salts  of  calcium  and  magnesium.  It,  however,  leaves  in 
the  water  the  corresponding  salt  of  sodium  and  this  is  objection- 
able. Examples  of  the  reactions  are  expressed  by  the  following 
equations  : 

Ca(HC03)2+Na2C03-^CaC03+2NaHCO3, 

CaSO4+Na2CO3-»CaCO3+Na2SO4,     . 
CaCl2+Na2C03->CaC03+2NaCl. 

The  best  procedure  is  to  treat  the  water  first  with  the  amount  of 
calcium  hydroxide  necessary  to  react  with  bicarbonates,  allowing 
a  small  excess,  and  then  with  sufficient  sodium  carbonate  to 
precipitate  all  remaining  calcium  and  magnesium.  The  reactions 
should  be  carried  out  in  tanks  large  enough  to  provide  water 
for  the  plant,  allowing  the  necessary  time  for  settling.  The 
initial  cost  of  the  purifying  plant  is  almost  the  only  cost  because 
of  the  relatively  low  cost  of  the  small  amount  of  lime  and  soda 
ash  necessary. 

In  calculating  the  necessary  treatment  the  purity  of  the  lime 
or  lime  water  must  be  known  and  also  that  of  the  soda  ash.  The 
results  may  be  expressed  according  to  custom  as  pounds  of 
reagent  per  100,000  gallons  of  water.  The  quantity  of  (C03)", 
expressed  as  grains  per  gallon,  is  multiplied  by  the  fraction 
equivalent  weight  of  CaO  x 

equivalent  weight  of  (CO.)"'  the  TeSult  bemg  *""*  °f  ^  ^ 
quired  for  one  gallon  of  water.  One  pound  avoirdupois  contains 

,       grains  CaO  per  gallon  X  100,000 
7000  grains.    Therefore-  ^T^TT  -  =  pounds 


of  calcium  oxide  required  for  100,000  gallons  of  water.  Instead 
of  this  the  calculation  may  be  made  to  gallons  of  lime  water  per 
100,000  gallons  of  water,  where  a  saturated  solution  of  calcium 
hydroxide  is  first  made  and  its  concentration  determined. 


WATER  403 

The  amount  of  sodium  carbonate  necessary  for  the  precipita- 
tion of  other  salts  of  calcium  and  magnesium  may  be  calculated 
in  a  similar  manner. 

An  inspection  of  the  statement  of  the  analysis  as  given  on  page 
395  will  show  that  calcium  and  magnesium  as  carbonates  may  be 
precipitated  by  lime  water,  while  calcium  as  sulphate  may  be 
removed  by  sodium  carbonate.  Since  no  other  bicarbonates  are 
present  the  amount  of  calcium  oxide  required  may  be  calculated 
directly  from  the  amount  of  carbonate  radical. 

56X160. 21X0. 0583  X  100,000  _10/<  ft 
60X7000 

Therefore  124.6  pounds  of  calcium  oxide  will   be  required  to 
precipitate  calcium  and  magnesium  carbonates. 

53X27.90X0.0583X100,000 


68X7000 


18.1. 


Therefore  18.1  pounds  of  sodium  carbonate  will  be  required  to 
remove  calcium  sulphate  from  100,000  gallons  of  water. 

The  remaining  salts  are  classed  as  corrosives  and  they  cannot 
be  removed  by  chemical  treatment. 

Problem 

74.  Calculate  the  treatment  for  the  water  already  analyzed  in  the 
laboratory. 

Boiler  Compounds. — Many  commercial  mixtures  are  now  on 
the  market,  designed  to  be  mixed  with  feed  water  as  it  enters  the 
boiler  to  prevent  the  formation  of  scale  or  to  loosen  scale  already 
formed.  Most  of  these  mixtures  are  solutions  of  sodium  carbon- 
ate or  sodium  hydroxide  with  or  without  the  addition  of  tannin  or 
some  other  colloidal  organic  compound.  The  base  precipitates 
bicarbonates  but  this  precipitation  occurs  within  the  boiler 
where  it  would  occur  if  no  base  had  been  added.  The  base  is  a 
strong  corrosive  and  may  be  highly  injurious  to  the  boiler  if 
added  in .  excess.  Colloidal  bodies,  such  as  tannin,  starch  or 
dextrine,  have  a  certain  loosening  effect  upon  the  scale  already 
present  and  to  some  extent  prevent  the  formation  of  compact 


404  QUANTITATIVE  ANALYSIS 

scale.  This  action  can  produce  only  temporary  relief,  and  a 
preliminary  treatment  such  as  has  already  been  described  is 
much  cheaper  and  better. 

Examination  of  Water  for  Sanitary  Purposes. — The  examina- 
tion of  water  to  be  used  for  drinking  may  be  made  to  determine 
the  quantity  of  mineral  salts  that  have  a  supposed  medicinal 
value  or  to  determine  its  potability.  The  examination  for  the 
first  purpose  will  follow  the  lines  of  methods  already  described 
for  boiler  water  or  it  may  be  extended  to  include  other  substances, 
such  as  lithium,  free  carbonic  acid,  hydrogen  sulphide,  etc. 
It  is  practically  certain  that  none  of  the  mineral  waters  that  are 
exploited  in  a  commercial  way  contains  enough  of  any  salt  or  gas 
to  have  any  appreciable  effect  upon  the  human  system  and  that 
the  beneficial  effects  that  are  noticed  by  those  who  take  treatment 
at  the  mineral  springs  are  due  largely  or  entirely  to  other  causes, 
such  as  enforced  dieting,  bathing,  relaxation  from  care,  good 
exercise  and  the  drinking  of  plenty  of  water.  This  being  the 
case  it  is  apparent  that  the  chemist's  report  on  the  analysis  of 
mineral  waters  can  have  little  value  except  as  a  commercial 
document.  If  this  analysis  is  demanded  the  methods  already 
given  may  be  used,  proper  modifications  being  made  in  quantity 
of  water  and  reagents  according  to  large  variations  in  the  percent 
of  mineral  matter  present.  Additional  determinations  will  be 
described. 

Free  Carbonic  Acid. — Water  containing  free  carbonic  acid  will 
rapidly  lose  carbon  dioxide  and  it  therefore  becomes  necessary 
to  make  this  determination  as  soon  as  the  water  sample  is  taken 
or  to  preserve  the  sample  in  tightly  closed  bottles  entirely  filled 
with  water.  The  determination  depends  upon  the  fact  that  as 
sodium  carbonate  is  added  to  carbonic  acid  in  presence  of  phenol- 
phthalein  a  color  change  occurs  at  the  moment  that  all  of  the  car- 
bonic acid  has  been  used  in  forming  sodium  bicarbonate,  sodium 
carbonate  having  a  basic  reaction  toward  phenolphthalein. 
The  end  point  is  shown  when  the  following  reaction  is  completed : 

Na2CO3+H2C03-+2NaHCO3. 

Determination. — Calculate  the  normality  of  a  solution  of  sodium  car- 
bonate so  made  that  1  cc  is  equivalent  to  0.001  gm  of  carbon  dioxide. 
Make  1000  cc  of  such  a  solution,  using  sodium  carbonate  prepared  by 


WATER  405 

heating  recrystallized  sodium  bicarbonate  to  about  300°  until  it  ceases 
to  lose  weight.  Do  not  allow  the  solution  to  come  into  contact  with  air 
more  than  is  necessary. 

Titrate  100  cc  of  water  as  rapidly  as  possible  with  the  standard  sodium 
carbonate  solution  and  report  milligrams  per  liter  of  carbon  dioxide. 
The  report  is  often  made  as  volume  of  gas  per  unit  volume  of  water  at 
some  specified  temperature  as,  for  example,  cubic  inches  of  gas  per  gallon 
of  water,  at  60°  F.  Reference  to  tables  of  density  of  carbon  dioxide 
will  give  the  necessary  data  for  this  calculation. 

Hydrogen  Sulphide.  —  The  determination  of  hydrogen  sulphide 
must  be  made  as  soon  as  the  water  is  taken  and  for  the  same 
reasons  that  apply  to  carbon  dioxide.  Titration  is  made  with 
standard  iodine  solution  according  to  the  reaction: 


Determination.  —  Make  a  solution  of  iodine,  1  cc  of  which  is  equivalent 
to  0.001  gm  of  hydrogen  sulphide.  Standardize  by  titrating  with  stand- 
ard sodium  thiosulphate.  Titrate  100  cc  of  the  water  with  the  standard- 
ized iodine  solution,  using  starch  solution  as  indicator.  Calculate 
milligrams  per  liter  of  hydrogen  sulphide,  also  cubic  inches  per  gallon. 

Iron.  —  The  quantity  of  iron  in  water  is  usually  too  small  to 
make  ordinary  volumetric  methods  desirable  for  its  determina- 
tion and  a  more  sensitive  colorimetric  method  is  substituted. 
The  iron  is  obtained  in  the  ferric  state  and  is  treated  with  potas- 
sium thiocyanate.  The  red  color  so  produced  is  compared  in 
tubes  with  that  formed  by  a  standard  iron  solution.  A  distinc- 
tion may  be  made  between  ferrous  and  ferric  iron  by  using  potas- 
sium ferricyanide  instead  of  potassium  thiocyanate.  In  this  case 
a  blue  color  is  produced  by  ferrous  iron  and  no  visible  color  re- 
sults from  the  reaction  of  the  small  amount  of  ferric  iron  usually 
present. 

Determination  of  Total  Iron.  —  Prepare  the  following  reagents: 

1.  Standard  Iron  Solution.  —  Calculate   the  weight  of  ferrous  am- 

monium sulphate  required  for  1000  cc  of  solution,  1  cc  of  which  shall 

contain  0.0001  gm  of  iron.     Dissolve  this  quantity  of  the  salt  in  about 

50  cc  of  distilled  water  and  add  20  cc  of  dilute  sulphuric  acid.     Warm 


406  QUANTITATIVE  ANALYSIS 

slightly  and  add  potassium  permanganate  solution  until  the  iron  is 
completely  oxidized,  using  the  smallest  possible  excess.  Dilute  to 
1000  cc. 

2.  Potassium  Thiocyanate. — Dissolve  20  gm  of  the  salt  in  1000  cc  of 
distilled  water. 

3.  Dilute  Hydrochloric  Acid. — Dilute  the  concentrated  acid  with  an 
equal  volume  of  distilled  water.     The  acid  must  be  free  from   nitric 
acid. 

4.  Potassium  Permanganate. — Make  500  cc  of  a  solution   approxi- 
mately fifth-normal. 

Evaporate  100  cc  of  the  sample  to  dryness,  or  use  the  residue  left 
after  the  determination  of  solids.  With  silt-bearing  waters  the  quantity 
of  iron  is  sometimes  so  great  that  it  is  necessary  to  use  as  little  as  10 
cc  of  the  sample.  With  such  waters  evaporation  should  be  made  in 
the  presence  of  5  to  10  cc  of  concentrated  hydrochloric  acid  to  effect 
complete  solution  of  the  iron.  If  the  sample  of  water  contains  much 
organic  matter,  destroy  this  by  ignition,  taking  care  not  to  prolong 
the  ignition  so  as  to  render  the  iron  too  difficultly  soluble. 

Cool  the  dish  and  add  5  cc  of  dilute  hydrochloric  acid  to  moisten  the 
whole  of  the  inner  surface  of  the  dish.  Place  the  dish  on  the  steam  bath 
for  two  or  three  minutes  and  again  moisten  the  whole  inner  surface  by 
allowing  the  hot  acid  to  flow  over  it.  Add  5  to  10  cc  of  distilled  water 
to  rinse  down  the  sides  of  the  dish,  and  again  place  on  the  steam  bath 
for  about  three  minutes. 

The  hot  acid  solution  is  washed  from  the  dish  with  distilled  water 
into  a  100-cc  Nessler  tube  (see  page  417).  Filter  the  sample  if  necessary, 
carefully  washing  the  filter  paper  with  hot  water.  Add  a  drop  or  two  of 
potassium  permanganate  solution  to  oxidize  the  iron  to  the  ferric  condi- 
tion. The  color  of  the  permanganate  should  persist  for  at  least  5 
minutes;  if  not,  add  more  permanganate  solution,  a  drop  at  a  time. 

To  the  cooled  solution  10  cc  of  potassium  thiocyanate  solution  is 
added,  and  the  volume  made  up  to  100  cc  and  well  mixed. 

Immediately  compare  the  resulting  color  with  that  in  a  series  of 
standards  prepared  side  by  side  with  the  sample  in  100-cc  Nessler  tubes 
in  which  amounts  of  standard  iron  solution  ranging  from  0.05  cc  to 
4  cc  are  first  diluted  with  water  to  about  50  cc.  5  cc  of  dilute  hydro- 
chloric acid  and  a  drop  or  two  of  potassium  permanganate  are  added 
to  each  tube  of  standard  solution  and  all  are  diluted  to  100  cc.  The 
number  of  standards  needed  is  governed  by  the  quantity  of  iron  likely 
to  be  present  in  the  sample  examined. 

Potassium  thiocyanate  is  added  to  each  of  the  standard  solutions  at 
the  same  time  that  this  reagent  is  added  to  the  samples  of  water  under 


WATER  407 

examination.  Comparison  of  the  sample  -with  the  standards,  which  are 
made  up  to  100  cc  after  adding  the  thiocyanate  and  mixing,  should 
be  made  immediately. 

Potability. — The  examination  of  water  to  determine  its  suit- 
ability for  drinking  (potability)  is  practically  always  directed 
toward  the  question  as  to  whether  pollution  by  sewage  has 
occurred.  This  examination  is  quite  different  in  principle  from 
any  of  the  processes  already  studied,  in  that  the  substances 
actually  determined  are  nearly  or  quite  harmless  and  are  sig- 
nificant only  as  they  point  to  the  probable  presence  of  patho- 
genic bacteria.  The  chemical  examination  goes  no  farther  than 
the  determination  of  certain  chemical  compounds  which  always 
accompany  sewage  and  which  therefore  indicate  a  danger  in  the 
use  of  the  water  for  drinking  because  disease-producing  micro- 
organisms also  generally  accompany  sewage. 

Since  this  is  the  case  it  might  be  supposed  that  the  examina- 
tion might  be  more  properly  made  by  the  bacteriologist,  who 
determines  directly  the  presence  or  absence  of  bacteria  in  ab- 
normal numbers  and  who  also  makes  a  direct  test  for  B.  coli 
communis,  an  organism  that  practically  always  accompanies 
faecal  discharges  and  is  therefore  found  in  all  water  polluted  by 
waste  matter  from  human  organisms.  If  the  results  of  the 
bacteriological  examination  were  unfailing  this  examination 
would  probably  suffice  for  all  cases.  It  should  be  noted,  however, 
that  the  bacteriologist  is  also  striving  for  indirect  rather  than 
direct  results.  It  is  not  practicable  to  make  a  direct  examina- 
tion of  the  species  of  every  organism  found  in  order  to  test  for 
the  presence  of  actual  pathogenic  forms  and  reliance  is  generally 
placed  upon  the  two  factors  noetd  above,  i.e.,  the  concentration 
of  bacteria  (number  per  cubic  centimeter)  and  the  presence  or 
absence  of  B.  coli  communis.  If,  for  some  reason,  conditions 
were  temporarily  unfavorable  to  the  growth  of  bacteria  at  the 
time  the  sample  was  taken,  the  number  of  organisms  might  be 
so  reduced  as  to  cause  no  suspicion  of  the  real  condition  of  the 
water.  It  is  conceivable  that 'the  chance  entrance  of  antiseptic 
substances  into  sewers  or  streams  or  the  action  of  sunlight  and 
air  should  bring  about  this  result  and  B.  coli  communis  might 
also  be  entirely  absent.  This  might,  for  instance,  happen  as  a 
result  of  mixing  with  factorywastes  such  as  those  from  the  manu- 


408  QUANTITATIVE  ANALYSIS 

facture  of  paper  and  textiles  and  from  plating,  bleaching  and 
other  chemical  industries.  In  such  a  case  the  water  would  be 
passed  by  the  bacteriologist  when  it  should  be  condemned.  On 
the  other  hand  such  influences  as  those  mentioned  would  not 
eliminate  the  chemical  products  of  putrescent  sewage  and  the 
chemical  examination  would  be  likely  to  show  pollution.  From 
this  examination  the  water  would  be  condemned.  The  conclu- 
sion is  that  neither  method  of  examination  is  infallible  and  both 
should  be  used  wherever  much  importance  attaches  to  the  results. 
If  one  method  must  be  omitted  it  is  preferable  that  this  should  be 
the  bacteriological  method,  provided  that  sufficient  data  are  at 
hand  for  the  proper  interpretation  of  the  results  of  the  chemical 
examination. 

Interpretation. — In  order  properly  to  interpret  thq  results  of 
the  analysis  it  is  necessary  to  know  the  normal  condition  of  the 
particular  class  of  water  under  examination  because  all  of  the 
substances  occur  normally  in  practically  all  waters  and  their 
proportions  vary  with  the  source  of  the  water.  For  example, 
chlorides  occur  in  all  ground  and  surface  waters  and  the  amount 
is  governed  largely  by  soil  conditions.  Near  the  sea  shore 
chlorides  occur  in  ground  waters  in  large  quantities.  Similar 
conditions  obtain  for  nitrogen  in  all  of  its  forms  and  for  organic 
matter  and  total  solids.  The  necessary  data  for  the  interpreta- 
tion of  a  single  analysis  cannot  well  be  collected  by  individuals 
without  great  expenditure  of  time  and  labor.  They  are  gener- 
ally obtained  as  a  result  of  organized  efforts  of  state  and  city 
boards  of  health  and  of  scientific  societies. 

The  details  of  manipulation  in  the  analytical  work  later 
described,  as  well  as  the  directions  for  taking  samples  and  making 
the  physical  examination,  are  essentially  those  recommended  by 
the  Joint  Committee  on  Standard  Methods  of  Water  Analysis  of 
the  American  Public  Health  Association,  American  Chemical 
Society  and  Association  of  Official  Agricultural  Chemists.  The 
report  of  this  Committee  forms  a  most  valuable  contribution  to 
the  scientific  phases  of  the  subject,  not  only  in  the  matter  of 
unification  of  practice  but  also  in  the  guidance  that  it  affords  in 
a  selection  of  the  best  methods  now  available.  The  third  edi- 
tion of  the  report  is  printed  as  a  special  volume  to  be  obtained 
from  The  American  Public  Health  Association. 


WATER  409 

For  the  directions  for  determinations  other  than  those  here 
described  reference  should  be  made  to  the  complete  report. 

Collection  of  Samples:  Quantity. — The  minimum  quantity  neces- 
sary for  making  the  ordinary  physical,  chemical  and  microscopical 
analyses  of  water  or  sewage  is  two  liters;  for  the  bacteriological  ex- 
amination, one  hundred  cubic  centimeters.  In  spe'cial  cases  larger 
quantities  may  be  required. 

Bottles. — The  bottles  for  the  collection  of  samples  must  be  of  halt, 
clear,  white  glass,  and  they  must  have  glass  stoppers.  Earthen  jugis 
or  metal  containers  must  not  be  used. 

Sample  bottles  must  be  carefully  cleansed  each  time  before  using. 
This  may  be  done  by  treating  with  sulphuric  acid  and  potassium 
dichromate,  or  with  a  basic  solution  of  potassium  permanganate, 
and  afterward  with  a  mixture  of  oxalic  and  sulphuric  acids,  then 
thoroughly  rinsing  with  water  and  draining. 

When  clean,  the  stoppers  and  necks  of  the  bottles  are  protected 
from  dirt  by  tying  cloth  or  thick  paper  over  them. 

For  shipment  they  are  packed  in  cases  with  a  separate  compartment 
for  each  bottle.  Wooden  boxes  may  be  lined  with  indented  fiber  paper, 
felt  or  some  similar  substance,  or  provided  with  spring  corner  strips, 
to  prevent  breakage.  Lined  wicker  baskets  also  may  be  used. 

Bottles  for  bacterial  samples,  besides  being  washed,  must  be  sterilized 
with  dry  heat  for  one  hour  at  160°  or  in  an  autoclave  at  115°  for  fifteen 
minutes.  For  transportation  they  may  be  wrapped  in  sterilized  cloth 
or  paper,  or  the  necks  may  be  covered  with  tin  foil  and  the  bottles  put 
in  tin  boxes.  When  bacterial  samples  must  of  necessity  stand  for  twelve 
hours  before  plating,  bottles  holding  more  than  four  ounces  must  be  used. 

The  bottles  used  for  chemical  samples  may  be  sterilized  and  the 
samples  so  collected  used  for  the  bacteriological  analysis.  When 
bacterial  samples  are  not  plated  at  the  time  of  collection  they  are  kept 
on  ice  at  a  temperature  not  higher  than  15°  and  preferably  as  low  as  10°. 

Time  Interval  between  Collection  and  Analysis. — Generally  speaking, 
the  shorter  the  time  elapsing  between  the  collection  and  the  analysis  of 
a  sample,  the  more  reliable  will  be  the  analytical  results.  Under  many 
conditions,  analyses  made  in  the  field  are  to  be  commended,  as  data  so 
obtained  are  frequently  preferable  to  those  made  in  a  distant  laboratory 
after  the  composition  of  the  water  has  changed  en  route. 

The  allowable  time  that  may  elapse  between  the  collection  of  a  sample 
and  the  beginning  of  its  analysis  cannot  be  stated  definitely,  as  it 
depends  upon  the  character  of  the  sample  and  upon  other  conditions, 
but  the  following  may  be  considered  as  fairly  reasonable  maximum 
limits  under  ordinary  conditions: 


410  QUANTITATIVE  ANALYSIS 

Physical  and  Chemical  Analysis. 

Ground  waters 72  hours 

Fairly  pure  surface  waters 48  hours 

Polluted  surface  waters 12  hours 

Sewage  effluents 6  hours 

Raw  sewages 6  hours 

Microscopical  Examination. 

Ground  waters 72  hours 

Fairly  pure  surface  waters 24  hours 

Waters  containing  fragile  organisms .  .  Immediate  examination 

Bacteriological  Examination. 

Samples  kept  at  less  than  10° 24  hours 

If  sterilized  by  the  addition  of  chloroform,  formaldehyde,  mercuric 
chloride,  or  some  other  disinfectant,  samples  for  chemical  and  micro- 
scopical examination  may  be  allowed  to  stand  for  longer  periods  than 
those  indicated,  but  as  this  is  a  matter  which  must  vary  according  to 
local  circumstances,  no  definite  procedure  is  recommended. 

If  unsterilized  samples  of  sewage,  sewage  effluents,  and  highly  polluted 
surface  waters  are  not  analyzed  on  the  day  of  their  collection,  caution 
must  be  used  in  regard  to  the  organic  contents,  which  frequently  change 
materially  upon  standing. 

The  gaseous  contents  of  samples,  especially  dissolved  oxygen  and 
carbonic  acid,  should  be  obtained  immediately,  in  accordance  with  the 
directions  given  in  connection  with  each  determination. 

Representative  Samples. — Care  should  be  taken  to  secure  a  sample 
which  is  truly  representative  of  the  liquid  to  be  analyzed.  In  the  case 
of  sewages  this  is  especially  important,  in  view  of  the  marked  variations 
in  composition  which  occur  from  hour  to  hour.  Frequently  satisfactory 
samples  can  be  obtained  only  by  mixing  together  several  portions  col- 
lected at  different  times  or  in  different  places — the  details  as  to  collecting 
and  mixing  depending  upon  local  conditions. 

Physical  Examination:  Temperature. — The  temperature  of  the 
sample  should  be  taken  at  the  time  of  collection,  and  should  be  expressed 
in  Centigrade  degrees,  to  the  nearest  degree  or  closer  if  for  any  reason 
more  exact  data  are  required.  For  obtaining  the  temperature  of  water 
at  various  depths  below  the  surface  the  thermophone  is  recommended. 

Turbidity. — The  turbidity  of  water  is  due  to  suspended  matter, 
such  as  clay,  silt,  finely  divided  organic  matter,  microscopic 
organisms,  etc.  The  increasing  use  of  filters  for  the  purification 
of  water  and  sewage  has  made  this  determination  one  of  great 
importance. 


WATER  411 

The  standard  of  turbidity  is  that  adopted  by  the  United  States 
Geological  Survey,  namely,  a  water  which  contains  100  parts  of  silica 
per  million  in  such  a  state  of  fineness  that  a  bright  platinum  wire  one 
millimeter  in  diameter  can  just  be  seen  when  the  center  of  the  wire  is 
100  millimeters  below  the  surface  of  the  water  and  the  eye  of  the  ob- 
server is  1.2  meters  above  the  wire,  the  observation  being  made  in  the 
middle  of  the  day,  in  the  open  air,  but  not  in  sunlight,  and  in  a  vessel 
so  large  that  the  sides  do  not  shut  out  the  light  so  as  to  influence  the 
results.  The  turbidity  of  such  water  is  taken  as  100. 

Preparation  of  Silica  Standard. — Dry  Pear's  "precipitated  fuller's 
earth"  and  sift  it  through  a  200-mesh  sieve. 

1  gm  of  this  preparation  in  one  liter  of  distilled  water  makes  a  stock 
suspension  which  contains  1000  parts  per  million  of  silica  and  which 
should  have  a  turbidity  of  1000.  Test  this  suspension,  after  diluting  a 
portion  of  it  with  nine  times  its  volume  of  distilled  water,  with  a  wire  to 
ascertain  if  the  silica  has  the  necessary  degree  of  fineness  and  if  the  sus- 
pension has  the  necessary  degree  of  turbidity.  If  not,  correct  by  adding 
more  silica  or  more  water  as  the  case  demands. 

Standards  for  comparison  are  prepared  from  this  stock  suspension 
by  dilution  with  distilled  water.  For  turbidity  readings  below  20, 
standards  of  0,  5,  10,  15  and  20  are  kept  in  gallon  bottles  made  of 
clear  white  glass;  for  readings  above  20,  standards  of  20,  30,  40,  50,  60, 
70,  80,  90  and  100  are  kept  in  100-cc  Nessler  tubes  approximately  20 
mm  in  diameter. 

Comparison  of  the  water  under  examination  with  the  standards  is 
made  by  viewing  them  sidewise  toward  the  light,  looking  at  some 
object  and  noting  the  distinctness  with  which  the  margins  of  the  object 
can  be  seen. 

The  standards  must  be  kept  stoppered  and  both  sample  and  standards 
thoroughly  shaken  before  making  the  comparison. 

In  order  to  prevent  any  bacterial  or  algal  growths  from  appearing  in 
the  standards,  a  small  amount  of  mercuric  chloride  may  be  added  to 
them. 

Platinum  Wire  Method. — This  method  requires  a  rod  with  a  platinum 
wire  having  a  diameter  of  1  mm  inserted  in  it  about  25  mm  from  the  end 
of  the  rod,  and  projecting  from  it  at  least  25  mm  at  a  right  angle.  Near 
the  end  of  the  rod,  at  a  distance  of  1.2  meters  from  the  platinum  wire, 
a  wire  ring  is  placed  directly  above  the  wire  through  which,  with  his 
eye  directly  above  the  ring,  the  observer  shall  look  when  making  the 
examination.  The  rod  is  graduated  as  follows : 

The  graduation  mark  of  100  is  placed  on  the  rod  at  a  distance  of 
100  mm  from  the  center  of  the  wire.  Other  graduations  are  made  ac- 
cording to  the  table  on  page  412.  These  graduations  are  the  ones  used 


412 


QUANTITATIVE  ANALYSIS 


to  construct  what  is  known  as  the  U.  S.  Geological  Survey  Turbidity 
Rod  of  1902. 

Procedure. — Push  the  rod  down  into  the  water  vertically  as  far  as 
the  wire  can  be  seen  and  then  read  the  level  of  the  surface  of  the  water  on 
the  graduated  scale.  This  will  indicate  the  turbidity. 

The  following  precautions  should  be  taken  to  insure  correct  results: 
Observations  should  be  made  in  the  open  air,  preferably  in  the 
middle  of  the  day  and  not  in  direct  sunlight.  The  wire  must  be  kept 
bright  and  clean.  Waters  which  have  a  turbidity  above  500  are  diluted 
with  clear  water  before  the  observations  are  made,  but  in  case  this  is 
done  the  degree  of  dilution  used  should  be  stated  and  should  form  a  part 
of  the  report. 


Turbidity,  parts 
per  million 

Vanishing  depth 
of  wire,  mm 

Turbidity,  parts 
per  million 

Vanishing  depth 
of  wire,  mm 

7 

1095 

*     70 

-   138 

8 

971 

75 

130 

9 

873 

80 

122 

10 

794 

85 

116 

11 

729 

90 

110 

12 

674 

95 

105 

13 

627 

100 

100 

14 

587 

110 

93 

15 

551 

120 

86 

16 

520 

130 

81 

17 

493 

140 

76 

18 

468 

150 

72 

19 

446 

160 

68.7 

20 

426 

180 

62.4 

22 

391 

200 

57.4 

24 

361 

250 

49.1 

26 

336 

300 

43.2 

28 

314 

350 

38.8 

30 

296 

400 

35.4 

35 

257 

500 

30.9 

40 

228 

600 

27.7 

45 

205 

800 

23.4 

50 

187 

1000 

20.9 

55 

171 

1500 

17.1 

60 

158 

2000 

14.8 

65 

147 

3000 

12.1 

WATER  413 

The  wire  method  is  used  for  testing  the  degree  of  fineness  of  the 
standard  silica,  and  this  degree  of  fineness  shall  be  such  that  when  added 
to  distilled  water  in  an  amount  equal  to  100  parts  per  million,  the  wire 
observed  under  standard  conditions  can  be  just  seen  at  a  depth  of  100 
mm  below  the  surface  of  the  water. 

Expression  of  Results. — The  results  of  turbidity  observations  are 
expressed  in  whole  numbers  which  correspond  to  parts  per  million  of 
silica,  and  recorded  as  follows: 

Turbidity  between        1  and  50,  recorded  to  nearest  unit. 

Turbidity  between      51  and  100,  recorded  to  nearest      5 

Turbidity  between    101  and  500,  recorded  to  nearest    10 

Turbidity  between    501  and  1000,  recorded  to  nearest    50 

Turbidity  between  1001  and  above,  recorded  to  nearest  100 

Coefficient  of  Fineness.- — The  number  obtained  by  dividing 
the  weight  of  suspended  matter  in  the  sample  (in  parts  per 
million)  by  the  turbidity  is  called  the  coefficient  of  fineness.  If 
greater  than  unity  it  indicates  that  the  matter  in  suspension  in 
the  water  is  coarser  than  the  standard;  if  less  than  unity,  that 
it  is  finer  than  the  standard. 

Color. — The  color  of  water  may  form  an  important  indication 
of  pollution.  There  is  little  of  value  to  be  obtained  from  a 
quantitative  measurement  of  color  although  the  determination 
is  discussed  at  length  in  the  report  of  the  Committee  on  Standard 
Methods. 

Odor. — The  observation  of  the  odor  of  cold  and  hot  samples  of 
surface  waters  is  very  important,  as  the  odors  are  usually  con- 
nected with  some  organic  growths  or  with  sewage  contamination 
or  both. 

The  odor  of  ground  waters  is  often  caused  by  the  earthy  con- 
stituents of  the  water  bearing  strata.  The  odor  of  a  contami- 
nated well  water  is  often  decisive  evidence  of  its  pollution. 

A  study  of  the  organisms  of  water  is  an  invaluable  adjunct  to 
the  physical  and  chemical  examination  of  water.  Certain  organ- 
isms can  be  distinguished  by  their  odor,  as,  for  example,  the 
" fishy"  odor  of  Uroglena  the  " aromatic"  or  "rose  geranium" 
odor  of  Asterionella  and  the  " pig-pen"  odor  of  Anabcena. 

Determination. — Observe  and  record  the  odor,  both  at  room  tempera- 
ture and  at  just  below  the  boiling-point,  as  follows: 

Cold  Odor. — Shake  the  sample  violently  in  one  of  the  collecting  bottles, 
when  it  is  about  half  or  two-thirds  full  and  when  the  sample  is  at  room 


414 


QUANTITATIVE  ANALYSIS 


temperature  (about  20°).  Remove  the  stopper  and  test  the  odor  at 
the  mouth  of  the  bottle. 

Hot  Odor. — Into  a  500  cc  Erlenmeyer  flask  pour  about  150  cc  of  the 
sample.  Cover  the  flask  with  a  well-fitting  watch  glass,  place  on 
a  hot  plate  and  bring  the  water  to  just  below  boiling.  Remove  the 
flask  from  the  plate  and  allow  it  to  cool  for  not  more  than  five  minutes. 
Then  shake  with  a  rotary  movement,  slip  the  watch  glass  to  one  side 
and  test  the  odor.  . 

Expression  of  Results. — Express  the  quality  of  the  odor  by  some 
such  descriptive  epithet  as  the  following,  which  for  purposes  of  record 
may  be  abbreviated : 


v  vegetable 
a  aromatic 
g  grassy 
f  fishy 
e  earthy 
c  free  chlorine 


m  moldy 
M  musty 

d  disagreeable 

P  peaty 

s  sweetish 

S  hydrogen  sulphide 


Express  the  intensity  of  the  odor  by  a  numeral  prefixed  to  the  term 
expressing  quality,  which  may  be  defined  as  follows: 


Numerical 
value 

Term 

Approximate  definition 

0 

1 

2 

None  
Very  faint  

Faint  

No  odor  perceptible. 
An  odor  that  would  not  be  ordinarily  detected  by  the 
average  consumer,  but  that  could  be  detected  in  the 
laboratory  by  an  experienced  observer. 
An  odor  that  the  consumer  might  detect  if  his  attention 

3 

4 
5 

Distinct  
Decided  

Very  strong.  .  . 

were  called  to  it,  but  that  would  not  otherwise  attract 
attention. 
An  odor  that  would  be  readily  detected  and  that  might 
cause  the  water  to  be  regarded  with  disfavor. 
An  odor  that  would  force  itself  upon  the  attention  and 
that  might  make  the  water  unpalatable. 
An   odor   of   such  intensity  that  the   water   would  be 
absolutely  unfit  to  drink.      (A  term  to  be  used  only  in 
extreme  cases.) 

Chemical  Examination. — The  following  determinations  may  be 
made:  Total  solids,  chlorine  of  chlorides,  albumenoid  nitrogen, 
total  organic  nitrogen,  nitrogen  of  ammonia  or  ammonium 
salts,  nitrogen  of  nitrites,  nitrogen  of  nitrates,  total  organic 
matter,  dissolved  oxygen  and  poisonous  metals.  Besides  the 
chemical  analysis  certain  purely  physical  tests  may  be  made  such 
as  temperature,  color,  odor  and  turbidity.  These  determinations 
have  just  been  described. 


WATER  415 

Total  Solids. — This  is  taken  as  the  residue  obtained  when  a 
measured  volume  of  water  is  evaporated.  The  general  character 
of  the  solids  may  be  sometimes  noted,  also  the  amount  of  loss 
suffered  by  igniting  in  air  and  the  odor  and  amount  of  charring 
afford  an  indication  as  to  the  quantity  and  character  of  solid 
organic  matter. 

Determination. — Ignite  and  weigh  a  platinum  dish,  then  evaporate 
in  it  100  cc  of  water,  using  the  steam  bath.  The  dish  need  not  be  large 
enough  to  hold  the  entire  100  cc  at  one  time.  A  small  dish  is  better. 
Heat  the  residue  at  about  103°  for  one-half  hour.  Cool  in  the  desiccator 
and  weigh.  Report  the  increase  in  weight  as  milligrams  per  liter  of 
total  solids.  Heat  the  dish  at  low  redness  until  all  organic  matter  is 
burned.  The  change  in  weight  is  loss  on  ignition. 

Suspended  Matter. — Filter  a  portion  of  the  sample  through  a  very 
fine,  close  filter  paper  or  a  well-formed  asbestos  mat  in  a  Gooch  crucible, 
rejecting  the  first  15  to  25  cc.  Determine  the  total  dissolved  solids  in 
the  filtrate  as  already  directed.  This,  subtracted  from  the  solids  of 
the  original  sample,  gives  the  total  suspended  matter. 

Chlorine  of  Chlorides. — Chlorine  occurs  to  some  extent  in  all 
natural  waters.  It  is  found  to  a  much  larger  extent  in  sewage 
where  it  enters  chiefly  as  sodium  chloride  of  urine  and  faeces, 
and  of  household  wastes.  Sewage  polluted  streams  or  wells, 
therefore,  always  carry  abnormally  large  quantities  of  chlorine. 
Also  the  chlorine  content  may  be  increased  by  ocean  vapors 
carried  inland  by  natural  deposits  or  by  factory  wastes.  If  the 
normal  chlorine  content  exceeds  20  mg  per  liter,  the  determina- 
tion will  have  little  significance  from  the  sanitary  standpoint. 

Determination. — Use  the  method  described  on  page  400.  Report 
milligrams  per  liter  of  chlorine. 

Nitrogen  in  Various  Forms. — Human  faeces  contains  large 
quantities  of  nitrogen  while  urine  has  a  normal  content  of  about 
0.85  percent  of  nitrogen.  The  entrance  of  sewage  therefore  im- 
parts abnormal  concentrations  of  nitrogen  to  water.  This 
nitrogen  is  at  first  practically  all  in  the  form  of  organic  com- 
pounds and  of  urea.  Part  of  the  organic  nitrogen  is  readily 
converted  into  ammonia  by  oxidizing  with  potassium  per- 


416  QUANTITATIVE  ANALYSIS 

manganate  in  basic  solution.  This  part  is  known  as  "albume- 
noid  nitrogen"  because  it  is  contained  in  albumenous  bodies. 

The  action  of  certain  forms  of  bacteria  (chiefly  anaerobic) 
causes  the  putrefaction  of  organic  matter  and  this  cleavage  of 
complex  compounds  results  in  the  formation  of  ammonia  from 
the  nitrogen.  Part  or  all  of  this  ammonia  may  combine  with 
acids  to  form  ammonium  salts.  All  such  nitrogen  is  known  as 
"  nitrogen  of  free  ammonia,"  whether  this  be  of  really  free 
ammonia  or  of  ammonium  salts. 

Where  sewage  or  water  polluted  by  it  is  exposed  to  air  and 
sunlight  the  simpler  organic  compounds  produced  by  putrefaction 
are  subjected  to  oxidation,  this  being  promoted  by  other  forms  of 
bacteria  (aerobic).  Ultimately  the  organic  compounds  are 
completely  oxidized.  The  two  processes,  putrefaction  and 
oxidation,  are  made  the  basis  of  the  septic  process  of  water  puri- 
fication. In  the  operations  of  water  analysis  the  changes  in 
the  forms  of  nitrogen  are  most  important.  "Free"  ammonia  is 
oxidized  to  nitrous  acid  which  usually  remains  combined  as 
nitrites  of  metals  or  of  ammonium.  Further  oxidation  produces 
nitric  acid  or  nitrates,  the  final  stage  in  the  series. 

The  analytical  estimation  of  the  nitrogen  in  different  forms  in 
water  offers  a  valuable  indication,  not  only  as  to  the  probability 
of  pollution  but  also  concerning  the  present  condition.  The 
presence  of  abnormal  quantities  of  "albumenoid  nitrogen" 
indicates  the  presence  of  unchanged  sewage  and  the  probable 
presence  of  dangerous  micro-organisms  in  their  most  virulent 
condition.  "Free  ammonia"  in  considerable  quantities  shows 
that  the  raw  sewage  has  become  fermented  and  that  it  must  have 
been  largely  diluted  in  the  time  that  has  elapsed  since  the 
entrance  of  sewage.  Nitrites  are  very  readily  oxidized  and  will 
not  be  found  in  more  than  traces  unless  free  ammonia  is  also 
present.  Abnormal  quantities  of  nitrates,  unless  these  are  of 
inorganic  origin,  are  the  result  of  complete  oxidation  of  organic 
matter  and  this  must  have  required  time  and  continued  action  of 
air  and  sunlight.  If  all  forms  of  nitrogen  are  found  in  abnormal 
quantities  continuous  pollution  is  occurring.  All  of  these  figures 
are  highly  significant  in  view  of  the  fact  that  the  same  influences 
that  promote  the  decomposition  and  oxidation  of  nitrogenous 
organic  matter  also  combine  for  the  partial  or  complete  steriliza- 


WATER  417 

tion  of  the  water.  It  is  not,  by  any  means,  to  be  concluded  that 
water  which  has  been  polluted  by  sewage  but  in  which  the  latter 
has  become  completely  oxidized  is  necessarily  safe  for  drinking. 
Indeed  if  the  analysis  shows  pollution,  even  at  a  remote  source  or 
time,  the  water  should  be  condemned  as  dangerous.  The  degree 
of  danger  is  still  indicated  and  the  indication  will  prove  of 
value. 

As  compared  with  most  other  substances  ordinarily  considered 
in  quantitative  analysis  the  different  forms  of  nitrogen  occur  in 
extremely  slight  concentrations.  Unusually  delicate  reactions 
must  be  used  in  order  to  cause  the  figures  to  have  any  value. 
Ordinary  gravimetric  or  volumetric  processes  are  rarely  used 
in  this  connection  but  very  sensitive  color  reactions  are 
made  the  basis  for  the  comparison  of  the  water  with  color 
standards. 

Free  Ammonia  is  made  evident  by  the  brown  color  produced 
when  a  solution  of  potassium  mercuriodide,  K2HgI4,  is  added. 
This  solution  is  known  as  "Nessler's  reagent,"  from  the  name  of 
the  discoverer  of  the  reaction.1  The  compound  that  is  produced 
when  ammonia  is  added  is  a  complex  substance,  thought  to 
have  the  composition  Hg2NI.  It  is  an  intensely  colored  brown 
substance  of  small  solubility  and  gives  a  visible  color  in  water 
containing  one  part  of  nitrogen  as  ammonia  in  ten  million  parts 
of  water. 

The  process  of  determining  free  ammonia  is  one  of  comparing 
the  color  produced  by  adding  Nessler's  reagent  to  water  with 
that  produced  by  the  reagent  with  a  standard  solution  of  ammo- 
nium chloride.  The  comparison  is  made  in  tubes  of  colorless 
glass,  the  two  that  are  being  compared  having  the  same  cross 
section  so  that  the  same  length  of  column  is  placed  in  the  line  of 
vision.  The  color  is  observed  by  looking  vertically  downward 
through  the  tubes  at  a  white  surface  placed  at  an  angle  in  front 
of  a  window  so  as  to  reflect  the  light  upward. 

If  Nessler's  reagent  is  added  directly  to  water  containing 
organic  matter,  iron  or  aluminium,  a  precipitate  is  produced  and 
an  accurate  color  comparison  is  impossible.  One  of  two  pre- 
liminary treatments  may  be  used.  The  free  ammonia  may  be 
separated  by  distillation  and  the  distillate  then  "  Nesslenzed " 

1  Z.  anal.  Chem.,  7,  415  (1868). 

27 


418  QUANTITATIVE  ANALYSIS 

or  reagents  may  be  added  to  the  water  sample  to  precipitate  inter- 
fering substances  and  "  direct  Nesslerization"  may  be  employed. 
Distillation  is  preferable,  but  where  apparatus  or  time  is  limited 
direct  Nesslerization  may  be  useful. 

Direct  Nesslerization. — For  precipitating  organic  matter  use 
is  made  of  the  power  of  flocculating  colloids  for  adsorbing  this 
material,  which  is  itself  chiefly  colloidal.  Cupric  sulphate  is 
added  to  the  water  which  is  then  made  basic  by  the  addition  of 
potassium  hydroxide.  The  precipitating  cupric  hydroxide  so 
clarifies  the  water  that  direct  Nesslerization  is  practicable. 
Instead  of  adding  cupric  sulphate  a  solution  of  magnesium  chlo- 
ride may  be  substituted.  Colloidal  magnesium  hydroxide  accom- 
plishes the  same  result  as  does  cupric  hydroxide.  If  the  water 
already  contains  much  magnesium  it  is  unnecessary  to  add  even 
magnesium  chloride.  Boiling  the  water  with  potassium  hydrox- 
ide will  cause  the  precipitation  of  magnesium  hydroxide.  If 
hydrogen  sulphide  is  present  it  will  cause  the  precipitation  of 
mercuric  sulphide  when  Nessler's  reagent  is  added.  This  inter- 
ference is  prevented  by  the  addition  of  lead  acetate  before  the 
removal  of  colloids  by  cupric  hydroxide. 

The  chief  objection  to  direct  Nesslerization  is  the  tendency  of 
the  precipitating  colloids  to  adsorb  small  amounts  of  ammonia. 
However,  the  process  is  recommended  for  raw  sewages,  sewage 
effluents  and  highly  polluted  surface  waters. 

Whether  direct  Nesslerization  or  distillation  processes  are  used 
for  free  ammonia  either  an  accurately  prepared  standard  solution 
of  an  ammonium  salt  or  a  standard  color  solution  of  a  permanent 
nature  is  required.  This  must  have  a  very  slight  concentration 
and  it  is  best  made  by  successive  dilutions  of  a  more  concentrated 
solution.  The  solvent  used  is  water  that  has  been  shown  to  be 
free  from  ammonia  by  a  test  with  Nessler's  reagent.  The  labora- 
tory supply  of  distilled  water  is  often  free  from  ammonia.  If 
it  is  not  it  may  be  purified  by  the  addition  of  basic  potassium 
permanganate  solution  and  distilling.  After  the  distillate  no 
longer  gives  a  test  for  ammonia  it  is  collected  and  kept  in  well- 
stoppered  bottles. 

For  a  permanent  color  standard  mixtures  of  potassium  chlor- 
platinate  and  cobalt  chloride  are  recommended.  By  properly 
varying  the  relative  concentrations  of  the  two  salts,  solutions  are 


WATER  419 


obtained  in  which  the  color  accurately  corresponds  with  that  of 
Nesslerized  ammonia  solutions  of  known  concentrations.  These 
solutions  are  to  be  preferred  to  the  standard  ammonium  chloride 
solutions  in  laboratories  where  many  determinations  are  to  be 
made,  because  of  their  permanency. 

Albumenoid  Nitrogen  cannot  be  determined  by  direct  Nessleri- 
zation.  It  is  determined  after  the  distillation  of  free  ammonia  by 
adding  to  the  residue  a  basic  solution  of  potassium  permanganate 
and  distilling.  The  organic  matter  is  oxidized  and  remaining 
nitrogen  is  distilled  and  Nesslerized. 

As  the  ratio  of  nitrogenous  organic  matter  to  the  ammonia 
obtained  by  distillation  is  decidedly  variable  in  sewages  and 
other  substances  containing  much  nitrogenous  organic  matter, 
albumenoid  nitrogen  results  on  such  materials  are  less  accurate 
than  total  organic  nitrogen,  obtained  by  the  Kjeldahl  process. 
Therefore  in  sewage  work,  including  analysis  of  influents  and 
effluents  of  purification  plants  and  the  water  of  highly  polluted 
streams,  it  is  recommended  by  the  joint  committee  that  deter- 
minations of  total  organic  nitrogen  be  substituted  for  determina- 
tions of  albumenoid  nitrogen.  For  ground  waters  and  surface 
waters  containing  but  little  pollution,  the  albumenoid  nitrogen 
is  approximately  one-half  the  organic  nitrogen;  accordingly  the 
continuance  of  albumenoid  nitrogen  determinations  for  this  class 
of  work  is  approved. 

All  determinations  of  nitrogen  must  be  made  in  a  laboratory 
in  which  the  air  is  free  from  ammonia. 

Determination. — Prepare  the  following  reagents: 

1.  Nessler's  Reagent. — Dissolve  25  gm  of  potassium  iodide  in  the 
minimum  quantity  of  cold  water.     Add  a  saturated  solution  of  mercuric 
chloride  until  a  slight  but  permanent  precipitate  persists.     Add  200  cc 
of  50  percent  solution  of  potassium  hydroxide  made  by  dissolving  the 
potassium  hydroxide  and  allowing  it  to  clarify  by  sedimentation  before 
using.     Dilute  to  500  cc,  allow  to  settle  and  decant.     This  solution 
should  give  the  required  color  with  ammonia  within  five  minutes  after 
addition,  and  should  not  precipitate  with  small  amounts  of  ammonia 
within  two  hours. 

2.  Basic  Potassium  Permanganate. — Pour  600  cc  of  distilled  water 
into  a  porcelain  dish  holding  1500  cc,  boil  10  minutes  and  turn  off  the 
gas.     Add  8  gm  of  potassium  permanganate  and  stir  until  dissolved. 


420  QUANTITATIVE  ANALYSIS 

Add  400  cc  of  50  percent  clarified  solution  of  potassium  or  sodium 
hydroxide  and  enough  distilled  water  to  fill  the  dish.  Boil  down  to  1000 
cc.  Test  this  solution  for  albumenoid  ammonia  by  making  a  blank 
determination.  Correction  should  be  made  accordingly. 

3.  Ammonia-free  Water. — Test  the  laboratory  supply  of  distilled  water 
by  rinsing  a  clean  Nessler  tube  several  times,  filling  to  the  mark  and  add- 
ing 2  cc  of  Nessler 'B  reagent.     Cover  and  allow  to  stand  for  5  minutes. 
If  the  color  produced  at  the  end  of  this  time  is  more  intense  than  that  of 
the  diluted  Nessler's  reagent  at  first,  the  water  must  be  purified.     In 
this  case  add  10  cc  of  basic  potassium  permanganate  solution  to  each 
1000  cc  of  distilled  water  and  distill,  using  a  tin  or  aluminium  condenser 
if  one  is  available.     After  the  distillate  ceases  to  give  a  test  for  ammonia 
it  is  collected  in  a  clean,  glass-stoppered  bottle,  which  is  first  rinsed  with 
the  distillate  and  the  rinsings  tested  for  ammonia. 

4.  Standard  Solution  for  Color  Comparisons. — Use  either  {a)  or  (b). 

a.  Ammonium  Chloride  Solution. — Dissolve  3.82  gm  of  ammonium 
chloride  in  ammonia-free  water  and  dilute  to  1000  cc.     Dilute  10  cc  of 
this  to  1000  cc  with  ammonia-free  water.     1  cc  contains  0.00001  gm 
of  nitrogen. 

b.  Platinum  Solution  and  Cobalt  Solution. — Weigh  2  gm  of  potassium 
chlorplatinate,  dissolve  in  a  small  amount  of  distilled  water,  add   100 
cc  of  concentrated  hydrochloric  acid  and  make  up  to  1000  cc. 

Weigh  12  gm  of  cobalt  chloride  and  dissolve  in  distilled  water;  add 
100  cc  of  concentrated  hydrochloric  acid  and  make  up  to  1000  cc. 

Nitrogen  of  Free  Ammonia. — A  750-cc  Kjeldahl  digestion  flask, 
connected  with  a  tin  or  aluminium  condenser  in  such  a  way  that  the 
distillate  may  be  conveniently  delivered  from  the  condenser  tube 
directly  into  the  Nessler  tubes,  is  freed  from  ammonia  by  boiling  dis- 
tilled water  in  it,  until  the  distillate  shows  no  further  traces  of  free 
ammonia.  When  this  has  been  done,  empty  the  distilling  flask  and 
measure  into  it  500  cc  of  the  sample,  or  a  smaller  portion  diluted  to  500  cc 
with  ammonia-free  water.  Apply  heat  so  that  the  distillation  will  be 
at  the  rate  of  not  more  than  10  cc  nor  less  than  6  cc  per  minute. 

Collect  four  Nessler  tubes  of  the  distillate,  50  cc  to  each  portion; 
these  contain  the  free  ammonia  to  be  measured  as  described  below. 

Use  only  Nessler  tubes  which  do  not  show  a  variation  of  more  than 
6  mm  (0.25  inch)  in  the  distance  which  the  graduation  mark  (50  cc)  is 
above  the  bottom.  The  tubes  should  be  of  clear  white  glass,  with  pol- 
ished bottoms.  The  residue  from  the  distillation  is  immediately  used 
for  the  determination  of  albuminoid  nitrogen  as  described  below. 

The  measurement  may  be  made  either  by  (1)  comparison  with  Ness- 
lerized  solutions  containing  known  quantities  of  nitrogen  as  ammonium 


WATER  421 

chloride,  or  (2)  comparison  of  the  Nesslerized  distillates  with  permanent 
standards. 

Comparison  with  Ammonia  Standards. — Prepare  a  series  of  16  Nessler 
tubes  which  contain  the  following  numbers  of  cubic  centimeters  of  the 
standard  ammonium  chloride  solution,  diluted  to  50  cc  with  ammonia- 
free  water,  namely:  0.0,  0.1,  0.3,  0.5,  0.7,  1.0,  1.4,  1.7,  2.0,  2.5,  3.0, 
3.5,  4.0,  4.5,  5.0,  and  6.0.  These  will  contain  0.00001  gm  of  nitrogen 
for  each  cc  of  the  standard  solution  used. 

Nesslerize  the  standards  and  also  the  distillates  by  adding  approxi- 
mately 2  cc  of  Nessler's  reagent  to  each  tube.  Do  not  stir  the  contents 
of  the  tubes. 

Have  the  temperature  of  the  tubes  practically  the  same  as  that  of  the 
standards,  otherwise  the  colors  will  not  be  directly  comparable. 

Compare  the  color  produced  in  these  tubes  with  that  in  the  standards 
by  looking  vertically  downward  through  them  at  a  white  surface  placed 
at  an  angle  in  front  of  a  window  so  as  to  reflect  the  light  upward.  Allow 
the  tubes  to  stand  for  at  least  10  minutes  after  Nesslerizing  before  mak- 
ing the  comparison. 

In  case  the  color  obtained  by  Nesslerizing  the  distillates  is  greater  than 
that  of  the  darkest  tube  of  the  standards,  mix  the  contents  of  the  tube 
thoroughly  and  pour  out  half  of  the  liquid,  making  up  the  remainder 
to  the  original  volume  with  ammonia-free  water,  then  make  the  color 
comparison  and  multiply  the  result  by  two.  If,  after  pouring  out  half 
of  the  liquid,  the  color  is  still  too  dark,  repeat  this  process  of  division 
until  a  reading  can  be  made. 

In  case  the  color  of  the  distillates  is  too  high,  this  process  may  be 
shortened  by  mixing  together  all  of  the  distillates  from  one  sample 
before  making  the  comparison,  subsequently  taking  an  aliquot  portion 
for  comparing  with  the  standards. 

After  the  readings  have  been  made  and  recorded,  add  together  the 
results  obtained  by  Nesslerizing  each  portion  of  the  entire  distillate 
from  each  sample.  Calculate  milligrams  per  liter  of  nitrogen  as  free 
ammonia  in  the  sample. 

Comparison  with  Permanent  .Standards. — Prepare  standards  by  put- 
ting varying  amounts  of  potassium  chlorplatinate  and  cobalt  chloride 
solutions  (page  420)  in  Nessler  tubes,  filling  up  to  the  mark  with  distilled 
water  as  follows: 


422 


QUANTITATIVE  ANALYSIS 


Equivalent  volume  of  standard 
ammonium  chloride,  cc 

Platinum  solution, 
cc 

Cobalt  solution, 
cc 

0.0 

1.2 

0.0 

0.1 

1.8 

0.0 

0.2 

2.8 

0.0 

0.5 

4.7 

0.1 

0.7 

5.9 

0.2 

1.0 

7.7 

0.5 

1.4 

9.9 

1.1 

1.7 

11.4 

1.7 

2.0 

12.7 

2.2 

2.5 

15.0 

3.3 

3.0 

17.3 

4.5 

3.5 

19.0 

45.7 

4.0- 

19.7 

7.1 

4.5 

19.9 

8.7 

5.0 

20.0 

10.4 

6  0 

20.0 

15.0 

7.0 

20.0 

22.0 

It  is  necessary  to  use  tubes  which  have  the  50  cc  mark  not  less  than 
20  nor  more  than  22  cm  above  the  bottom.  These  standards  may  be 
kept  for  several  months  if  protected  from  dust.  The  method  of  cal- 
culating results  is  practically  the  same  as  with  the  ammonia  standards. 

Albumenoid  Nitrogen. — Interrupt  the  distillation  (made  as  already 
described)  after  the  collection  of  the  distillate  for  free  ammonia.  Add 
50  cc  or  more  of  basic  potassium  permanganate  solution  and  conduct 
this  distillation  until  at  least  four  portions  of  50  cc  each  and  preferably 
five  portions  of  the  distillate  have  been  collected  in  separate  tubes. 
Have  enough  permanganate  solution  present  to  insure  the  maximum 
oxidation  of  the  organic  matter.  These  distillates  contain  the  albu- 
menoid  nitrogen  as  ammonia,  measurement  of  which  will  be  made  as 
described  in  connection  with  nitrogen  as  free  ammonia. 

Nitrogen  of  Free  Ammonia  by  Direct  Nesslerization. — Prepare  the 
following  solutions. 

1.  A  10  percent  solution  of  cupric  sulphate. 

2.  A  50  percent  solution  of  sodium  hydroxide. 

If  hydrogen  sulphide  is  present  in  the  water  prepare  also : 

3.  A  10  percent  solution  of  lead  acetate. 

50  cc  of  the  sample  to  be  tested,  mixed,  if  necessary,  with  an  equal 
volume  of  ammonia-free  water,  is  placed  in  a  Nessler  tube  and  a  few 


WATER  423 

drops  of  cupric  sulphate  solution  added.  After  thorough  mixing  1  cc 
of  the  sodium  hydroxide  solution  is  added  and  the  solution  again  thor- 
oughly mixed.  The  tube  is  then  allowed  to  stand  for  a  few  minutes, 
when  a  heavy  precipitate  should  fall  to  the  bottom  leaving  a  colorless 
supernatant  liquid.  Nesslerize  an  aliquot  portion  of  this  liquid. 

If  hydrogen  sulphide  is  present  add  a  few  drops  of  lead  acetate  solu- 
tion before  the  addition  of  potassium  hydroxide. 

Organic  Nitrogen. — This  determination  may  be  conveniently  com- 
bined with  that  of  ammonia  nitrogen,  by  the  procedure  for  water  and  the 
first  procedure  for  sewage,  described  below. 

Procedure  for  Water. — Distill  the  ammonia  from  500  cc  of  the  sample 
exactly  as  already  directed  for  the  determination  of  nitrogen  of  free 
ammonia,  page  420.  This  will  usually  involve  the  loss  of  200  cc  of  the 
sample.  Cool  somewhat  and  rinse  into  a  Kjeldahl  digestion  flask,  unless 
such  a  flask  was  used  for  the  first  distillation.  Add  5  cc  of  concentrated 
sulphuric  acid  which  is  free  from  ammonium  sulphate,  also  a  small  piece 
of  ignited  pumice,  dropped  in  while  hot.  Mix  and  digest  over  a  free 
flame,  using  a  suitable  apparatus  for  removing  the  sulphuric  acid 
fumes  (see  Figure  116,  page  514).  The  digestion  should  be  continued 
until  copious  fumes  are  evolved  and  the  liquid  is  finally  colorless  or 
very  pale  yellow.  Remove  from  the  flame  and  add  to  the  hot  solution 
potassium  permanganate  crystals  until  a  heavy  green  precipitate  per- 
sists. Cool  and  dilute  to  about  300  cc  with  ammonia-free  water.  Make 
basic  by  adding  10  percent  sodium  hydroxide  which  has  been  made  free 
from  ammonia  by  boiling  for  a  short  time.  Distill  the  ammonia  and 
Nesslerize  as  directed  for  the  determination  of  nitrogen  of  free  ammonia. 

First  Procedure  for  Sewage. — Use  100  cc  or  less  of  the  sample,  according 
to  the  amount  of  ammonia  expected.  Dilute  to  500  cc  with  ammonia- 
free  water  in  the  distilling  flask  and  distill  the  free  ammonia,  Nesslerizing 
the  distillate.  Cool  somewhat  and  add  5  cc  of  nitrogen-free  concen- 
trated sulphuric  acid  and  1  cc  of  10  percent  copper  sulphate  solution. 
Digest  the  solution  for  a  half  hour  after  it  has  become  colorless  or  pale 
yellow.  Add  0.5  gm  of  potassium  permanganate  crystals  to  the  hot 
acid  solution,  cool,  transfer  to  a  500  cc  volumetric  flask,  dilute  to 
the  mark  and  mix.  By  means  of  a  pipette  transfer  10  cc  of  this  solution 
to  a  Kjeldahl  distilling  flask  and  dilute  to  about  300  cc  with  ammonia- 
free  water.  Make  basic  with  10  percent  sodium  hydroxide  solution, 
distill  and  Nesslerize.  (With  some  samples  direct  Nesslerization  may 
be  employed.) 

Second  Procedure  for  Sewage. — Omit  the  separation  of  ammonia 
nitrogen  and  determine  this  and  organic  nitrogen  together.  Determine 
the  ammonia  nitrogen  on  a  separate  sample  by  direct  Nesslerization  as 


424  QUANTITATIVE  ANALYSIS 

directed  on  page  422  .     The  difference  between  these  results  is  the  organic 
nitrogen. 

Nitrogen  as  Nitrites.  —  It  has  already  been  explained  that 
nitrites  will  not  normally  occur  in  more  than  traces  in  water 
because  of  the  readiness  with  which  they  oxidize.  In  order  to 
give  this  determination  any  significance  it  is  necessary  to  use  a 
very  delicate  test.  Use  is  made  of  the  ready  action  of  nitrous 
acid  with  aromatic  amines,  forming  diazo  compounds,  and  of 
the  latter  with  naphthylamine,  forming  azo  dyes  of  intense  color- 
ing power.  When  water  containing  nitrites  is  acidified  and 
sulphanilic  acid  (p-amidobenzenesulphonic  acid)  is  added  there 
is  formed  the  anhydride  of  p-diazobenzenesulphonic  acid,  thus: 


HN02+C6H4<  ->C6H4<  +2H50. 

\S03H  X  S03  / 

If  to  this  solution  a-amidonaphthalene  is  added,  an  azo   dye, 
a-amidonaphthaleneazobenzene-p-sulphonic    acid,    is  produced. 

/N  =  Nx  ,N  =  NC10H6NH2 

C6H4<  >  +C10H7NH2->C6H4< 

X  S03  /  \S03H 

This  dye  possesses  a  very  intense  red  color  and  one  part  of  nitro- 
gen as  nitrite  can  be  detected  in  1,000,000,000  parts  of  water. 

The  amino  compounds  entering  into  these  reactions  are  not 
easily  soluble  and  their  soluble  salts  are  used.  The  hydrochlo- 
rides  may  be  employed  but  the  reactions  appear  to  proceed  more 
rapidly  if  the  acetates  are  used. 

Determination.  —  Prepare  the  following  reagents: 

1.  Sulphanilic  Add.  —  Dissolve  4  gm  of  the  purest  sulphanilic  acid 
in  500  cc  of  5-normal  acetic  acid  (sp.  gr.  1.041)  or  in  a  mixture  of  950 
cc  of  nitrite-free  water  and  50  cc  of  concentrated  hydrochloric  acid. 
This  is  a  practically  saturated  solution. 

2.  a-amidonaphthalene  Acetate  or  Hydrochloride.  —  Dissolve  2.5  gm  of 
solid  a  -naphthylamine  in  500  cc  of  5-normal  acetic  acid  or  in  1000  cc 
of  nitrite-free  water  containing  8  cc  of  concentrated  hydrochloric  acid. 
Filter  the  solution  through  washed  absorbent  cotton  or  an  alundum 
crucible. 

3.  Sodium  Nitrite,  Stock  Solution.  —  Dissolve  1.1  gm  of  silver  nitrite 
in   nitrite-free   water.     Precipitate   the   silver   with   sodium    chloride 
solution  and  dilute  the  whole  to  1000  cc. 


WATER  425 

4.  Standard  Sodium  Nitrite  Solution. — Dilute  100  cc  of  solution  (3) 
to  1000  cc  and  dilute  10  cc  of  the  resulting  solution  to  1000  cc  with 
sterilized  nitrite-free  water.     Add  1  cc  of  chloroform  and  preserve  in 
a  sterilized  bottle.     Calculate  and  record  the  weight  of  nitrogen  in  1  cc 
of  this  last  solution. 

5.  Fuchsine  Solution. — 0.1  gm  per  liter  of  water. 

Measure  50  cc  or  100  cc  of  the  water  to  be  tested  into  a  Nessler 
tube.  These  Nessler  tubes  should  be  of  clear,  white  glass,  with  the 
graduation  mark  not  varying  more  than  6  mm  in  its  distance  above  the 
bottom.  At  the  same  time  make  a  set  of  standards  by  diluting  various 
volumes  of  the  standard  nitrite  solution  in  Nessler  tubes  to  50  or  100  cc 
with  nitrite-free  water,  for  example,  0,  1,  3,  5,  7,  10,  14,  17,  20  and  25  cc. 
Add  2  cc  of  reagents  (1)  and  (2)  to  each  100  cc  of  the  sample  and  to 
each  standard.  Mix  and  allow  to  stand  10  minutes.  Compare  the 
samples  with  the  standards.  Do  not  allow  the  samples  to  stand  over 
one-half  hour  before  being  compared.  Make  a  blank  determination  in 
all  cases  to  correct  for  the  presence  of  nitrites  in  the  air,  the  water  and 
the  reagents.  Dilute  all  samples  which  develop  more  color  than  the 
30  cc  standard  before  comparing.  Mixing  is  important.  Calculate 
milligrams  per  liter  of  nitrite  nitrogen. 

Permanent  standards  may  be  prepared  by  matching  the  nitrite 
standards  as  above  made  against  dilutions  of  the  fuchsine  solution. 
Fuchsine  standards  have  been  found  to  be  sufficiently  accurate 
for  waters  high  in  nitrite  and  for  sewage.  Such  standards  should 
be  checked  once  a  month  and  should  be  kept  out  of  bright 
sunlight  to  avoid  bleaching. 

Nitrogen  as  Nitrates. — When  a  soluble  aromatic  sulphonic 
acid  is  mixed  with  nitrates  and  sulphuric  acid  the  nitric  acid  so 
liberated  acts  upon  the  aromatic  compound  and  produces  nitro- 
derivatives  which  are  faintly  yellow  in  most  cases.  If  a  base  is 
now  added  the  sulphonate  is  formed  and  this  is  much  more  in- 
tensely colored.  These  reactions  are  applied  to  the  determina- 
tion of  nitrates  in  water  by  what  was  originally  known  as  Spren- 
gePs  method.1 

The  method  was  further  modified  by  Grandval  and  Lajoux.2 
A  measured  volume  of  water  ife  evaporated  to  dryness,  sodium 
carbonate  having  been  first  added  if  the  water  contains  free  acid. 
The  dry  residue  is  treated  with  a  small  amount  of  a  phenolsul- 

1  Pogg.  Ann.,  121,  188  (1863). 

2  Compt.  rend.,  62,  101  (1885). 


426  QUANTITATIVE  ANALYSIS 

phonic  acid,  the  mono-nitro  derivative  being  formed.  The 
reagent  is  made  by  heating  phenol  with  sulphuric  acid  in  the  pro- 
portions indicated  below.  These  interact  with  the  formation 
phenol-o-p-disulphonic  acid.  The  reaction  of  this  acid  with  ni- 
tric acid  results  in  the  formation  of  o-nitrophenol-o-p-disulphonic 
acid: 

C6H3.OH.(S03H)2+HN03->C6H2.OH.N02.(S03H)2+H20. 

Treatment  with  a  base  produces  the  highly  colored  sulpho- 
nate,  e.g., 

C6H2.OK.N02.(S03K)2. 

Three  important  sources  of  error  may  render  impossible  a 
determination  of  nitrate  by  the  phenolsulphonic  acid  method  or 
may  cause  incorrect  results  to  be  obtained.  These  may  be 
enumerated  as  follows: 

Interference  of  Organic  Matter.- — If  the  water  contains  any 
considerable  amount  of  organic  matter,  as  is  always  the  case 
with  surface  streams,  sewage  or  waters  contaminated  by  sewage, 
the  addition  of  sulphuric  acid  will  cause  a  charring  of  the  organic 
matter  and  the  color  comparison  cannot  be  made  accurately 
because  of  the  resulting  brown  coloration.  Of  course  the  same 
interference  will  result  from  any  color  that  may  have  been  in  the 
sample  before  treatment.  Organic  matter  may  also  cause  the 
reduction  of  the  nitrates  during  evaporation.  This  interference 
may  be  prevented  by  first  removing  the  organic  matter  by  means 
of  colloidal  aluminium  hydroxide. 

Interference  of  Chlorides.- — If  the  sample  contains  more  than 
about  30  mg  of  chlorine  as  chlorides  per  liter  there  is  a  possibility 
of  reactions  occurring  between  chlorides  and  nitrates  during  evapo- 
ration, resulting  in  the  reduction  of  nitrates.  This  may  be 
avoided  by  the  addition  of  silver  sulphate  solution  to  the  slightly 
acidified  sample,  silver  chloride  being  precipitated. 

Interference  of  Nitrites. — Nitrites  likewise  cause  variable 
results  to  be  obtained  unless  certain  precautions  are  taken. 
During  the  evaporation  of  water  containing  nitrites  some  of  the 
latter  will  be  decomposed  and  nitrogen  lost,  while  some  may  be 

i  Chamot  and  Pratt:  J.  Am.  Chem.  Soc.,  31,  922  (1909);  32,  630  (1910); 
33,  366  and  381  (1911). 


WATER  427 

oxidized  to  nitrates.  On  account  of  the  uncertain  extent  to 
which  these  reactions  occur  it  is  necessary  either  to  remove  the 
nitrites  entirely  or  to  oxidize  them  quantitatively  to  nitrates. 
.The  latter  is  accomplished  by  treatment  of  the  sample  with 
hydrogen  peroxide. 

All  of  these  interferences  may  be  avoided  by  using  a  method 
based  upon  the  reduction  of  nitrates  to  ammonia  by  nascent 
hydrogen.  The  sample  is  made  basic  and  concentrated  by  boil- 
ing. By  this  means  all  of  the  ammonia,  free  or  combined,  as 
well  as  all  of  the  nitrogen  as  nitrites  is  removed.  Nascent  hydro- 
gen is  then  generated  by  adding  aluminium  to  the  basic  solution. 
The  resulting  ammonia  is  later  distilled  and  determined  by 
Nesslerization. 

Determination. — For  the  phenolsulphonic  acid  method  the  following 
reagents  will  be  required: 

1.  Phenoldisulphonic  Acid. — Dissolve  25  gm  of  pure  white  phenol  in 
150  cc  of  concentrated  sulphuric  acid.     Add  75  cc  of  fuming  sulphuric 
acid  containing  15  percent  of  "free"  sulphur  trioxide,  stir  well  and  heat 
for  2  hours  at  about  100°. 

2.  Sodium  or  Potassium  Hydroxide  Solution. — This  solution  should  be 
made  approximately  12  normal.     About  5  cc  will  then  be  required  to 
neutralize  2  cc  of  the  phenolsulphonic  acid. 

3.  Standard  Nitrate  Solution. — Dissolve  0.72  gm  of  pure  recrystal- 
lized  potassium  nitrate  and  dilute  to   1000  cc  with  distilled  water. 
Evaporate  cautiously  10  cc  of  this  solution  in  a  dish  placed  on  a  steam 
bath.     Moisten  the  residue  quickly  and  thoroughly  with  2  cc  of  phenol- 
sulphonic acid,  dissolve  and  dilute  to  1000  cc.     Calculate  the  weight  of 
nitrogen  in  1  cc  of  the  last  solution. 

4.  Standard  Silver  Sulphate  Solution. — Dissolve  4.397  gm  of  silver 
sulphate,  free  from  nitrate,  in  nitrate-free  water  and  dilute  to  1000  cc. 
If  a  good  grade  of  silver  sulphate  has  been  used  this  solution  will  require 
no  other  standardization  for  the  purpose  for  which  it  is  here  to  be  used. 
Otherwise  it  should   be  standardized  against  pure  sodium  chloride. 
1  cc  should  be  equivalent  to  1  mg  of  chlorine. 

5.  Standard  Sulphuric  Acid  Solution. — A  solution,  N/50,  standardized 
by  titration  against  pure  sodium  carbonate  (pages  224  and  231).     The 
solution  must  be  free  from  nitrates. 

6.  Aluminium  Hydroxide.- — This  must  be  freshly  prepared.     From 
a  solution  of  nitrate-free  aluminium  sulphate  or  alum  precipitate  hy- 
droxide by  adding  dilute  ammonium  hydroxide.     Filter  and  wash  several 
times.     The  water  and  ammonium  hydroxide  must  be  free  from  nitrates. 


428  QUANTITATIVE  ANALYSIS 

The  aluminium  hydroxide  so  prepared  is  used  without  drying  and 
before  it  has  had  time  to  change  to  the  crystallqidal  form. 

Measure  into  an  evaporating  dish  enough  sample  to  furnish  not  more 
than  0.01  mg  of  nitrate  nitrogen.  100  cc  will  be  suitable  for  ordinary 
unpolluted  waters.  Add  sufficient  N/50  sulphuric  acid  to  make  the 
water  nearly  neutral,  as  determined  by  a  separate  titration  with  methyl 
orange  as  indicator.  (See  the  determination  of  carbonates,  page  398.) 
Now  add  silver  sulphate  solution  in  quantity  sufficient  to  precipitate 
all  but  about  0.1  mg  of  chlorine.  (This  determination  is  described 
on  page  400.)  This  treatment  may  be  omitted  if  the  quantity  of  sample 
used  contains  less  than  about  30  mg  of  chlorine.  If  silver  sulphate 
has  been  added  or  if  the  water  sample  is  perceptibly  colored  heat  the 
mixture  to  boiling,  add  a  little  aluminium  hydroxide,  stir,  filter  and 
wash  with  small  amounts  of  nitrate-free  hot  water.  Evaporate  over 
the  steam  bath  and  add  2  cc  of  phenoldisulphonic  acid,  rubbing  with  a 
glass  rod  to  insure  intimate  contact.  If  the  residue  becomes  packed  or 
appears  vitreous  because  of  the  presence  of  much  iron,  heat  the  dish  on 
the  steam  bath  for  a  few  minutes. 

Dilute  the  mixture  with  distilled  water  and  add  slowly  solution  (2) 
until  the  maximum  color  is  developed.  Transfer  to  a  Nessler  tube, 
filtering  if  necessary.  Compare  the  color  with  that  of  standards 
made  by  adding  2  cc  of  the  basic  solution  to  various  amounts  of  the 
standard  nitrate  solution  and  diluting  to  the  mark  in  Nessler  tubes. 
The  following  amounts  of  standard  are  suggested:  0.5,  1.0,  1.5,  2.0, 
4.0,  6.0,  8.0,  10.0,  15.0,  20.0  and  40.0  cc.  These  standards  may  be 
kept  for  several  weeks  without  deterioration  if  the  tubes  are  kept  corked 
to  prevent  evaporation  or  contamination. 

The  amount  of  nitrite  nitrogen  that  will  remain  after  evaporation  is 
not  sufficient  to  alter  materially  the  results  unless  present  in  excess  of 
1  mg  per  liter.  In  case  such  quantity  is  found  the  nitrite  is  oxidized  to 
nitrate  by  repeatedly  heating  with  a  few  drops  of  hydrogen  peroxide 
which  is  free  from  nitrate.  Proper  correction  in  the  nitrate  nitrogen 
figure  must  then  be  made. 

Calculate  milligrams  per  liter  of  nitrate  nitrogen. 

For  the  reduction  method  for  nitrates  prepare  the  following  reagents : 

1.  Sodium  Hydroxide  Solution. — Dissolve  50  gm  of  the  purest  ob- 
tainable sodium  hydroxide  in  250  cc  of  distilled  water,  add  several 
strips  of  aluminium  foil  and  leave  over  night.     Evaporate  to  200  cc  by 
boiling. 

2.  Aluminium  Foil. — Use  strips  about  10  cm  long,  0.33  mm  thick 
and  6  mm-  wide. 

Place  100  cc  of  the  water  in  a  300  cc  casserole  or  dish.  Add  2  cc 
of  sodium  hydroxide  solution  and  boil  until  the  volume  is  20  cc.  Using 


WATER  429 

nitrogen-free  water,  rinse  into  a  test-tube  about  15  cm  long  and  3  cm 
in  diameter.  The  volume  of  the  solution  should  now  be  about  75  cc. 
Add  a  strip  of  aluminium  foil  and  close  the  tube  with  a  rubber  stopper 
through  which  passes  a  bent  glass  tube,  the  shorter  end  of  which  is 
flush  with  the  lower  end  of  the  stopper,  the  longer  end  dipping  beneath 
the  surface  of  distilled  water  in  a  beaker.  This  serves  as  a  trap  to  pre- 
vent the  entrance  of  oxygen. 

Allow  to  stand  for  four  hours  or  more.  If  the  supernatant  liquid  is 
then  clear  and  colorless,  Nesslerize  at  once,  otherwise  rinse  into  the 
apparatus  used  for  ammonia  determinations,  dilute  to  250  cc,  distill 
and  Nesslerize  the  distillate.  Calculate  milligrams  per  liter  of  nitrate 
nitrogen. 

Required  Oxygen. — Besides  the  indirect  estimation  of  organic 
matter  through  the  determination  of  nitrogen  in  its  various 
forms  a  more  direct  estimation  may  be  made  by  oxidizing  with 
standard  potassium  permanganate.  It  is  readily  seen  that  no 
calculation  of  organic  matter  can  be  made  as  a  result  of  such  a 
titration  because  the  great  variety  of  organic  substances  present 
in  polluted  water  gives  rise  to  a  great  variety  of  reactions.  On 
the  other  hand  the  calculation  of  the  oxygen  required  to  oxidize 
all  reducing  agents  in  the  water  gives  a  fair,  though  inexact  idea  of 
the  amount  of  organic  pollution.  Differences  in  procedure  will 
cause  the  reduction  of  varying  quantities  of  potassium  per- 
manganate and  it  is  therefore  necessary  rigidly  to  standardize 
the  method.  It  is  also  necessary  to  correct  the.  results  according 
to  the  amount  of  nitrites,  ferrous  salts  and  hydrogen  sulphide, 
if  these  are  found  in  considerable  quantities. 

The  determination  is  carried  out  by  treating  a  measured 
volume  of  water  with  an  excess  of  standard  potassium  perman- 
ganate solution,  at  a  specified  temperature,  and  titrating  the 
excess  after  a  stated'  period.  If  the  solution  is  heated  during  the 
treatment  the  excess  of  permanganate  is  determined  by  adding  a 
measured  excess  of  a  standard  solution  of  oxalic  acid  or  ammo- 
nium oxalate,  titrating  the  excess  by  standard  potassium  per- 
manganate solution.  If  cold  treatment  is  used  the  excess  of 
potassium  permanganate  cannot  be  determined  in  this  manner 
because  oxalic  acid  reduces  permanganates  very  slowly  unless 
heated  to  at  least  60°.  In  this  case  the  excess  of  potassium 
permanganate  is  reduced  by  adding  potassium  iodide  and  titrat- 


430  QUANTITATIVE  ANALYSIS 

ing   the   liberated   iodine   with   standard   sodium   thiosulphate 
solution : 

2KMn04+10KI+8H2SO4->6K2S04+2MnS04+8H20+10I; 
2Na2S2O3 + 2I-+Na2S4O6 + 2NaI . 

The  method  involving  digestion  at  1006  will  be  described. 

Determination. — Prepare  the  following  reagents: 

1.  Sulphuric  Acid. — Dilute  the  concentrated  acid  with  three  volumes 
of  distilled  water.     Add  potassium  permanganate  until  a  faint  pink 
color  persists  after  standing  for  several  hours. 

2.  Standard  Potassium  Permanganate  Solution. — Calculate  the  weight 
of  crystallized  potassium  permanganate  required  for  1000  cc  of  a  solu- 
tion, 1  cc  of  which  shall  be  equivalent  to  0.1  mg  of  oxygen.     Dissolve 
this  weight  of  salt  and  dilute  to  1000  cc. 

Standardize  as  follows :  Add  10  cc  of  this  solution  and  10  cc  of  solution 
(1)  to  100  cc  of  distilled  water  in  a  flask,  immersing  the  flask  in  boiling 
water  for  30  minutes.  This  destroys  the  oxygen -consuming  capacity 
of  the  distilled  water.  Add  10  co  of  solution  (3)  and  then  potassium 
permanganate  solution  until  a  faint  pink  color  persists.  Now  add  10 
cc  more  of  the  oxalate  solution  and  titrate  with  the  permanganate  solu- 
tion for  the  standardization  of  the  latter.  Calculate  its  value  in  milli- 
grams of  available  oxygen  per  cubic  centimeter. 

3.  Ammonium  or  Sodium  Oxalate  Solution. — Use  the  purest  obtain- 
able salt.     Make  a  solution  of  which  1  cc  is  equivalent,  as  a  reducing 
agent,  to  0.1  mg  of  oxygen. 

Measure  100  cc  or  less  of  the  water  into  a  200-cc  flask,  add  10  cc  of 
solution  (1)  and  10  cc  (exactly  measured)  of  solution  (2)  and  immediately 
place  the  flask  in  a  bath  of  boiling  water,  the  water  level  of  which  is 
kept  above  the  level  of  the  contents  of  the  flask.  Digest  for  exactly 
30  minutes.  Remove  the  flask  and  add  exactly  10  cc  of  solution  (3). 
Titrate  with  standard  potassium  permanganate  solution  and  calculate 
milligrams  per  liter  of  required  oxygen. 

If  10  cc  of  permanganate  solution  is  insufficient  for  complete  oxidation 
repeat  the  digestion  with  a  larger  quantity.  At  least  5  cc  excess  of 
permanganate  should  remain  after  the  digestion. 

Interfering  Substances. — If  oxidizable  mineral  substances, 
such  as  ferrous  iron,  sulphide  or  nitrite,  are  present  in  appreciable 
quantities  corrections  should  be  applied  as  accurately  as  possible 
by  suitable  procedures.  Direct  titration  of  the  acidified  sample 
while  cold,  using  a  three-minute  period  of  digestion,  serves  this 


WATER  431 

purpose  quite  well  for  polluted  surface  waters  and  fairly  well 
for  purified  sewage  effluents.  Few  raw  sewages  containing  no 
trade  wastes  need  such  a  correction  but  raw  sewages  containing 
"pickling"  liquors  do  need  it.  If  the  sample  contains  both  oxi- 
dizable  mineral  compounds  and  gaseous  organic  substances  the 
latter  should  be  driven  off  by  heating,  the  sample  being  allowed 
to  cool  before  applying  this  test  for  the  correction  factor.  If 
such  corrections  are  made  the  fact  should  be  stated,  with  the 
amount  of  correction. 


CHAPTER  XV 
STEEL  AND  ALLOYS 

STEEL  AND  CAST  IRON 

The  impurities  contained  in  iron  and  steel  usually  form  a  very 
small  portion  of  the  total  mass.  Wrought  iron  and  steel  often 
contain  a  total  of  less  than  one  percent  of  elements  other  than 
iron  while  even  pig  iron  does  not  often  contain  as  much  as  ten 
percent  of  other  elements.  It  is  therefore  not  custorriary  to  make 
determinations  of  the  percent  of  iron  but  rather  of  the  small 
amounts  of  other  elements,  which  give  certain  very  important 
properties  to  the  metal  in  which  they  are  contained.  Elements 
occurring  in  iron  and  steel  and  commonly  determined  are  carbon, 
silicon,  phosphorus,  sulphur,  manganese  and  titanium.  In  alloy 
steels  for  special  purposes  determinations  are  also  made  of  tung- 
sten, nickel,  chromium,  molybdenum,  vanadium  and  copper. 

Exact  and  Rapid  Methods. — For  the  determination  of  each 
element  there  are  available  certain  well  known  methods  and  these 
are  continually  being  revised  and  supplemented  by  other  newer 
methods.  Considerable  experience  is  therefore  necessary  if  the 
analyst  is  to  be  able  to  intelligently  select  the  method  best 
adapted  to  his  purpose.  There  is  a  certain  distinction  to  be 
made  between  what  may  be  classed  as  " exact"  methods  and 
others  that  are  more  properly  called  " rapid"  methods.  Thus  a 
determination  of  carbon  in  steel,  made  by  an  approved  exact 
method,  may  require  at  least  two  hours  and  sometimes  longer 
while  a  less  exact  determination  might  be  made  by  another 
method  in  ten  or  fifteen  minutes.  Conversations  with  works 
chemists  will  often  give  the  student  the  erroneous  impression 
that  the  longer  methods  are  impracticable  and  are  taught  in 
colleges  but  not  used  in  practice,  while  the  rapid  methods  are 
improperly  neglected  in  the  students'  college  courses.  It  is  true 
that  more  emphasis  is  laid  upon  the  exact  method,  as  a  rule.  If 

432 


STEEL  AND  ALLOYS  433 

the  science  and  careful  manipulation  involved  in  the  longer 
method  are  properly  appreciated  and  learned  the  student  will 
have  no  difficulty  in  learning  the  shorter  and  less  exact  method 
after  he  enters  his  professional  career. 

It  is  highly  important  that  one  should  understand  the  proper 
place  of  each  class  of  methods.  In  the  steel  works  samples  may 
be  taken  from  the  melted  iron  as  it  runs  from  the  blast  furnace 
or  from  the  steel  ladles  which  receive  the  product  of  the  steel 
furnaces.  These  samples  are  taken  directly  to  the  works  labora- 
tory where  the  analysis  must  be  made  very  quickly  in  order  to 
furnish  information  which  will  serve  as  a  guide  in  mixing  charges 
for  the  steel  furnace  or  for  properly  disposing  of  or  modifying  the 
product  of  a  given  furnace.  The  results  of  such  an  analysis  do 
not  often  serve  as  a  guarantee  to  the  steel  consumer  but  rather 
as  a  check  upon  the  various  stages  in  the  process  of  steel  manu- 
facture. For  this  reason  rapid  methods  are  quite  suitable  for 
the  purpose. 

When  the  steel  is  placed  upon  the  market  as  a  finished  product 
the  case  is  quite  different.  Modern  industrial  development  has 
created  new  and  rigorous  requirements  regarding  the  quality  of 
steel  entering  into  machinery  and  structural  work.  It  becomes 
necessary  for  the  steel  manufacturer  to  guarantee  the  percents 
of  the  elements  in  his  steel  within  very  narrow  limits  and  a 
method  of  analysis  that  will  not  give  results  having  a  high  degree 
of  accuracy  is  quite  useless  for  this  purpose. 

Standard  Methods. — An  inspection  of  the  methods  for  analysis 
of  steel  as  practised  in  the  various  works  laboratories  will  show 
that  while  the  several  standard  methods  are  quite  universally 
used,  many  variations  have  been  introduced  by  individual  chem- 
ists. Each  laboratory  usually  has  its  methods  described  and 
specified  and  these  must  be  rigidly  followed  by  all  chemists 
working  in  that  laboratory.  There  is,  of  course,  much  difference 
of  opinion  concerning  the  relative  merits  of  different  methods  and 
it  is  inevitable — indeed  it  is  even  desirable — that  modifications 
should  be  made  whenever  any  improvement  is  seen  to  be  possible. 
It  is  also  'true,  however,  that  many  modifications  of  good  methods 
have  made  poor  methods  because  the  modifications  have  been 
made  without  an  adequate  knowledge  of  the  scientific  principles 
underlying  the  analytical  process.  Many  chemists  have  con- 

28 


434  QUANTITATIVE  ANALYSIS 

fidence  in  their  methods  when  this  confidence  is  based  upon  little 
more  than  the  ability  to  obtain  close  agreement  of  duplicate 
determinations.  The  error  involved  in  such  conclusions  has 
been  discussed  in  an  earlier  section.  Many  of  the  analytical 
methods  for  iron  and  steel  have  been  in  use  for  a  long  time. 
Some  of  these  have  been  retained  in  practically  their  original 
form  and  still  bear  the  names  of  the  chemists  who  first  proposed 
them.  Others  have  been  so  modified  that  they  bear  little 
resemblance  to  the  original  method. 

Sampling. — Analysis  may  be  required  of  either  works  samples, 
taken  from  the  metal  as  it  runs  from  the  furnace,  or  of  the  finished 
product.  Samples  of  the  first  class  are  dipped  from  the  melted 
metal  by  means  of  a  small  ladle  and  are  poured  onto  a  clean  iron 
plate  or  into  a  small  iron  mold.  The  sample  is  crushed,  if  a  brittle 
product  like  pig  iron,  or  drilled  if  steel.  Pieces  of  already  solidi- 
fied metal  are  drilled  to  obtain  a  sample  for  analysis.  The  outer 
case  should  be  first  removed  because  it  may  contain  iron  oxide  or 
sand,  or  the  percent  of  carbon  may  have  been  lowered  by  oxida- 
tion. The  drill  should  be  set  to  make  as  fine  drillings  as  possible 
and  if  powder  is  at  the  same  time  produced  it  should  be  well 
mixed  with  the  larger  pieces  before  weighing  for  analysis. 

Solution  and  Evaporation. — Steel  and  iron  analysis  involves 
many  operations  of  dissolving  and  of  evaporating  solutions. 
The  work  of  the  iron  and  steel  laboratory  must  usually  be  done 
as  quickly  as  possible  and  the  analyst  must  therefore  give 
considerable  attention  to  the  best  manipulation,  from  the  stand- 
point of  speed  and  accuracy. 

Dissolving  metals  in  strong  acids  is  always  attended  with 
the  evolution  of  disagreeable  and  often  poisonous  fumes  and 
good  draught  hoods  are  absolutely  essential  for  carrying  on 
this  work.  The  evolution  of  gases  and  the  boiling  of  solutions 
for  evaporation  will  occasion  loss  of  the  dissolved  matter  unless 
proper  attention  is  given  to  the  prevention  of  such  loss.  When 
evaporation  is  not  to  follow  solution  of  the  sample  an  Erlenmeyer 
flask  is  usually  the  best  vessel  for  the  purpose.  Loss  by  spat- 
tering is  thus  reduced  to  a  minimum.  Casseroles  should  be  used 
if  the  solution  of  the  sample  is  later  to  be  evaporated.  These 
are  covered  with  glasses  until  the  sample  has  all  dissolved  and 
evolution  of  gases  is  completed.  The  cover  glasses  are  then 


STEEL  AND  ALLOYS  435 

rinsed  down  and  removed  and  the  solution  is  placed  on  a  steam 
bath  or  hot  plate  or  it  is  held  over  a  free  flame  and  rotated  con- 
tinuously by  hand. 

The  choice  of  method  to  be  used  for  evaporating  will  depend 
upon  the  requirements  of  the  case.  If  time  does  not  press  and 
work  may  be  so  fitted  together  as  to  carry  on  a  large  number 
of  analyses  at  a  time,  evaporation  on  the  steam  bath  will  prove 
to  be  a  convenient  and  safe  process,  except  for  solutions  of  high 
boiling  points  such  as  those  containing  sulphuric  acid.  If  the 
opposite  is  true  and  speed  is  a  matter  of  prime  importance, 
evaporations  must  be  hurried  but  the  resulting  danger  of  loss 
through  bumping  and  spattering  requires  that  the  casserole 
should  be  held  in  the  hand  and  kept  in  continuous  motion. 
This  constant  agitation  considerably  increases  the  rate  of  evapora- 
tion, particularly  because  of  the  spreading  of  the  solution  over 
the  sides  of  the  casserole  and  the  consequent  increase  in  the 
effective  surface.  Of  course  this  process  is  carried  out  with 
an  uncovered  casserole  as  otherwise  little  advantage  would  be 
derived  from  forced  boiling. 

Standard  Samples. — In  the  description  of  the  volumetric 
determinations  that  follow,  methods  are  given  for  the  standardi- 
zation of  the  solutions  against  suitable  primary  standards.  In 
many  of  these  cases  it  is  also  convenient  to  standardize  the  solu- 
tion against  a  steel  or  iron  in  which  the  percent  of  the  element  in 
question  is  accurately  known ,  the  standard  sample  being  weighed 
and  treated  exactly  as  is  the  sample  that  is  being  analyzed. 
This  procedure  possesses  a  further  advantage  in  that  it  auto- 
matically corrects  for  any  deviation  from  the  theoretical  course 
of  the  reactions  occurring  during  the  preparation  of  the  solution 
or  during  the  titration  itself. 

The  steel  laboratory  may  prepare  and  carefully  analyze  its  own 
standard  samples.  It  is  also  possible  to  obtain  most  of  the 
necessary  standard  steels  and  irons  from  the  Bureau  of  Standards. 
These  standard  samples  may  also  be  used  for  checking  the  accu- 
racy of  gravimetric  methods  and  in  this  way  they  will  serve  for 
control  work. 

Carbon. — The  most  important  element  occurring  in  steel  is 
carbon.  This  is  because  it  is  the  element  which  makes  possible 
the  formation  of  steel  by  imparting  to  iron  the  capability  of  being 


436  QUANTITATIVE  ANALYSIS 

hardened  by  suitable  heat  treatment.  The  development  of 
alloy  steels  has  lately  brought  nickel,  chromium,  vanadium, 
tungsten  and  other  metals  into  prominence  as  constituents  of 
special  steels,  but  without  carbon,  alloys  of  these  elements  with 
iron  would  be  of  little  value.  The  effect  of  carbon  upon  iron 
with  which  it  is  combined  is  to  increase  the  tensile  "strength  and 
hardness  and  to  decrease  the  ductility.  Carbon  is  present  in 
steel  chiefly  as  a  carbide,  FeaC,  although  small  quantities  may 
occur  as  free  carbon.  In  cast  iron  large  quantities  of  carbon  are 
free,  particularly  in  gray  cast  iron.  A  more  extended  discussion 
of  the  properties  of  steel,  as  dependent  upon  the  condition  of  the 
carbon,  will  be  taken  up  later  (page  478) . 

The  determination  of  total  carbon  is  the  only  carbon  determina- 
tion that  is  usually  required  in  steel  analysis.  In  4  the  analysis 
of  cast  iron  determinations  also  of  free  and  combined  carbon  may 
be  required.  The  determination  of  total  carbon  is  generally 
made  by  a  combustion  process,  the  carbon  being  oxidized  and  the 
resulting  carbon  dioxide  determined,  although  oxidation  in  solu- 
tion has  been  employed,  the  oxidizing  agent  being  chromic  acid. 
The  details  of  the  combustion  processes  vary  widely.  Fine 
drillings  of  steel  may  be  burned  directly  or  a  preliminary  separa- 
tion of  the  carbon  may  be  made.  The  apparatus  for  combustion 
may  be  a  furnace  and  combustion  tube  or  a  special  form  of  closed 
crucible,  through  which  air  and  oxygen  may  be  passed.  The 
resulting  carbon  dioxide  may  be  measured  at  an  accurately 
observed  temperature  and  pressure  and  its  weight  calculated  or 
it  may  be  absorbed  in  a  basic  solution  and  either  weighed  or  the 
excess  of  base  determined  by  titration. 

Direct  Combustion. — Iron  or  steel  may  be  completely  burned 
in  oxygen  at  900°  to  1100°.  The  method  is  desirable  because 
it  avoids  a  rather  tedious  process  of  preliminary  solution,  nitra- 
tion and  washing.  For  the  combustion  there  is  required  a  tube 
of  quartz  or  porcelain,  24  inches  long  and  having  an  inside 
diameter  of  3/4  inch.  It  must  be  protected  from  contamination 
by  iron  oxide  by  placing  an  alundum  cylinder  in  the  section 
which  is  to  hold  the  boat.  The  combustion  furnace  may  be 
heated  by  gas  or  electricity  and  should  be  12  to  14  inches  in 
length.  The  combustion  is  carried  out  in  a  manner  quite  similar 
to  the  combustion  of  coal  (page  311).  The  long  furnace  and 


STEEL  AND  ALLOYS  437 

tube  there  used  are  not  required  in  this  case  because  no  volatile 
hydrocarbons  are  produced  and  long  contact  of  the  gases  with 
cupric  oxide  is  not  necessary.  The  small  amount  of  carbon 
monoxide  that  may  be  formed  at  first  is  completely  oxidized 
by  passing  the  mixture  with  oxygen  over  a  small  amount  of 
cupric  oxide  or  through  platinized  asbestos.  The  platinum 
black  in  the  latter  case  catalyzes  the  combination  of  carbon 
monoxide  with  oxygen. 

The  train  of  apparatus  necessary  for  the  gravimetric  determina- 
tion is  as  follows:  (1)  Oxygen  tank,  (2)  bubble  tube  of  30  percent 
potassium  hydroxide  solution,  (3)  tube  of  soda  lime  to  retain 
spray  from  the  potassium  hydroxide  solution  and  to  complete 
the  removal  of  carbon  dioxide,  (4)  combustion  tube  containing 
(4a)  space  of  about  3J  inches  inside  the  furnace,  lined  with 
an  alundum  cylinder  to  hold  the  combustion  boat,  (4b)  platinized 
asbestos  to  the  end  of  the  furnace,  leaving  the  projecting  ends 
of  the  tube  empty,  (5)  U-tube  filled  with  granular  zinc  or  with 
glass  beads  moistened  with  chromic  acid  solution  for  the  re- 
tention of  oxides  of  sulphur,  (6)  U-tube  filled  with  calcium  chlo- 
ride and  (7)  absorption  apparatus  for  carbon  dioxide,  discussed 
below.  The  method  of  preparing  and  assembling  this  apparatus 
should  be  clear  from  the  discussion  of  the  determination  of  carbon 
dioxide  in  carbonates  (page  129)  and  of  the  determination  of 
carbon  and  hydrogen  in  coal  (page  311). 

Absorption  Apparatus. — The  nature  of  the  absorption  ap- 
paratus (7)  will  depend  upon  the  method  chosen  for  the  final 
determination.  This  may  be  either  gravimetric  or  volumetric 
in  principle.  One  of  the  following  variations  is  recommended: 

(a)  Absorption  in  Weighed  Bulbs  of  the  Geissler  or  a  Similar 
Type,  Filled  with  33  Percent  Potassium  Hydroxide  Solution 
and  Carrying  a  Prolong  Filled  with  Calcium  Chloride. — The 
manipulation  of  these  bulbs  is  carried  with  precautions  similar 
to  those  observed  in  the  determination  of  carbon  dioxide  in 
carbonates.  (See  pages  133  to  136.) 

(6)  Absorption  in  Weighed  Tubes  Carrying  Soda  Lime  and 
Calcium  Chloride  or  Phosphorus  Pentoxide. — The  Fleming  tube 
(figure  95)  is  satisfactory  for  this  purpose  and  a  determination 
can  be  made  very  rapidly  by  this  method.  In  filling,  the  part 
(s)  is  to  contain,  first,  a  loose  plug  of  asbestos  then  alternating 


438 


QUANTITATIVE  ANALYSIS 


layers,  about  one-fourth  inch  thick,  of  20-,  40-  and  60-mesh  soda 
lime.  Part  (d)  is  filled  with  either  phosphorus  pentoxide  on 
glass  wool  or  calcium  chloride.  (If  phosphorus  pentoxide  is 
used  at  this  point  it  must  also  be  substituted  for  calcium  chloride 
in  tube  (6)  of  the  absorption  train.) 

Phosphorus  pentoxide  should  not  be  used 
after  it  has  become  visibly  moist.  On  this 
account  the  charging  of  tubes  with  this  sub- 
stance should  not  be  undertaken  on  days 
when  the  humidity  is  high  as  it  is  then  im- 
possible to  work  rapidly  enough  to  prevent 
absorption  of  considerable  quantities  of 
moisture  from  the  air.  To  prepare  phos- 
phorus pentoxide  for  this  purpose  a  loose 
ribbon  of  glass  wool  is  first  spread  on  a 
clean  piece  of  paper  and  the  dry  material  is 
sifted  over  the  ribbon  in  an  even  layer. 
The  edges  of  the  glass  wool  are  then  turned 
over  and  the  material  is  rolled  into  the 
proper  shape  to  fit  loosely  into  the  tube. 
By  this  means  the  drying  agent  is  given  the 
maximum  exposure  to  gases  passing  through 
the  tube. 

(c)  Absorption  in  Standard  Barium 
Hydroxide  Solution,  the  Unused  Base  being 
Titrated  in  Presence  of  Phenolphthalein 
after  the  Absorption  is  Finished. — The 
Meyer  tube  (figure  96)  is  suitable  for  this 
purpose  and  it  possesses  the  important  merit 
of  being  easily  emptied  and  rinsed  just  before  the  titration. 
The  drying  tube  (6)  should  be  omitted  in  this  and  all  other 
volumetric  methods  as  the  drying  of  gases  entering  the  carbon 
dioxide  absorption  tube  is  quite  unnecessary. 

A  saturated  solution  of  barium  hydroxide  in  carbon  dioxide-free 
water  is  prepared  and  kept  as  a  stock  solution.  This  will  contain 
approximately  3.9  gm  of  barium  hydroxide,  calculated  as  the 
anhydrous  base,  in  each  100  cc  of  solution  and  it  will  be  approxi- 
mately 0.45  normal.  550  cc  of  this  solution  is  diluted  to  1000 
cc.  The  resulting  solution  is  then  approximately  0.25  normal, 


FIG.    95. — Fleming 
absorption  tube. 


STEEL  AND  ALLOYS  439 

so  that  1  cc  is  equivalent  to  about  0.0015  gm  of  carbon.  This 
solution  should  be  kept  in  a  bottle  which  is  provided  with  a 
siphon  or  with  an  outlet  at  the  bottom  and  protected  in  such  a 
manner  as  that  entering  air  shall  be  drawn  through  a  tube  of 
soda  lime  or  of  saturated  barium  hydroxide  solution,  to  remove 
carbon  dioxide. 

As  carbon  dioxide  is  absorbed  in  barium  hydroxide  solution 
barium  carbonate  is  formed  and  almost  entirely  precipitated. 
One  liter  of  a  saturated  solution  of  barium  carbonate  in  water 
contains  0.022  gm  (corresponding  to  0.00134  gm  of  carbon) 
at  20°.  Therefore  it  is  possible  to  titrate  the  excess  of  unused 


FIG.  96. — Meyer  absorption  bulbs. 

base  after  the  absorption  is  finished,  using  a  standard  acid  of 
concentration  equivalent  to  that  of  the  barium  hydroxide  and 
without  nitration,  provided  that  the  solution  was  previously 
saturated  with  barium  carbonate  and  that  the  titration  is 
carried  out  rapidly  and  with  stirring,  so  as  to  avoid  resolution 
of  the  precipitate  by  local  excess  of  acid.  Phenolphthalein 
is  used  as  the  indicator.  The  first  condition,  above,  is  usually 
met  by  the  fact  that  commercial  barium  hydroxide  always 
contains  small  amounts  of  carbonate  and  that  more  is  formed 
by  carbon  dioxide  in  the  water  that  is  used  for  preparing  the 
standard  solution. 

(d)  Absorption  in  Standard  Barium  Hydroxide  Solution, 
the  Unused  Excess  being  Titrated  after  Filtration  and  Washing 
with  Carbon  Dioxide-free  Water. — This  procedure  has  no  ad- 
vantage over  that  outlined  as  (c).  Absorption  of  carbon  dioxide 
from  the  air  takes  place  during  filtration  and  washing  of  the 
precipitate.  Also  the  addition  of  wash  water  that  is  not 
saturated  with  barium  carbonate  causes  resolution  of  traces  of 
the  precipitate.  The  second  error  tends  to  compensate  the 


440  QUANTITATIVE  ANALYSIS 

first  but  a  method  that  depends  for  accuracy  upon  mutual  com- 
pensation of  variable  errors  is  not  ideal. 

(e)  Absorption  in  Saturated  Barium  Hydroxide  Solution, 
the  Precipitated  Barium  Carbonate  being  Removed  by  Filtration, 
Washed,  Dissolved  in  an  Excess  of  Standard  Acid  and  the 
Unused  Excess  of  Acid  Titrated  by  Standard  Base,  with  Methyl 
Orange  as  Indicator.1—  The  barium  hydroxide  solution  that  is 
used  in  this  method  may  be  much  more  concentrated  than  that 
used  in  methods  (c)  and  (d)  because  it  is  not  to  be  titrated.  On 
this  account  absorption  is  more  certain  when  the  gas  is  bubbled 
rapidly  through  the  solution  and  in  this  lies  the  only  important 
advantage  over  the  other  volumetric  methods.  The  sources  of 
error  noted  under  (d)  obtain  here  also. 

Drying  tubes  are  omitted  from  the  absorption  tr,ain  in  volu- 
metric methods  (c),  (d)  and  (e). 

Combustion,  Preceded  by  Solution  of  the  Iron  and  Separation 
of  the  Carbon.  —  Iron  or  steel  dissolves  easily  in  a  solution  of  the 
double  chloride  of  potassium  and  copper.  The  cupric  chloride 
is  the  active  agent  and  the  double  salt  is  used  only  because  it  is 
more  easily  purified  and  preserved.  The  reactions  are  : 

Fe+2CuCl2->FeCl2+2CuCl  and 
Fe3C+6CuCl2-»3FeCl2+6CuCl+C. 

Free  carbon  is  also  left  undissolved.  The  residue  contains  organic 
compounds,  formed  during  the  process  of  solution,  and  the  total 
residue  cannot,  therefore,  be  weighed  directly  for  the  determina- 
tion of  total  carbon.  If  the  solution  is  not  well  stirred  or  if  not 
enough  cupric  potassium,  chloride  is  used,  copper  will  separate, 
returning  to  the  solution  upon  stirring  or  addition  of  more  of  the 
solution  of  cupric  salt: 

Fe+CuCl2-+FeCl2+Cu, 


These  reactions  illustrate  the  principle  of  replacement  of  one 
metal  of  a  salt  solution  by  another  which  has  a  higher  solution 
tension. 

The  solution  of  cupric  potassium  chloride  must  contain  hy- 

1  Cain:  J.  Ind.  Eng.  Chem.,  6,  465  (1914). 


STEEL  AND  ALLOYS  441 

drochloric  acid  in  order  to  prevent  the  precipitation  of  cuprous 
chloride,  a  substance  having  very  small  solubility  in  water.  If 
too  much  acid  is  present  carbon  may  be  lost  through  the  forma- 
tion of  hydrocarbons  during  the  process  of  solution.  This  is 
typified  by  the  hypothetical  reaction  shown  by  the  following 
equation : 

2Fe3C  +  12HCl-*6FeCl2+C2H2+5H2. 

Concerning  the  choice  of  methods  it  may  be  said  that  direct 
combustion  is  much  more  rapid  and  is  accurate  if  the  metal  is  in  a 
proper  state  of  division.  The  method  of  preliminary  solution  is 
fully  as  accurate,  except  for  high-speed  tool  steels,  and  is  safer  if 
the  nature  of  the  steel  is  not  accurately  known.  There  is  little 
to  choose  between  the  gravimetric  method,  weighing  the  absorp- 
tion bulbs  before  and  after  the  absorption  of  carbon  dioxide, 
and  the  volumetric  methods.  The  so-called  "  moist  combustion  " 
processes  and  the  methods  involving  measuring  the  volume  of 
carbon  dioxide  evolved,  while  attractive  in  principle,  are  trouble- 
some in  execution  and  are  subject  to  large  errors  unless  great 
care  is  exercised.  These  are  therefore  little  used. 

Determination  by  Direct  Combustion. — The  apparatus  train  described 
on  page  437  is  assembled,  drying  tube  (6)  being  used  only  in  the  gravi- 
metric modifications.  If  a  high-pressure  oxygen  cylinder  is  used  to 
supply  this  gas  a  special  control  valve  must  be  provided. 

The  entire  apparatus  is  first  tested  for  leaks.  With  the  carbon  dioxide 
absorption  tube  temporarily  removed  the  furnace  is  brought  to  a 
temperature  of  about  1100°  while  oxygen  is  passed  through  slowly  for 
one-half  hour.  This  preliminary  heating  to  expel  traces  of  moisture  and 
organic  matter  may  be  dispensed  with  if  the  apparatus  has  been  in  use. 

Unless  the  apparatus  has  been  in  continuous  use  one  or  more  blank 
determinations  must  be  made  in  order  to  correct  for  any  constant  small 
gain  of  carbon  dioxide  from  imperfect  apparatus  or  reagents.  The 
determination  of  carbon  in  steel  follows  the  blank.  The  different 
modifications,  with  blanks,  will  be  referred  to  by  the  letters  that  have 
already  been  used  in  the  discussion  of  the  principles  of  these  methods. 

Method  (a). — The  potassium  hydroxide  bulbs  are  carefully  wiped  and 
the  outlets  are  closed  by  short  pieces  of  rubber  tubing  bearing  glass 
plugs.  They  are  allowed  to  stand  in  the  balance  case  for  10  minutes, 
after  which  the  rubber  tubes  are  removed  and  the  bulbs  are  weighed. 


442  QUANTITATIVE  ANALYSIS 

Crucible  forceps  tipped  with  rubber  should  be  used  for  handling  the 
bulbs  in  this  and  all  subsequent  operations.  After  weighing,  the  bulbs 
are  inserted  in  the  train  and  oxygen  is  passed  through  at  the  rate  of 
about  three  bubbles  per  second  for  15  minutes.  At  the  end  of  this 
period  the  gas  flow  is  stopped  and  the  bulbs  are  removed,  plugged  and 
placed  in  the  balance  case.  After  10  minutes  the  rubber  caps  are  re- 
moved and  the  bulbs  are  immediately  weighed.  If  this  blank  determi- 
nation gives  a  change  of  more  than  0. 1  mg  in  the  weight  of  the  bulbs  it 
must  be  repeated  until  the  change  drops  to  zero  or  becomes  constant 
within  this  limit.  When  this  condition  is  reached  the  determination  of 
carbon  may  be  made. 

While  the  blank  determinations  are  running,  prepare  an  alundum 
boat  by  placing  a  layer  of  granular  alundum  (grade  RR)  on  the  bottom 
and  sides,  then  igniting  for  5  minutes.  After  this  has  cooled  about  2  gm 
of  steel  sample  is  weighed  on  a  counterpoised  glass,  brushed  into  the 
boat  and  distributed  in  a  uniform  layer. 

Insert  the  weighed  bulbs  into  the  train  and  adjust  the  flow  of  oxygen 
to  the  same  rate  as  that  used  in  the  blank.  Without  stopping  this 
flow,  open  the  end  of  the  combustion  tube  next  to  the  oxygen  tank  and 
insert  the  boat  containing  the  sample,  pushing  it  by  means  of  a  wire  into 
the  alundum  thimble  which  is  next  to  the  platinized  asbestos.  Close 
the  tube  as  quickly  as  possible  and  carefully  twist  the  stopper  into  the 
end  so  that  no  leak  can  occur.  The  combustion  of  the  steel  begins 
almost  immediately  and  is  usually  completed  within  a  very  short  time. 
Now  continue  the  passage  of  oxygen  for  15  minutes,  at  the  end  of  which 
period  remove  the  absorption  bulbs,  stopper,  place  in  the  balance 
case  and  weigh,  without  the  rubber  caps,  after  10  minutes. 

In  the  meantime  another  sample  of  steel  should  have  been  made  ready 
in  a  second  boat  and  another  absorption  bulb  should  have  been  weighed. 
As  soon  as  the  first  bulb  is  removed  the  second  is  inserted  in  the  train. 
The  combustion  tube  is  opened  as  before,  the  first  boat  is  drawn  out 
and  the  second  is  inserted.  A  continuous  series  of  determinations  may 
be  made  in  this  way  without  stopping  the  flow  of  oxygen  or  cooling  the 
furnace.  Weighings  may  be  made  while  the  combustion  is  proceeding. 
Bulbs  may  be  used  without  refilling  until  exhausted,  following  the  rule 
given  on  page  133. 

From  the  weights  of  samples  and  of  carbon  dioxide  calculate  the  per- 
cent of  carbon  in  the  steel  samples. 

Method  (b). — The  manipulation  is  exactly  like  that  of  method  (a) 
The  rate  of  flow  of  oxygen  is  judged  by  bubbling  through  the  potassium 
hydroxide  tube  following  the  oxygen  tank. 

Method  (c). — A  saturated  solution  of  barium  hydroxide  is  first  pre- 
pared by  warming  and  stirring  the  solid  base  with  recently  boiled  water, 


STEEL  AND  ALLOYS  443 

using  a  ratio  of  70  to  100  gm  of  base  to  1000  cc  of  water  according  to  the 
purity  of  the  barium  hydroxide  obtainable.  Cool  to  room  temperature 
and  siphon  into  a  bottle  which  is  then  closed  with  a  rubber  stopper. 
Dilute  550  cc  of  this  solution  to  1000  cc  with  distilled  water,  mix  and 
place  in  a  bottle  which  is  provided  with  a  guard  tube  of  soda  lime  and  a 
siphon  or  similar  outlet. 

Prepare  a  solution  of  hydrochloric  acid,  1  cc  of  which  is  equivalent 
to  0.002  gm  of  carbon,  standardizing  against  pure  sodium  carbonate  in 
presence  of  methyl  orange. 

Rinse  the  Meyer  bulbs  with  boiled  water,  then  measure  into  them 
from  a  burette  or  an  automatic  pipette  attached  to  the  bottle  50  cc  of 
the  dilute  solution  of  barium  hydroxide,  first  discarding  the  few  drops 
that  are  in  the  outlet  of  the  measuring  instrument.  Add  to  the  bulbs 
from  a  graduated  cylinder  enough  cold,  carbon  dioxide-free  water  to 
bring  the  liquid  just  to  the  lower  edge  of  the  upper  bulb  when  the  gas 
is  flowing.  The  quantity  necessary  should  be  determined,  once  for  all, 
so  that  it  may  be  added  without  delay  in  subsequent  determinations. 
With  the  furnace  already  heated,  connect  the  bulbs  in  place  while  the 
oxygen  is  flowing  and  conduct  the  blank  experiment  as  in  method  (a), 
to  the  end  of  the  absorption  period,  then  disconnect  the  bulbs  without 
stopping  the  flow  and  rinse  the  solution  into  a  250  cc  Erlenmeyer  flask 
by  means  of  boiled  water,  paying  no  attention  to  the  precipitate.  Add 
a  drop  of  phenolphthalein  and  titrate  at  once  (not  too  rapidly  but  stir- 
ring vigorously)  with  standard  hydrochloric  acid,  to  the  disappearance 
of  the  pink  color.  When  this  point  is  reached  the  volume  of  required 
acid  is  read.  The  pink  color  will  return  after  standing,  due  to  the 
gradual  resolution  of  barium  carbonate,  but  this  is  not  considered  in  the 
titration. 

At  the  time  when  the  absorption  bulbs  are  removed  from  the  train 
of  apparatus  another  tube,  charged  like  the  first,  is  inserted  for  use  in 
the  next  blank  determination,  the  barium  hydroxide  solution  having 
been  measured  just  before  inserting  the  bulbs.  Run  this  blank  deter- 
mination like  the  first. 

While  the  gas  is  flowing  for  the  second  blank  the  steel  sample  should 
be  weighed  and  placed  in  the  prepared  boat,  as  in  method  (a).  When 
the  blank  is  finished  the  first  set  of  absorption  bulbs,  charged  as  before, 
is  substituted  and  the  boat  with  the  steel  sample  is  inserted  into  the 
combustion  tube.  Continue  the  combustion  as  directed  for  method 
(a).  While  this  is  proceeding  the  base  from  the  second  blank  is  titrated. 
This  titration  should  agree  with  the  first  to  within  0.1"  cc  of  standard 
acid. 

A  continuous  series  of  determinations  may  be  made  without  stopping 
the  flow  of  oxygen  or  cooling  the  furnace.  Enough  blanks  must  be 


444  QUANTITATIVE  ANALYSIS 

run  to  obtain  agreement  of  titrations  and  at  least  two  determinations 
of  carbon  should  be  made  on  each  sample  of  steel. 

The  volume  of  acid  required  in  the  determination  is  subtracted  from 
that  used  in  the  blanks  and  the  remainder  is  multiplied  by  the  carbon 
equivalent  of  the  standard  acid.  From  this  and  the  weight  of  sample 
calculate  the  percent  of  carbon  in  the  steel. 

Method  (e). — The  saturated  solution  of  barium  hydroxide  whose 
preparation  was  described  for  method  (c)  is  used.  About  25  cc  is  placed 
in  the  Meyer  bulbs  and  carbon  dioxide-free  water  is  added  so  as  to  fill 
all  but  the  upper  bulb  when  the  gas  is  flowing.  The  blank  experiment 
and  the  carbon  determination  are  conducted  as  in  method  (c),  with  the 
following  exceptions : 

Barium  hydroxide  need  not  be  accurately  measured,  although  approxi- 
mately equal  volumes  should  be  used  in  all  experiments.  A  graduated 
cylinder  is  suitable  for  measuring  the  solution  but  it  should  first  be 
rinsed  with  boiled  water. 

At  the  end  of  the  absorption  the  solution  is  filtered  rapidly  on  a  paper 
filter  and  the  Meyer  tube,  the  precipitate  and  the  paper  are  washed 
several  times  with  cold,  carbon  dioxide-free  water.  The  precipitate 
is  then  dissolved  by  adding  exactly  25  cc  of  tenth-normal  hydrochloric 
acid  (or  the  solution  described  for  method  (c))  the  solution  being  caught 
in  the  Meyer  tube  so  that  all  adhering  barium  carbonate  is  dissolved. 
The  paper  is  well  washed  with  hot  water  and  the  combined  filtrate 
and  washings  rinsed  into  an  Erlenmeyer  flask.  The  excess  of  acid  is 
then  titrated  by  tenth-normal  sodium  hydroxide,  using  methyl  orange 
as  indicator. 

Determination  by  Combustion,  Preceded  by  Solution. — Prepare  a 
solution  of  cupric  potassium  chloride  containing  500  gm  of  the  crystal- 
lized salt  and  75  cc  of  concentrated  hydrochloric  acid  in  1000  cc  of  solu- 
tion. Filter  through  ignited  asbestos  into  the  bottle. 

Weigh  1  gm  of  the  steel  or  iron  drillings,  place  in  a  250-cc  beaker  and 
add  100  cc  of  the  cupric  potassium  chloride  solution.  Stir  until  the 
metal  is  all  dissolved,  warming  to  about  65°.  If  many  determinations 
are  to  be  made  a  stirring  machine  is  desirable. 

Filter  the  solution  through  a  Gooch  crucible  or  a  carbon  tube  (shown 
in  Fig.  67,  page  250).  The  asbestos  used  in  the  filter  must  have  been 
previously  ignited  to  remove  all  organic  matter.  Wash  with  warm  (50°) 
dilute  hydrochloric  acid  until  the  washings  are  free  from  color,  then  with 
cold  water  until  free  from  chlorides.  It  is  desirable  that  most  of  the 
water  be  removed  from  the  filter  and  carbon,  although  the  latter  need  not 
be  absolutely  dry.  After  partial  drying  by  means  of  the  pump  and  dry- 
ing oven  the  asbestos  and  carbon  are  carefully  removed  and  placed  in  a 
combustion  boat,  using  a  small  pair  of  forceps.  This  operation  should 


STEEL  AND  ALLOYS  445 

be  performed  over  a  sheet  of  white,  glazed  paper.  The  inside  of  the 
crucible  or  carbon  tube  is  carefully  wiped  clean,  using  a  tuft  of  ignited 
asbestos. 

The  combustion  and  subsequent  determination  are  carried  out  as  in 
the  direct  combustion  process  except  that  directly  following  the  combus- 
tion tube  there  is  inserted  a  U-tube  containing  a  saturated  solution  of 
ferrous  sulphate,  acidified  by  sulphuric  acid.  If  traces  of  hydrochloric 
acid  are  retained  by  the  filter  or  carbon  this  is  partly  oxidized,  chlorine 
being  produced,  and  partly  carried  over  without  change.  Chlorine  is 
absorbed  and  reduced  by  the  ferrous  sulphate  while  small  quantities 
of  hydrochloric  acid  are  absorbed  by  the  water  of  the  solution. 

Free  (Graphitic)  Carbon. — When  steel  or  iron  containing  both 
free  and  combined  carbon  is  dissolved  in  nitric  acid  of  specific 
gravity  1.2  the  combined  carbon  passes  into  solution  as  hydro- 
carbons, the  graphitic  carbon  being  left  as  an  insoluble  residue. 
The  latter  may  be  separated  by  filtration  and  used  for  the  deter- 
mination of  free  carbon.  If  it  is  to  be  weighed  directly  the  silica 
which  is  also  left  must  be  removed  by  washing  with  potassium 
hydroxide  solution,  then  with  water.  A  better  method  is  to 
wash  the  carbon  and  silica  free  from  iron  salts  and  acids  and  then 
determine  by  combustion,  exactly  as  in  the  case  of  total  carbon. 

Graphitic  carbon  may  also  be  determined  by  difference,  sub- 
tracting the  combined  carbon  from  the  total  carbon. 

Determination. — Weigh  1  gm  of  pig  iron  or  10  gm  of  steel  and  dis- 
solve in  nitric  acid,  specific  gravity  1.2,  using  15  cc  of  acid  for  each 
gram  of  sample.  Filter  through  ignited  asbestos  in  a  Gooch  crucible 
or  a  carbon  tube  and  wash  with  dilute  hydrochloric  acid,  then  with  hot 
water  until  free  from  chlorides.  Burn  in  the  combustion  tube  used  for 
total  carbon  and  determine  in  the  same  way. 

Combined  Carbon. — -The  percent  of  combined  carbon  may  be 
determined  indirectly  by  subtracting  the  percent  of  graphite 
from  that  of  total  carbon.  The  only  reliable  method  for  the 
direct  determination  of  combined  carbon  is  that  of  Eggertz.1 
This  method  depends  upon  the  fact,  noted  in  the  discussion  of  free 
carbon,  that  when  steel  or  iron  containing  combined  carbon  is 
dissolved  in  dilute  nitric  acid  the  combined  carbon  forms  soluble 
organic  compounds  which  impart  a  color  to  the  solution,  the  inten- 

XZ.  anal.  Chem.,  2,  433  (1862);  Chem.  News,  7,  254  (1863). 


446  QUANTITATIVE  ANALYSIS 

sity  of  which  varies  with  the  percent  of  combined  carbon.  The 
solution  is  then  compared  in  tubes  with  the  solution  of  a  stand- 
ard steel  whose  carbon  content  is  known,  the  unknown  per- 
cent being  then  calculated.  It  will  be  seen  later,  when  a  more 
extended  study  of  carbon  conditions  is  taken  up,  that  combined 
carbon  may  exist  in  more  than  one  physical  state,  although 
probably  always  present  as  the  carbide  FeaC.  This  difference 
in  physical  state  is  influenced  by  the  presence  of  other  elements 
and  also  by  the  mechanical  and  thermal  treatment  which  the 
steel  has  received.  The  color  of  the  acid  solution  is  affected  by 
all  of  these  factors  and  it  therefore  becomes  necessary  to  use  for  a 
standard  steel  one  in  which  not  only  the  percent  of  combined 
carbon  is  known  to  be  approximately  the  same  as  that  of  the  steel 
being  examined  but  also  one  that  has  nearly  the  same  rjercents  of 
other  impurities  and  that  has  been  subjected  to  the  same  thermal 
and  mechanical  treatment.  All  of  these  factors  cannot  well  be 
known  in  general  testing  and  the  method  is  therefore  of  little 
value  for  this  class  of  work.  Its  chief  value  is  to  the  steel  works 
chemist  who  knows  in  every  case  the  nature  of  the  steel  with 
which  he  is  dealing  and  who  is  thereby  enabled  to  select  his 
standard  steel  with  due  regard  to  all  of  the  variable  factors. 

Determination. — Treat  the  standard  steel  and  the  steel  being  ex- 
amined as  follows:  Weigh  1  gm  of  the  drillings  and  dissolve  in  a  beaker 
in  30  cc  of  nitric  acid  whose  specific  gravity  is  1.2  and  which  is  free  from 
chlorine.  Warm  the  acid  toward  the  end  of  the  process,  to  complete 
the  solution.  Filter  to  separate  free  carbon  and  silica,  receiving  the 
filtrate  in  a  100-ec  volumetric  flask.  Wash  the  residue,  dilute  to  the 
mark  and  mix.  Transfer  30  cc  of  the  solution  of  lighter  color  to  an 
Eggertz  tube,  which  is  a  tube  graduated  from  1  cc  to  30  cc  and  having 
an  internal  diameter  of  about  1  cm.  Add  the  darker  solution  to  another 
similar  tube  until  the  color  of  the  two  tubes  appears  to  be  equal,  viewed 
from  above.  In  case  the  color  is  very  dark,  less  solution  must  be  used 
or  the  color  observed  from  one  side,  or  else  the  color  of  the  darker 
solution  is  lightened  by  dilution. 

Calculate  the  percent  of  combined  carbon. 

Silicon. — Silicon  occurs  in  all  steels,  generally  in  quantities 
less  than  0.3  percent.  Certain  silicon  steels  contain  as  much  as 
20  percent.  Cast  iron  contains  as  much  as  3  percent  of  silicon. 
Silicon  occurs  as  a  silicide  which  is  probably  to  be  represented  by 


STEEL  AND  ALLOYS  447 

the  formula  FeSi,  this  forming  a  solid  solution  with  the  remainder 
of  the  iron.  Inclusions  of  slag  also  may  contain  silicon  as  silicates 
of  iron  and  manganese.  Silicon  has  little  effect  upon  the  me- 
chanical properties  of  steel  but  is  desired  in  cast  iron  because  of 
its  tendency  toward  throwing  carbon  out  of  its  combination  with 
iron,  thus  forming  gray  iron  which  has  a  greater  fluidity  when 
melted  than  does  white  iron,  and  which  is  therefore  better  suited 
for  foundry  purposes. 

When  iron  silicide  is  dissolved  in  nitric  acid  the  silicon  is 
entirely  converted  into  silicon  dioxide,  largely  in  the  state  of 
colloidal  silicic  acid.  If  the  silicic  acid  is  dehydrated  and  the 
resulting  silicon  dioxide  made  insoluble  by  heating  with  acids  it 
may  be  separated  by  filtration.  As  obtained  from  pig  iron  the 
silicon  dioxide  so  obtained  contains  all  of  the  free  carbon  of  the 
iron.  This  is  removed  by  ignition.  In  the  adaptation  of  this 
process  to  the  quantitative  determination  of  silicon  in  iron  and 
steel  the  chief  difficulties  encountered  are  due  to  the  tendency  of 
silica  to  change  from  the  gel  to  the  sol  and  also  to  incomplete 
washing  of  the  silica.  In  order  to  assist  in  the  separation  of 
silica  in  an  insoluble  condition  Drown  suggested1  the  addition  of 
sulphuric  acid  to  the  solution  during  the  evaporation  to  render 
silica  insoluble.  This  materially  shortens  the  time  required  fora 
determination  as  otherwise  the  solution  must  be  evaporated  and 
heated  for  some  time  in  order  to  completely  separate  the  silica. 

During  the  washing  of  the  silica,  if  pure  water  is  used,  iron  salts 
hydrolyze  and  insoluble  basic  salts  are  retained  by  the  filter. 
Alternate  washing  with  water  and  hydrochloric  acid  will  remove 
all  but  traces  of  iron  salts  and  a  correction  may  be  made  for  these 
by  the  common  process  of  volatilization  of  silica  by  hydrofluoric 
acid. 

Determination. — Prepare  a  mixture  of  375  cc  of  concentrated  nitric 
acid,  125  cc  of  concentrated  sulphuric  acid  and  500  cc  of  water  or  as 
much  of  this  mixture  as  is  needed.  Weigh  1  gm  of  pig  iron  or  5  gm  of 
steel  and  dissolve  by  warming  in  a  casserole  or  platinum  dish  with  75  cc 
of  the  acid  mixture.  The  solution  is  evaporated  by  agitation  of  the  un- 
covered casserole  over  a  flame  until  pronounced  fumes  of  the  sulphuric 
acid  appear.  Allow  the  solution  t:>  cool,  then  add  10  cc  of  dilute  hydro- 
chloric acid  and  50  cc  of  water.  Warm  until  iron  salts  are  dissolved 

1  Trans.  Am.  Inst.  Min.  Eng.,  7,  346  (1879). 


448  QUANTITATIVE  ANALYSIS 

then  filter  and  wash  alternately  with  hot  dilute  hydrochloric  acid  and 
water  until  practically  free  from  iron.  Ignite  in  a  platinum  crucible, 
cool  and  weigh.  Volatilize  the  silica  by  treatment  with  sulphuric  acid 
and  hydrofluoric  acid  (see  page  293),  and  from  the  loss  in  weight  calcu- 
late the  percent  of  the  element  silicon. 

Sulphur. — Sulphur  occurs  in  iron  and  steel  as  ferrous  sulphide, 
FeS,  unless  manganese  is  present,  in  which  case  it  forms  mangan- 
ous  sulphide  MnS.  Ferrous  sulphide  is  itself  brittle.  It  also 
shows  a  tendency  toward  the  formation  of  envelopes  surrounding 
the  crystalline  grains  of  steel,  reducing  their  cohesion  and  result- 
ing in  " shortness,"  particularly  when  hot.  Sulphur  is  therefore 
said  to  cause  "red  shortness"  of  steel.  Manganese  sulphide 
usually  occurs  as  small  rounded  masses  instead  of  envelopes 
and  it  is  therefore  much  less  objectionable  than  ferrous  sulphide. 
The  steel  maker  therefore  relies  upon  manganese  Ho  correct 
largely  the  bad  effects  of  sulphur,  although  the  latter  should  not 
be  present  in  steel  in  quantities  greater  than  0.05  percent.  In 
the  best  steel  its  quantity  is  much  less  than  this. 

The  determination  of  sulphur  may  be  accomplished  by  oxida- 
tion to  sulphuric  acid,  followed  by  precipitation  as  barium 
sulphate,  or  by  evolution  methods;  in  the  latter  the  metal  is 
dissolved  in  hydrochloric  acid,  ferrous  sulphide  or  manganous  sul- 
phide forming  hydrogen  sulphide.  The  latter  is  distilled  into 
some  absorbing  solution  and  subsequently  determined  by  gravi- 
metric or  volumetric  methods. 

Oxidation  Method. — Steel  or  iron  dissolves  more  readily  in 
dilute  nitric  acid  than  in  the  concentrated  acid  and  the  former  is 
therefore  used  for  dissolving  the  sample  for  nearly  all  other  deter- 
minations. But  the  dilute  acid  will  not  serve  for  dissolving  the 
metal  as  a  preliminary  to  the  gravimetric  determination  of 
sulphur  because  a  part  of  the  sulphur  will  be  evolved  as  hydrogen 
sulphide  and  will  then  escape.  Concentrated  nitric  acid  com- 
pletely oxidizes  the  sulphide  to  sulphate. 

6FeS+24HNO3->2Fe2(S04)3+2Fe(NO3)3+18NO+12H2O. 

The  sulphur  is  then  precipitated  as  barium  sulphate. 

The  separation  from  the  large  amount  of  iron  involves  some 
difficulty.  Unless  a  considerable  excess  of  acid  is  present  basic 
ferric  salts,  products  of  hydrolysis,  are  retained  by  the  precipitate 


STEEL  AND  ALLOYS  449 

of  barium  sulphate.  If  too  much  acid  is  present  the  precipitation 
of  barium  sulphate  is  incomplete.  For  the  solubility  of  barium 
sulphate  in  hydrochloric  acid,  see  page  92.  Nitric  acid  must  not 
be  present  at  all  because  of  its  effect  upon  the  occlusion  of  iron 
salts  by  the  precipitate. 

Silica  must  be  separated  by  evaporation  and  nitration  before 
the  precipitation  of  barium  sulphate,  and  during  the  evaporation 
and  heating  that  are  necessary  for  this  purpose  there  is  danger  of 
loss  of  sulphur  through  decomposition  of  ferric  sulphate: 


In  order  to  prevent  loss  of  sulphur  trioxide  by  this  means,  a 
small  amount  of  sodium  carbonate  is  added  before  the  evapora- 
tion. This  immediately  forms  sodium  nitrate  or  chloride  (hy- 
drochloric acid  also  having  been  added)  and  this  reacts  during  the 
evaporation,  thus: 

Fe2  (SO4)  a  +  6NaCl->2FeCl3  +  3Na2SO4. 
Sodium  sulphate  is  not  decomposed  by  moderate  heating. 

Determination.  —  Weigh  5  gm  of  drillings  or  powder  into  a  casserole. 
Place  under  a  hood  and  add  50  cc  of  concentrated  nitric  acid  which  is 
free  from  sulphuric  acid.  Action  may  not  begin  at  once  unless  the  cas- 
serole is  warmed  but  after  the  metal  begins  to  dissolve  the  action  may 
become  violent.  In  this  case  the  casserole  should  be  placed  in  cold 
water.  In  the  later  stages  it  may  again  be  necessary  to  heat  the  cas- 
serole. Add  1  gm  of  sodium  carbonate,  free  from  sulphate,  and  evapo- 
rate to  dryness,  holding  the  casserole  over  the  flame  and  giving  it  a 
rotary  motion  to  prevent  bumping  and  to  hasten  evaporation.  When 
the  residue  is  dry,  heat  for  15  minutes  at  a  temperature  just  below  red- 
ness, then  add  30  cc  of  concentrated  hydrochloric  acid,  and  again 
evaporate  to  dryness  and  heat  as  before.  Cool,  add  30  cc  of  concen- 
trated hydrochloric  acid,  warm  until  all  iron  salts  are  in  solution,  then 
evaporate  in  the  same  manner  as  before  until  ferric  chloride  begins  to 
crystallize.  Add  a  very  small  amount  of  hydrochloric  acid  to  redissolve 
these  crystals,  then  add  25  cc  of  water,  filter  and  wash  with  water, 
then  with  a  very  small  amount  of  hot,  dilute  hydrochloric  acid,  repeating 
the  water  and  acid  washing  until  the  paper,  silica  and  carbon  are  free 
from  the  red  or  brown  stains  of  ferric  chloride.  Finally  wash  with  hot 
water  until  the  volume  of  the  filtrate  is  about  200  cc.  If  this  residue 
is  large  in  quantity  it  will  contain  an  appreciable  amount  of  sulphur. 

29 


450  QUANTITATIVE  ANALYSIS 

In  this  case  transfer  the  paper  with  the  residue  to  a  platinum  crucible, 
burn  until  paper  and  carbon  have  been  removed  and  fuse  with  2  gm  of 
sodium  carbonate.  Cool,  dissolve  the  fusion  in  dilute  hydrochloric 
acid,  using  no  more  than  is  necessary,  and  wash  into  the  main  solution. 

Heat  to  boiling  and  add,  a  drop  at  a  time  and  stirring  continuously, 
10  cc  of  10  percent  barium  chloride  solution.  Digest  at  a  temperature 
near  the  boiling  point  for  30  minutes,  then  allow  to  stand  for  2  hours. 
Filter  and  wash,  alternately  with  dilute  hydrochloric  acid  and  water, 
until  the  filter  and  precipitate  are  white  and  finally  with  water  until 
free  from  chlorides.  In  this  washing  use  as  little  hydrochloric  acid  as 
possible.  Ignite  the  paper  and  precipitate  in  a  platinum  crucible  at  a 
low  temperature  and  weigh  the  barium  sulphate.  If  the  ignited  precipi- 
tate is  not  white  some  iron  oxide  is  contained  in  it.  In  this  case  add  1  gm 
of  sodium  carbonate,  fuse,  dissolve  the  fusion  in  water  and  dilute  hydro- 
chloric acid  and  precipitate  as  before. 

Calculate  the  percent  of  sulphur  in  the  sample. 

Evolution  Method. — The  determination  of  sulphur  by  evolu- 
tion depends  upon  the  decomposition  of  metallic  sulphides  by 
hydrochloric  acid,  the  resulting  hydrogen  sulphide  being  distilled 
and  absorbed  in  another  solution.  The  absorbing  solution  may 
form  an  insoluble  sulphide  with  the  hydrogen  sulphide  or  it  may 
oxidize  the  latter  to  sulphuric  acid  which  is  then  determined 
gravimetrically.  Absorbents  of  the  first  class  are  basic  solutions 
of  salts  of  lead,  cadmium  or  silver.  Absorbents  of  the  second 
class  are  bromine  in  hydrochloric  acid,  potassium  permanganate 
and  hydrogen  peroxide.  A  solution  of  cadmium  chloride  in  excess 
of  ammonium  hydroxide  is  to  be  preferred.  The  precipitate  of 
cadmium  sulphide  may  be  washed,  dried  and  weighed,  but  it  is 
better  to  decompose  it  with  hydrochloric  acid  and  titrate  the 
liberated  hydrogen  sulphide  with  standard  iodine  solution.  The 
solubility  of  cadmium  sulphide  in  hydrochloric  acid  is  not  large 
unless  the  resultant  hydrogen  sulphide  is  removed,  as  in  this  case 
by  oxidation. 

The  evolution  method  may  be  performed  in  less  time  than  the 
oxidation  method.  It  has  been  shown  by  Phillips  to  be  inaccu- 
rate,1 however,  for  white  pig  iron  because  of  the  formation  of 
organic  sulphur  compounds,  of  which  methyl  sulphide,  (CHs^S, 
was  isolated.  Such  sulphides  are  difficult  to  expel  from  the 

1  J.  Am.  Chem.  Soc.,  17,  891  (1895). 


STEEL  AND  ALLOYS  451 

evolution  flask  and  require  as  much  as  two  hours  of  boiling, 
during  which  time  air,  carbon  dioxide  or  hydrogen  is  drawn 
through  the  apparatus.  Phillips  found  that  the  organic  sulphides 
could  be  decomposed  by  passing  the  vapors  through  a  tube, 
heated  to  redness.  The  additional  time  necessary  for  the  ex- 
pulsion of  organic  sulphides  from  the  evolution  flask  makes  the 
method  impracticable  for  white  pig  iron.  For  steel  and  gray 
pig  iron,  containing  relatively  low  percents  of  combined  carbon, 
the  method  is  satisfactory. 

Determination. — Prepare  an  approximately  fiftieth-normal  solution 
of  iodine  by  dissolving  the  calculated  weight  of  iodine  and  twice  its 
weight  of  potassium  iodide  or  sodium  iodide  in  1000  cc  of  water.  Stand- 
ardize just  before  making  the  sulphur  determination  by  titrating 
against  a  standard  solution  of  sodium  thiosulphate  or  of  sodium  arsenite 
(As203  and  half  its  weight  of  NaOH).  The  iodine  dissolves  rather 
slowly  unless  well  powdered.  It  is  well  to  decant  the  solution  into 
another  bottle  in  order  to  avoid  the  possibility  of  particles  of  undissolved 
iodine  changing  the  concentration  of  the  solution  after  standardization. 

Determination. — Use  a  300-cc  flask,  having  a  round  bottom,  for 
the  evolution  flask.  Connect,  through  a  2-hole  rubber  stopper,  a  100- 
cc  separatory  funnel  and  a  short  tube,  bent  at  a  right  angle  with  the 
flask.  The  separatory  funnel  should  reach  to  the  bottom  of  the  flask 
and  should  have  the  bottom  turned  up,  as  in  the  apparatus  for  the 
determination  of  carbon  dioxide  in  carbonates.  The  short  exit  tube  is 
connected  with  another  tube  which  reaches  to  the  bottom  of  a  cylinder 
having  a  capacity  of  about  100  cc,  in  which  is  placed  50  cc  of  a  solution 
made  as  follows:  Cadmium  chloride  10  gm,  water  375  cc,  concentrated 
ammonium  hydroxide  625  cc.  This  cylinder  is  similarly  connected 
with  a  second  cylinder  containing  the  same  kind  of  solution.  Both 
cylinders  should  stand  in  a  large  beaker  of  cold  water.  Instead  of  the 
two  cylinders  a  Meyer  tube  (figure  96)  may  be  used. 

Weigh  into  the  evolution  flask  5  gm  of  steel  or  iron  drillings,  close 
the  flask  and  place  75  cc  of  hydrochloric  acid  (1:1)  in  the  separatory 
funnel.  Admit  the  acid  fast  enough  to  cause  a  rapid  evolution  of 
hydrogen.  Finally  add  all  but  about  5  cc  of  the  acid  and  warm  to  assist 
the  solution.  When  the  metal  is  all  dissolved  boil  for  5  minutes  at 
a  rate  that  will  permit  absorption  of  the  hydrogen  sulphide.  This 
boiling  should  completely  expel  hydrogen  sulphide  and  hydrogen  from 
the  flask. 

Disconnect  the  delivery  tube,  then  remove  the  source  of  heat  and  rinse 
the  tubes,  allowing  the  washings  to  run  into  the  absorption  cylinders. 


452  QUANTITATIVE  ANALYSIS 

If  the  tubes  contain  any  cadmium  sulphide  wash  with  dilute  hydrochloric 
acid  and  then  with  water  but  do  not  agitate  the  solution.  Rinse  the 
contents  of  the  absorption  cylinder  into  a  500-cc  beaker,  dissolving 
adhering  precipitate  by  means  of  dilute  hydrochloric  acid,  allowing  this 
solution  to  run  immediately  into  the  main  body  of  solution.  Usually  all 
of  the  hydroge"n  sulphide  is  absorbed  in  the  first  cylinder  and  the  contents 
of  the  second  need  not  be  used  if  no  trace  of  yellow  cadmium  sulphide 
appears  in  it.  Add  water  until  the  volume  is  about  300  cc,  then  add 
dilute  hydrochloric  acid  until  the  liquid  is  distinctly  acid  in  character, 
stirring  gently  meanwhile.  The  disappearance  of  turbidity  is  sufficient 
indication  of  an  acid  reaction.  Rapid  stirring  and  undue  agitation  will 
cause  a  loss  of  hydrogen  sulphide.  Add  1  cc  of  starch  solution  and  ti- 
trate at  once  with  decinormal  iodine  solution. 
Calculate  the  percent  of  sulphur  in  the  sample. 

Phosphorus. — The  proportion  of  phosphorus  in  steel  of 
satisfactory  quality  is  not  usually  higher  than  0.1  percent  and 
is  frequently  required  to  be  less.  Acid  open  hearth  and  acid 
Bessemer  steel  contain  larger  quantities  of  phosphorus  than 
steel  made  by  basic  processes.  Phosphorus  occurs  in  steel  as 
the  phosphide  Fe3P.  Its  effect  is  to  cause  brittleness  of  the 
steel,  this  being  at  least  partly  due  to  the  promotion  of  coarse 
granulation. 

The  determination  of  phosphorus  in  iron  or  steel  may  follow 
either  gravimetric  or  volumetric  methods.  In  any  case  the  final 
determination  must  be  preceded  by  separation  from  the  relatively 
large  excess  of  iron.  The  separation  is  usually  made  by  either 
a  modification  of  the  method  of  Fresenius1  known  as  the  "acetate 
method,"  or  the  molybdate  method  of  Sonnenschein.2 

Acetate  Method. — This  method  of  separating  iron  and  phos- 
phorus depends  upon  the  relatively  large  solubility  of  ferrous 
acetate  as  compared  with  that  of  basic  ferric  acetate  and  ferric 
phosphate.  The  iron  is  first  reduced  entirely  to  the  ferrous 
condition  by  sulphurous  acid,  then  either  a  small  amount  reoxi- 
dized  by  bromine  or  a  small  amount  of  ferric  chloride  is  added. 
The  solution  is  now  made  slightly  basic,  then  an  excess  of  acetic 
acid  and  water  is  added.  A  precipitate  forms,  consisting  of 
ferric  phosphate  and  basic  ferric  acetate,  the  latter  being  present 

1  J.  prakt.  Chem.,  45,  258  (1848), 

2  Ibid.,  53,  339  (1851). 


STEEL  AND  ALLOYS  453 

in  very  small  quantity.  The  larger  part  of  the  iron  has  remained 
in  solution  as  ferrous  acetate  and  is  separated  by  filtration. 

This  method  necessarily  leaves  a  small  amount  of  iron  in  the 
phosphorus  precipitate.  In  order  to  separate  this,  advantage 
is  taken  of  the  fact  that  small  quantities  of  iron  are  not  pre- 
cipitated by  ammonium  hydroxide  if  organic  acids  are  present. 
Either  citric  acid  or  ammonium  citrate  is  added  and  the  phos- 
phorus is  precipitated  by  " magnesia  mixture"  in  presence  of 
ammonium  hydroxide.  The  ionization  of  ferric  citrate  is  so 
small  that  the  solubility  product  of  neither  ferric  hydroxide  nor 
basic  ferric  citrate  is  attained. 

The  acetate  method  is  accurate  if  carefully  performed,  but  is 
complicated  in  detail  and  is  more  liable  to  fail  than  the  next 
method  to  be  described. 

Molybdate  Method. — The  molybdate  method  of  separating 
iron  and  phosphorus  depends  upon  the  insolubility  of  ammonium 
phosphomolybdate  and  the  solubility  of  iron  in  nitric  acid.  The 
iron  or  steel  is  dissolved  in  nitric  acid,  carbon  is  oxidized  by  potas- 
sium permanganate,  the  solution  is  nearly  neutralized  and  a  solu- 
tion of  ammonium  molybdate  in  nitric  acid  is  added.  The  pre- 
cipitate of  ammonium  phosphomolybdate  is  separated  by 
filtration  and  is  then  treated  according  to  the  method  which  has 
been  selected  for  the  final  determination.  The  removal  of 
carbon  by  oxidation  is  necessary  in  order  that  precipitation  shall 
be  complete. 

The  determination  of  phosphorus  may  now  be  made  (1)  by 
drying  and  weighing  the  yellow  precipitate  of  ammonium  phos- 
phomolybdate, (2)  by  measuring  its  volume,  (3)  by  titrating  its 
molybdic  oxide  by  means  of  a  standard  base,  (4)  by  reducing 
its  molybdic  oxide  to  molybdenum  sesquioxide  and  titrating  by 
standard  potassium  permanganate  solution,  or  (5)  by  dissolving 
the  yellow  precipitate  in  ammonium  hydroxide  and  precipitating 
as  magnesium  ammonium  phosphate. 

If  method  (1)  is  to  be  followed  it  is  necessary  that  care  be 
exercised  in  precipitating  the  ammonium  phosphomolybdate  in 
order  that  its  composition  may  be  constant.  Precipitated  under 
the  conditions  later  described  its  composition  is  represented  by 
the  formula  (NH4)3P04.12MoO3.  The  composition  is  somewhat 
altered  by  variation  in  temperature,  excess  of  ammonium 


454  QUANTITATIVE  ANALYSIS 

molybdate,  excess  of  nitric  acid  and  time  of  precipitation.  The 
precipitate  may  contain  also  small  amounts  of  free  molybdic  acid, 
especially  if  too  much  nitric  acid  is  present,  or  of  ammonium 
silicomolybdate  if  silicon  has  not  been  removed.  This  method 
of  direct  weighing  is  not  often  followed. 

Method  (2)  is  a  rapid  but  inaccurate  method.  The  precipita- 
tion is  carried  out  in  a  pear-shaped  bulb  having  a  graduated  stem. 
The  precipitate  is  packed  into  the  stem  by  centrifugal  action 
and  its  volume  is  read  and  converted  into  weight  percent  by  a 
previously  determined  factor. 

Method  (3)  was  suggested  by  Pemberton.1  In  this  method  the 
yellow  precipitate  is  dissolved  in  an  excess  of  a  standard  solution 
of  potassium  hydroxide  or  sodium  hydroxide,  the  excess  being 
then  titrated  by  a  standard  acid  solution,  phenolphthalein  being 
used  as  the  indicator.  The  reaction  between  the  phosphomolyb- 
date  and  the  base  is  as  follows: 


12H20. 

Upon  the  addition  of  standard  acid,  phenolphthalein  changes 
color  when  the  excess  of  base  has  been  neutralized  and  the 
following  reaction  has  occurred: 

(NH4)  3P04+  HC1^(NH4)  2HPO4+  NH4C1. 

Twenty-three  equivalents  of  base  have  therefore  apparently  been 
used  at  the  end  point  and  in  order  to  express  this  fact  in  one  equa- 
tion the  reaction  is  often  represented  as  follows: 

2(NH4)3P04.12Mo03  +  46KOH->2(NH4)2HP04  +  (NH4)2Mo04 

+23K2MoO4+22H2O. 

This  is  seen  to  be  really  a  direct  titration  of  molybdic  acid  instead 
of  a  titration  of  phosphorus  and  it  is  therefore  an  indirect  estima- 
tion of  phosphorus  and  can  be  correct  only  in  case  the  composition 
of  the  precipitate  is  constant.  It  is  also  essential  that  no  free 
molybdic  acid  should  be  present  with  the  phosphomolybdate. 

There  is  some  difference  in  opinion  concerning  the  accuracy 
of  this  method.  If  the  precipitation  is  carefully  performed  it  is 
probably  as  accurate  as  the  gravimetric  method  (5). 

1  J.  Chem.  Soc.,  15,  382  (1893);  16,  278  (1894). 


STEEL  AND  ALLOYS  455 

Determination  by  Pemberton's  Method. — Prepare  the  following 
reagents : 

(a)  Acid  Solution  of  Ammonium  Molybdate. — Dissolve  100  gm  of 
molybdic  acid  in  a  mixture  of  144  cc  of  ammonium  hydroxide  (specific 
gravity  0.90)  and  271  cc  of  water.  Pour  this  solution,  slowly  and  with 
vigorous  stirring,  into  a  mixture  of  590  cc  of  concentrated  nitric  acid 
(specific  gravity  1.42)  and  1148  cc  of  water.  Allow  to  stand  at  a  tem- 
perature of  about  40°  for  several  days  and  then  decant  from  sediment 
and  preserve  in  glass-stoppered  bottles. 

(6)  Standard  Potassium  Hydroxide  Solution,  1  cc  of  which  is  equiva- 
lent to  0.1  mg  of  phosphorus.  This  should  be  as  nearly  free  from 
carbonates  as  possible  and  is  made  as  follows :  Dissolve  2  percent  more 
than  the  calculated  quantity  for  1000  cc,  dilute  to  100  cc  and  add  1  cc 
of  a  saturated  solution  of  barium  hydroxide.  Stopper  the  flask  and 
allow  to  stand  until  the  precipitate  of  barium  carbonate  has  settled. 
Decant  and  dilute  to  1000  cc.  Standardize  by  titration  against  solution 
(c),  using  phenolphthalein.  Adjust  so  that  1  cc  is  equivalent  to  0.1 
mg  of  phosphorus. 

(c)  Standard  Hydrochloric  Acid  Solution,  equivalent  in  concentra- 
tion to  the  standard  base;  use  boiled  water. 

(d)  Potassium  Permanganate  Solution,  1.5  gm  in  100  cc. 

(e)  Potassium  Nitrate  Solution,  1.0  percent. 

Weigh  2  gm  of  iron  or  steel  into  a  250-cc  Erlenmeyer  flask  and  add 
100  cc  of  nitric  acid  (specific  gravity  1.13)  and  warm  until  the  sample 
is  dissolved  (see  note  on  page  457:  ''Interference  of  Titanium").  Boil 
to  expel  oxides  of  nitrogen,  then  add  10  cc  of  solution  (d)  and  boil 
until  the  combined  carbon  is  completely  oxidized  and  the  excess  of 
potassium  permanganate  is  decomposed,  as  is  made  evident  by  the 
disappearance  of  the  pink  color.  Dissolve  the  precipitated  manganese 
dioxide  by  boiling  with  about  1  gm  of  ferrous  ammonium  sulphate. 
Add  ammonium  hydroxide  with  vigorous  stirring.  The  last  part  of 
this  operation  must  be  conducted  with  care  because  if  much  ferric 
hydroxide  is  allowed  to  form  it  will  not  readily  redissolve,  even  though 
the  solution  still  contains  an  excess  of  acid.  Redissolve  the  precipi- 
tate by  the  addition  of  the  least  necessary  quantity  of  nitric  acid. 
Place  a  thermometer  in  the  flask  and  warm  to  a  temperature  of  60°  to 
65°  by  placing  the  flask  in  a  water  bath  and  then  add  40  cc  of  freshly 
filtered  ammonium  molybdate  solution,  stir  well  and  allow  to  stand 
15  minutes.  Filter  immediately  and  wash  flask  and  precipitate  with 
solution  (e)  until  the  washings  are  neutral  to  phenolphthalein  but  with- 
out attempting  to  remove  all  precipitate  from  the  flask. 

Transfer  the  paper  and  precipitate  to  the  flask  in  which  the  precipita- 
tion was  made  and  add  enough  standard  solution  of  potassium  hydrox- 


456  QUANTITATIVE  ANALYSIS 

ide  to  dissolve  the  precipitate.  Dilute  to  about  75  cc  with  recently 
boiled  water,  add  a  drop  of  phenolphthalein  and  titrate  the  excess  of 
base  with  standard  acid  solution. 

Calculate  the  percent  of  phosphorus  in  the  steel. 

Method  (4).  This  also  is  an  indirect  method  for  the  determina- 
tion of  phosphorus,  since  it  also  depends  upon  reactions  of  molyb- 
denum oxides,  rather  than  of  phosphorus.  The  precipitate  of 
ammonium  phosphomolybdate,  obtained  as  in  method  (3),  is 
dissolved  in  ammonium  hydroxide,  the  solution  is  acidified  with 
sulphuric  acid  and  zinc  is  then  added.  Molybdenum  trioxide, 
MoOs,  is  reduced  to  molybdenum  sesquioxide  Mo203,  which  is 
again  oxidized  by  titration  with  standard  potassium  perman- 
ganate solution. 

Method  (5).  This  is  one  of  the  most  reliable  of  air  methods  if 
carefully  performed,  since  its  accuracy  does  not  depend,  in  any 
way,  upon  the  composition  of  the  yellow  precipitate.  A  some- 
what less  acid  solution  of  ammonium  molybdate  may  be  used  and 
this  keeps  better  than  the  solution  required  for  volumetric  proc- 
esses. The  yellow  precipitate  of  ammonium  phosphomolybdate 
is  dissolved  in  ammonium  hydroxide  and  the  phosphorus  is  then 
precipitated  as  ammonium  magnesium  phosphate  by  the  addition 
of  a  solution  of  magnesium  chloride.  The  ammonium  mag- 
nesium phosphate  is  ignited  and  weighed  as  magnesium  pyro- 
phosphate.  Potassium  permanganate  cannot  be  used  for  the 
oxidation  of  carbon  since  it  would  later  form  a  precipitate  of 
ammonium  manganese  phosphate. 

Determination. — Prepare  the  following  solutions: 

(a)  Acid  Solution  of  Ammonium  Molybdate. — Dissolve  100  gm  of 
molybdic  acid  in  a  mixture  of  144  cc  of  ammonium  hydroxide  (specific 
gravity  0.90)  and  271  cc  of  water.     Pour  this  solution  slowly  and  with 
vigorous  stirring  into  a  mixture  of  490  cc  of  concentrated  nitric  acid 
(specific  gravity  1.42)  and  1148  cc  of  water.     Allow  to  stand  at  a  tem- 
perature of  30°  to  40°  for  several  days  and  then  decant  and  preserve 
in  glass-stoppered  bottles. 

(b)  Ammonium  Citrate  Solution. — Dissolve  50  gm  of  citric  acid  in 
water,  add  350  cc  of  ammonium  hydroxide  (specific  gravity  0.90)  and 
dilute  to  1000  cc. 

(c)  Ammonium  Hydroxide  Solution  containing  2.5  percent  of  ammonia. 

(d)  "  Magnesia   Mixture  " — Dissolve  55  gm  of  crystallized  magne- 


STEEL  AND  ALLOYS  457 

slum  chloride  and  140  gm  of  ammonium  chloride  in  water,  add  130  cc 
of  ammonium  hydroxide  (specific  gravity  0.90)  and  dilute  to  1000  cc. 

(e)  Ammonium  Nitrate  Solution,  10  percent. 

Dissolve  1  to  2  gm  of  steel  in  20  cc  of  nitric  acid  (specific  gravity 
1.2)  in  a  casserole,  cover  and  boil  until  nitrogen  oxides  are  expelled. 
Evaporate  to  dryness  on  the  steam  bath  or  by  agitating  over  a  flame. 
Heat  for  15  minutes  over  the  direct  flame  in  order  to  oxidize  organic 
matter,  formed  from  combined  carbon.  Cool,  add  30  cc  of  concentrated 
hydrochloric  acid  and  heat  to  dissolve  iron  oxide.  Evaporate  with 
stirring  until  ferric  chloride  begins  to  crystallize  but  do  not  allow 
salts  to  dry  on  the  sides  of  the  casserole.  Add  10  cc  of  concentrated 
nitric  acid,  boil  to  expel  chlorine,  dilute  to  75  cc  and  filter  into  a  250-cc 
flask.  (If  titanium  is  present,  see  note  below:  "Interference  of  Tita- 
nium.") Wash  the  silica  and  carbon  on  the  paper  with  2  per  cent  nitric 
acid  and  water  until  the  iron  is  all  removed,  as  made  evident  by  the 
disappearance  of  brown  stains. 

Dilute  the  filtrate  to  about  100  cc  and  add  dilute  ammonium  hydrox- 
ide solution  very  slowly  and  with  vigorous  stirring  until  a  small  amount 
of  precipitate  remains  undissolved.  Redissolve  this  in  concentrated 
nitric  acid,  immerse  the  flask  in  water  and  warm  to  about  60°.  Add  50 
cc  of  ammonium  molybdate  solution  (a),  shake  and  allow  to  remain  at  a 
temperature  of  65°  for  an  hour.  Filter  and  wash  with  solution  (e) 
until  no  brown  stains  remain  on  the  paper.  It  is  not  necessary  to  re- 
move all  of  the  precipitate  from  the  sides  of  the  flask  at  this  point,  but  it 
must  be  well  washed.  Place  the  flask  in  which  precipitation  was  made 
under  the  funnel  and  dissolve  the  precipitate  by  adding  about  25  cc  of 
ammonium  citrate  solution  (6). 

Wash  the  paper  thoroughly  with  hot  water.  Rotate  the  flask  until 
all  of  the  precipitate  is  dissolved  from  the  sides  then  nearly  neutralize 
with  hydrochloric  acid.  Transfer  to  a  beaker,  dilute  to  100  cc,  add 
10  cc  of  magnesia  mixture,  slowly  and  with  vigorous  stirring.  After 
the  solution  has  stood  for  30  minutes  add  slowly  ammonium  hydroxide 
of  specific  gravity  0.90  in  quantity  equal  to  1/9  of  the  total  volume 
of  solution.  Allow  to  stand  for  two  hours,  filter  and  wash  with  dilute 
ammonium  hydroxide  solution.  Ignite  in  a  platinum  or  porcelain 
crucible  until  white  and  weigh  the  magnesium  pyrophosphate. 

Calculate  the  percent  of  phosphorus  in  the  steel. 

Interference  of  Titanium.— If  titanium  is  present,  as  it  fre- 
quently is  in  pig  iron  and  sometimes  in  steel,  the  phosphorus 
will  not  all  be  recovered  by  any  of  the  methods  already  described 
because  the  action  of  acids  upon  iron  leaves  an  insoluble  double 


458  QUANTITATIVE  ANALYSIS 


salt  of  phosphoric  acid,  titanic  acid  and  iron.  In  this  case  the 
residue  of  silica,  carbon,  ferric  phosphotitanate,  etc.,  obtained  by 
filtration  of  the  acid  solution  of  iron,  is  ignited  in  a  platinum 
crucible  to  burn  organic  matter,  the  silica  is  volatilized  by  mois- 
tening with  a  drop  of  sulphuric  acid  and  adding  1  cc  of  hydro- 
fluoric acid,  and  the  residue  is  then  fused  with  about  2  gm  of 
sodium  carbonate.  Sodium  phosphate,  ferric  oxide  and  sodium 
titanate,  Na2TiOs,  are  formed.  Sodium  phosphate  is  dissolved 
in  water  and  added  to  the  principal  solution  of  the  iron  in  nitric 
acid.  Sodium  titanate  and  ferric  oxide  are  insoluble  in  water. 

Titanium. — Titanium  is  often  present  in  pig  iron  as  an  impurity, 
being  derived  from  the  iron  ores.  There  is  now  an  increasing 
use  of  titanium,  in  the  form  of  iron-titanium  " alloys,"  as  an 
agent  to  promote  sound  castings  and  sound  steel.  Its  effect  is 
to  reduce  oxides,  combine  with  nitrogen  and  sulphur  and  thus  to 
prevent  blow  holes  and  flaws  by  the  formation  of  a  solid  oxide  or 
nitride  which  enters  the  slag.  If  this  action  were  ideal  there 
should  be  no  titanium  remaining  in  the  metal  at  the  end  of  the 
process  but  this  is  not  always  the  case  and  determinations  of 
titanium  may  be  required.  Titanium  is  now  also  used  to  some 
extent  as  an  essential  constituent  of  finished  alloy  steels. 

It  has  already  been  stated  that  much  of  the  titanium  will 
remain  as  an  insoluble  compound  with  iron  and  phosphorus 
when  iron  is  dissolved  in  acids.  This  is  freed  from  carbon  by 
ignition  and  from  silica  by  treatment  with  sulphuric  acid  and 
hydrofluoric  acid.  The  titanium  in  the  acid  solution  of  the  sam- 
ple is  recovered  by  neutralizing  the  excess  of  acid,  reducing  the 
iron  to  the  ferrous  state  by  sodium  thiosulphate  or  sulphurous 
acid  and  precipitating  titanic  acid  by  boiling.  Titanic  acid  is 
an  irreversible  colloid  (see  page  19)  and  becomes  insoluble 
when  its  solution  is  boiled  for  some  time.  This  precipitate  is 
removed  by  filtration  and  added  to  the  residue  already  in  the 
crucible.  The  whole  is  then  fused  with  sodium  carbonate  and 
the  sodium  acid  titanate,  NaHTiOs,  and  ferric  oxide  are  separated 
from  sodium  phosphate  by  dissolving  the  latter  and  filtering. 
One  of  two  methods  of  procedure  may  now  be  adopted: 

(a)  The  insoluble  sodium  acid  tifcanate  is  fused  with  potassium 
pyrosulphate,  forming  titanic  acid.  Sulphuric  acid  and  water 
are  added,  the  titanic  acid  forming  a  colloidal  sol.  The  iron  also 


STEEL  AND  ALLOYS  459 

passes  into  solution  as  ferric  sulphate.  This  is  reduced  to  the 
ferrous  condition  by  sulphurous  acid  or  ammonium  acid  sulphite, 
the  solution  is  largely  diluted  and  boiled,  when  the  sol  is  floccu- 
lated, titanic  acid  again  passing  into  the  irreversible  gel.  This  is 
separated  by  filtration,  washed  and  ignited  and  weighed  as  tita- 
nium dioxide. 

(6)  The  sodium  titanate  in  the  crucible  is  dissolved  in  hot, 
dilute  sulphuric  acid,  transferred  to  a  color  comparison  tube  (a 
Nessler  cylinder  or  similar  tube)  and  treated  with  hydrogen 
peroxide.  Titanium  is  oxidized  by  hydrogen  peroxide  to  the 
hexavalent  condition  and  forms  an  intensely  yellow  solution. 
The  color  is  compared  with  that  produced  by  a  standard  titan- 
ium solution  in  a  similar  tube. 

Determination. — Weigh  from  2  to  5  gm  of  iron  or  steel  and  place 
in  a  casserole.  Add  50  cc  of  concentrated  hydrochloric  acid,  cover  and 
warm  until  the  metal  is  dissolved.  Filter,  wash  twice  with  hot  water, 
transfer  to  a  platinum  crucible  and  burn  the  paper  and  all  carbon.  Add 
a  drop  of  sulphuric  acid  and  about  3  cc  of  hydrofluoric  acid  and  finally 
heat  to  expel  the  acids  and  silicon  tetrafluoride. 

To  the  filtrate  containing  most  of  the  iron  add  dilute  ammonium 
hydroxide,  slowly  and  with  continuous  stirring,  until  a  small  amount 
of  ferric  hydroxide  remains  undissolved.  Redissolve  this  in  hydro- 
chloric acid,  leaving  the  solution  with  a  small  excess  of  acid.  Add  a. 
20-percent  solution  of  sodium  thiosulphate  until  the  red  color  of  ferric 
chloride  disappears  and  sulphur  begins  to  precipitate.  Dilute  to  about 
400  cc,  add  20  gm  of  sodium  acetate  and  50  cc  of  30-percent  acetic  acid 
and  boil  for  15  minutes  or  until  precipitation  of  titanic  acid  seems  to 
be  complete.  Filter  and  wash  two  or  three  times  with  hot,  1-percent 
acetic  acid  and  place  the  paper  and  precipitate  in  the  crucible  contain- 
ing the  main  portion  of  titanium.  Burn  the  paper  and  carbon  then 
add  about  5  gm  of  sodium  carbonate  and  thoroughly  fuse.  Cool, 
place  the  crucible  in  a  beaker  and  cover  with  hot  water.  When  the 
fusion  is  entirely  disintegrated,  filter  and  wash  the  sodium  titanate  and 
iron  oxide  with  1-percent  sodium  carbonate  solution.  Proceed  by 
method  (a)  or  (6),  below. 

(a)  Gravimetric  Method.1 — Return  the  paper  containing  the  washed 
residue  of  sodium  titanate  to  the  crucible  in  which  the  fusion  was  made. 
Add  10  gm  of  potassium  pyrosulphate  and  heat  gently,  avoiding  loss  by 
effervescence.  Gradually  raise  the  temperature  until  the  crucible  is 

1  Blair:  The  Chemical  Analysis  of  Iron,  8th  ed.,  172. 


460  QUANTITATIVE  ANALYSIS 

finally  red  and  keep  at  this  temperature  until  all  the  iron  oxide  is  dis- 
solved. Cool,  add  15  cc  of  concentrated  sulphuric  acid  and  heat  until 
the  entire  contents  of  the  crucible  have  becomo  liquid.  Cool  and  pour, 
slowly  and  with  stirring,  into  400  cc  of  water  contained  in  a  500-cc 
beaker.  If  basic  ferric  salts  precipitate,  redissolve  in  hydrochloric 
acid.  Add  50  cc  of  a  20-percent  solution  of  sodium  thiosulphate.  Filter 
if  not  clear,  nearly  neutralize  with  ammonium  hydroxide,  redissolve 
any  precipitate  that  may  have  formed  and  add  a  clear  solution  contain- 
ing 20  gm  of  sodium  acetate  and  150  cc  of  30-percent  acetic  acid.  Boil 
and  filter  the  titanic  acid.  Wash  three  times  with  1-percent  acetic 
acid,  transfer  the  paper  and  precipitate  to  a  porcelain  or  platinum 
crucible  and  burn  the  carbon,  finally  igniting  for  five  minutes  over  the 
blast  lamp.  Weigh  the  titanic  oxide,  Ti02,  and  calculate  the  percent 
of  titanium. 

(b)  Colorimetric  Method. — Dissolve  the  residue  in  the  4  crucible  by 
heating  with  dilute  sulphuric  acid,  place  the  filter  paper  in  a  beaker 
and  pour  the  sulphuric  acid  upon  it.  Heat  until  the  sodium  titanate  is 
dissolved  then  remove  the  paper,  rinse  thoroughly  and  rinse  the  contents 
of  the  crucible  into  the  beaker.  Transfer  to  a  Nessler  tube,  filtering  if 
not  clear,  and  dilute  to  the  mark.  Prepare  a  standard  solution  of 
titanic  acid  as  follows:  Ignite  1  gm  of  the  purest  obtainable  titanic 
acid  in  a  weighed  platinum  crucible,  cool  and  weigh.  Dissolve  the 
titanic  acid  in  dilute  sulphuric  acid,  rinse  into  a  1000-cc  volumetric 
flask  and  dilute  to  the  mark.  Mix  well,  transfer  to  a  dry  glass-stop- 
•pered  bottle  and  record  the  concentration  of  the  solution.  Into  four 
similar  tubes,  having  the  same  capacity  and  the  mark  at  the  same 
height  as  the  first  tube,  containing  the  titanium  from  the  sample,  place 
1  cc,  3  cc,  5  cc  and  10  cc,  respectively,  of  the  standard  titanium  selu- 
tion  and  dilute  these  to  the  mark.  Add  to  each  of  the  five  tubes  so 
prepared  5  cc  of  hydrogen  peroxide.  Compare  the  color  of  the  tube 
containing  the  sample  with  that  of  the  four  tubes  of  standard,  looking 
vertically  downward  through  the  tubes  toward  a  white  surface,  placed 
near  a  window.  As  a  result  of  these  comparisons,  limits  will  be  found 
for  the  concentration  of  the  sample  tube.  Prepare  other  tubes  of 
solutions  whose  concentrations  lie  between  these  limits,  until  an  equality 
of  intensity  is  obtained.  Calculate  the  percent  of  titanium  in  the 
sample. 

Manganese. — Manganese  is  found  in  certain  quantities  in 
practically  all  iron  and  steel.  At  least  traces  of  this  metal  are 
derived  from  iron  ores  while  larger  quantities  are  intentionally 
added,  either  to  correct  the  undesirable  effects  of  other  elements 


rvr»   -fry    01 


STEEL  AND  ALLOYS  461 


or  to  add  desirable  properties  of  its  own.  It  has  already  been 
stated  that  manganese  overcomes  the  tendency  of  sulphur  to 
render  steel  " red-short."  Manganese  also  has  an  effect  upon 
carbon,  exactly  the  opposite  of  that  of  silicon,  which  is  to  increase 
the  formation  of  graphitic,  or  free,  carbon.  Manganese,  on  the 
other  hand,  increases  the  tendency  of  carbon  to  remain  combined 
with  iron  as  the  carbide,  Fe3C.  With  cast  iron  it  thus  favors  the 
formation,  of  "white"  iron.  Part  of  the  carbon  also  combines 
with  manganese  to  form  a  carbide,  Mn3C,  which  is  very  hard  and 
brittle.  On  this  account  the  addition  of  manganese  to  steel  in 
quantities  above  0.50  percent  renders  the  steel  increasingly  hard. 
Such  steel  is  frequently  used  for  apparatus  that  must  resist 
abrasion,  such  as  ore  crushers  and  grinders. 

Several  excellent  methods  are  in  use  for  the  determination  of 
manganese  in  iron  and  steel.  The  underlying  principles  of  the 
most  important  of  these  will  be  discussed  and  details  will  be 
given  for  some.  All  methods  involve  (a)  the  separation  of  the 
manganese  from  the  large  excess  of  iron  and  (b)  the  determi- 
nation of  the  separated  manganese. 

Bismuthate  Method. — Schneider1  discovered  the  reaction  upon 
which  this  method  is  based  and  the  details  of  the  method  have 
since  been  modified.  The  method  is  now  recognized  as  one  of 
the  most  accurate  and  easily  applied  of  all  methods  now  in  use. 
It  was  adopted  as  a  standard  method  in  1907  by  the  Committee 
on  Standard  Methods  for  the  Analysis  of  Iron,  of  the  American 
Foundrymen's  Association.2 

This  method  is  based  upon  the  oxidation  of  bivalent  manganese 
to  heptavalent  manganese  by  sodium  bismuthate.  The  solution 
of  pig  iron  or  steel  in  an  acid  contains  manganese  as  a  manganous 
salt.  This  is  oxidized  to  sodium  permanganate  by  sodium  bis- 
muthate, the  bismuth  being  reduced  to  the  trivalent  condition. 
Sodium  bismuthate  is  derived  from  bismuth  pentoxide  and  is  a 
salt  of  the  hypothetical  acid  HBiO3.  The  reaction  between 
sodium  bismuthate  and  manganous  nitrate  may  be  represented 
thus: 

5NaBiO3+.2Mn(NO3)2+14HN03->2NaMn04+5Bi(NO3)3+ 

3NaN03+7H20. 

1  Dingl.  polyt.  J.,  269,  224  (1888);  Monatsh.  9,  242  (1888) 

2  J.  Am.  Chem.  Soc.,  29,  1372  (1907). 


462  QUANTITATIVE  ANALYSIS 

The  method  usually  involves  a  solution  in  nitric  acid.  Metzger 
and  McCrackan1  proposed  a  sulphuric  acid  solution,  the  reaction 
taking  place  as  had  already  been  shown  by  Schneider. 

Determination. — Prepare  a  solution  of  potassium  permanganate,  1 
cc  of  which  contains  1  mg  of  manganese.  Standardize  against  ferrous 
ammonium  sulphate  by  the  method  given  on  page  250. 

Prepare  also  a  solution  of  ferrous  ammonium  sulphate  or  sodium 
arsenite  (page  451)  in  recently  boiled  water  equivalent  in  concentration 
to  the  permanganate  solution,  the  former  containing  50  cc  of  concen- 
trated sulphuric  acid  in  each  1000  cc.  This  solution  will  slowly  oxidize, 
even  if  stoppered  when  not  in  use,  and  it  will  be  necessary  to  obtain  its 
relation  to  the  permanganate  solution  at  the  time  the  determination  of 
manganese  is  made,  by  a  blank  determination. 

For  pig  iron,  which  contains  much  silicon  and  free  carbon,  weigh  1 
gm  of  drilled  or  crushed  sample,  dissolve  in  50  cc  of  nitric  acid  of  specific 
gravity  1.13  and  filter,  receiving  the  filtrate  in  a  250-cc  flask.  Wash 
the  paper  and  residue  with  dilute  nitric  acid  until  free  from  iron. 

For  steel,  weigh  1  gm  of  drillings,  placing  in  a  250-cc  flask,  and  dissolve 
in  50  cc  of  nitric  acid  of  specific  gravity  1.13.  For  steels  with  excep- 
tionally high  manganese  content  modify  the  sample  weight  accord- 
ingly. For  either  pig  iron  or  steel  proceed  with  the  clear  solution  as 
follows : 

Add  about  0.5  gm  of  sodium  bismuthate  which  is  free  from  manganese. 
Heat  until  the  permanganate,  which  is  formed  at  first,  is  decomposed 
by  nitric  acid  and  the  pink  color  disappears.  This  insures  the  oxida- 
tion of  organic  matter  from  combined  carbon.  Add  enough  powdered 
ferrous  ammonium  sulphate  to  redissolve  any  precipitated  manganese 
dioxide  and  boil  until  all  nitrogen  oxides  are  expelled.  Cool  and  add 
0.5  gm  of  sodium  bismuthate  and  agitate.  Add  50  cc  of  3-percent 
nitric  acid  and  filter  through  an  ignited  asbestos  filter  in  a  Gooch 
crucible  or  a  carbon  tube,  washing  the  excess  of  sodium  bismuthate 
with  50  cc  of  3-percent  nitric  acid.  Immediately  add  from  a  burette 
the  ferrous  ammonium  sulphate  or  sodium  arsenite  solution  until  the 
permanganate  is  reduced  and  an  excess  of  the  ferrous  salt  is  present. 
Titrate  this  excess  by  means  of  standard  potassium  permanganate. 

The  ratio  of  concentrations  of  reducing  solution  and  potassium 
permanganate  solution  should  be  determined  each  day,  or  more  often 
if  it  changes  rapidly.  A  direct  titration  should  not  be  made  because 
the  presence  of  sodium  bismuthate  exerts  a  slight  disturbing  effect  due 
to  partial  oxidation  of  iron.  In  order  to  correct  this  error  the  blank 

i  J.  Am.  Chem.  Soc.,  32,  1250  (1910). 


STEEL  AND  ALLOYS  463 

determination  is  made  by  treating  the  solutions  as  they  are  treated  in 
the  determination  of  manganese. 

Measure  25  cc  of  nitric  acid  having  the  same  concentration  as  the 
acid  used  in  dissolving  the  sample  and  50  cc  of  3-percent  nitric  acid. 
Add  0.25  gm  of  sodium  bismuthate.  Heat  to  boiling,  cool  and  filter 
through  asbestos  into  a  250-cc  flask,  washing  with  50  cc  of  3-percent 
nitric  acid.  Add  35  cc,  accurately  measured,  of  ferrous  ammonium 
sulphate  or  sodium  arsenite  solution  and  titrate  immediately  with 
standard  potassium  permanganate  solution.  Calculate  the  number  of 
cubic  centimeters  of  potassium  permanganate  solution  equivalent  to 
1  cc  of  reducing  solution  and  record  this  as  the  value  of  the  secondary 
standard. 

Calculate  the  percent  of  manganese  in  the  sample. 

Ford's  Method. *• — The  separation  of  manganese  from  most  of 
the  iron  is  accomplished,  in  this  method,  by  precipitating  the 
former  metal  from  an  acid  solution,  using  nitric  acid  and  potas- 
sium chlorate.  The  nitric  acid  must  be  quite  free  from  nitrous 
acid  as  the  latter  will  redissolve  manganese  dioxide: 

MnO2+HN02+HNO3-+Mn(NO3)2+H20. 

Manganese  is  oxidized  to  the  dioxide  and  precipitates,  carrying  a 
small  amount  of  iron  with  it.  The  precipitate  is  filtered  on  asbes- 
tos that  has  been  washed  with  acids  and  ignited,  is  washed  and 
dissolved  in  sulphurous  acid  or  ammonium  acid  sulphite  and  the 
excess  of  sulphur  dioxide  is  removed  by  boiling.  The  iron  is 
reoxidized  by  bromine  and  is  then  precipitated  as  basic  acetate 
by  boiling  with  acetic  acid  or  ammonium  acetate  and  water.  In 
the  filtrate  from  this  precipitate  the  manganese  is  precipitated 
as  manganese  ammonium  phosphate  and  ignited  to  the  form  of 
manganese  pyro phosphate  by  a  method  described  on  page  114. 
The  Ford-Williams  Method. — Williams  proposed2  the  precipi- 
tation of  manganese  dioxide  by  Ford's  method,  following  this  by 
the  volumetric  determination  of  the  manganese  by  adding  a 
measured  excess  of  ferrous  ammonium  sulphate  or  sodium 
arsenite  and  titrating  the  excess  by  means  of  standard  potassium 
permanganate  solution.  The  time  necessary  for  a  complete 
determination  is  thereby  much  shortened  and  the  method  is 

1  Trans.  Am.  Inst.  Min.  Eng.,  9,  397  (1879). 

2  Ibid.,  10,  100  (1880). 


464  QUANTITATIVE  ANALYSIS 

fully  as  accurate  as  Ford's  method.  In  neither  method  is  the 
iron  precipitated,  so  that  large  samples  may  be  used  when  the 
percent  of  manganese  is  low. 

The  reaction  between  manganese  dioxide  and  ferrous  sulphate 
is  expressed  by  the  following  equation: 

MnO2+2FeSO4+2H2S04->MnS04+Fe2(S04)3+2H20. 

An  objection  to  both  the  Ford  and  the  Ford- Williams  methods 
is  in  the  difficulty  that  is  experienced  in  filtering  the  manganese 
dioxide  when  the  steel  or  iron  contains  much  silicon.  A.  P. 
Ford  and  Bregowsky  showed1  that  the  addition  of  a  few  drops 
of  hydrofluoric  acid  to  the  solution  before  filtering  eliminated  the 
silica  without  materially  injuring  the  beaker.  If  this  is  not 
done  it  is  necessary  to  evaporate  such  solutions  to  dryness  with 
hydrochloric  acid  in  order  to  render  silica  insoluble. 

Determination. — Prepare  the  following  solutions: 

(a)  Potassium  Permanganate  Solution,  1  cc  of  which  is  equivalent 
to  0.001  gm  of  manganese  by  the  reaction  just  given  and  assuming 
that  the  excess  of  ferrous  sulphate  is  to  be  titrated  by  potassium 
permanganate. 

(6)  Ferrous  Ammonium  Sulphate  Solution,  approximately  equivalent 
in  concentration  to  the  potassium  permanganate  solution  and  contain- 
ing 50  cc  of  concentrated  sulphuric  acid  in  each  1000  cc. 

Dissolve  about  5  gm  of  sample  in  a  250-cc  beaker  in  75  cc  of  nitric 
acid  of  specific  gravity  1.2.  Evaporate  until  the  solution  becomes 
viscous  then  add  75  cc  of  concentrated  nitric  acid  and  5  gm  of  potassium 
chlorate.  The  nitric  acid  must  be  free  from  nitrous  acid  as  indicated 
by  the  absence  of  a  brown  coloration.  If  not  perfectly  colorless,  draw 
a  current  of  air  through  the  acid  until  all  oxides  are  removed. 

After  the  addition  of  concentrated  nitric  acid  and  potassium  chlo- 
rate, boil  the  solution  for  15  minutes,  remove  the  flame  and  add  about 
5  or  6  drops  of  hydrofluoric  acid,  dropping  it  near  the  center  of  the 
beaker  and  mixing  it  well  with  the  solution  at  once.  Boil  for  10  minutes 
to  expel  silicon  tetrafluoride  and  the  excess  of  hydrofluoric  acid,  then  add 
1  gm  of  potassium  chlorate  and  boil  again  until  chlorine  oxides  are  no 
longer  evolved.  Filter  on  a  pad  of  acid-washed  and  ignited  asbestos 
and  wash  two  or  three  times  with  concentrated  nitric  acid  which  is  free 
from  nitrous  acid.  The  precipitate  need  not  all  be  removed  from  the 
beaker  but  all  must  be  washed.  Remove  as  much  acid  as  possible 

1  J.  Am.  Chem.  Soc.,  20,  504  (1898). 


STEEL  AND  ALLOYS  465 

from  the  filter  by  suction,  then  wash  the  beaker  and  residue  on  the  filter 
with  cold  water  until  the  washings  are  free  from  acid. 

Remove  the  asbestos  pad  and  manganese  dioxide  to  the  beaker  in 
which  the  latter  was  precipitated,  wiping  the  interior  of  the  Gooch  cru- 
cible or  filtering  tube  with  a  tuft  of  asbestos.  Measure  into  the  beaker 
enough  ferrous  ammonium  sulphate  or  sodium  arsenite  solution  to  dis- 
solve completely  the  manganese  dioxide  and  stir  until  solution  is 
complete.  Titrate  at  once  with  standard  potassium  permanganate 
solution. 

Measure  35  cc  of  standard  reducing  solution  into  another  beaker 
and  titrate  with  standard  potassium  permanganate  solution.  Calcu- 
late the  number  of  cubic  centimeters  of  potassium  permanganate 
solution  equivalent  to  1  cc  of  the  standard  reducing  solution  and 
record  this  as  the  value  of  the  secondary  standard. 

Calculate  the  percent  of  manganese  in  the  sample. 

Volhard's  Method. — This  method  is  discussed  on  page  253. 
Applied  to  iron  and  steel  it  is  somewhat  difficult  of  execution 
because  of  the  large  amount  of  ferric  hydroxide  that  is  produced 
when  zinc  oxide  is  added.  Either  the  bismuthate  or  the  Ford- 
Williams  method  is  to  be  preferred  to  it. 

Acetate  Method. — This  title  really  applies  only  to  the  sepa- 
ration of  manganese  and  iron  and  it  is,  in  practice,  followed  by 
any  one  of  several  methods  of  determination. 

In  the  discussion  of  the  acetate  method  for  separating  iron  and 
phosphorus  (page  452)  it  was  noticed  that  the  object  is  to  precipi- 
tate the  phosphorus  as  ferric  phosphate,  along  with  the  least- 
possible  excess  of  basic  ferric  acetate,  leaving  the  greater  portion 
of  the  iron  in  solution  in  the  ferrous  condition.  In  the  acetate 
method  for  manganese  separation  the  object  is  to  precipitate  all 
of  the  iron  as  basic  ferric  acetate,  leaving  the  manganese  in  solu- 
tion to  be  subsequently  determined  as  manganese  pyrophosphate 
or  manganese  tetroxide,  or  by  one  of  the  volumetric  processes. 
The  method  is  not  applicable  to  the  use  of  more  than  1  gm  of 
sample  because  of  the  difficulty  that  is  experienced  in  the  filtra- 
tion of  the  large  quantity  of  colloidal  basic  ferric  acetate  and  in 
washing  this  precipitate  free  from  manganese. 

Walters'  Method.-^-Marshall  showed1  that  ammonium  per- 
sulphate, in  presence  of  silver  nitrate,  oxidizes  manganese  from 

1  Chem.  News,  83,  76  (1901). 

30 


466  QUANTITATIVE  ANALYSIS 

the  bivalent  to  the  heptavalent  condition,  thus  producing  per- 
manganates from  manganese  salts.  His  interpretation  of  the 
reaction  is  as  follows: 

(NH4)2S208+2AgN03-»Ag2S2O8+2NH4NO3, 
Ag2S208+2H2O-*2H2S04+Ag2O2. 

According  to  this  the  silver  peroxide,  formed  momentarily  in 
small  amount,  is  responsible  for  the  oxidizing  action: 

5Ag202+2Mn(N03)2+6HN03_,2HMn04+10AgN03+2H20. 

Walters  applied  these  reactions  to  the  quantitative  determina- 
tion of  manganese  in  iron  and  steel.1  The  sample  is  dissolved  in 
nitric  acid,  silver  nitrate  and  ammonium  persulphate  are  added 
and  the  intensity  of  color  is  compared  with  that  produced  by  a 
standard  steel  in  which  the  manganese  has  been  determined  by 
another  method.  The  relative  volumes  required  to  produce  the 
same  intensity  of  color  in  the  two  provide  a  basis  for  the  calcula- 
tion of  the  percent  of  manganese  in  the  sample. 

Determination. — Weigh  0.2  gm  of  the  sample  and  the  same  amount 
of  a  standard  steel  of  known  manganese  content,  placing  in  different 
test  tubes  having  a  capacity  of  50  cc.  Add  to  each  10  cc  of  nitric  acid 
(specific  gravity  1.2)  and  immerse  the  tubes  in  hot  water  until  solution 
is  complete  and  brown  oxides  of  nitrogen  are  expelled.  Add  15  cc  of  a 
solution  containing  0.02  gm  of  silver  nitrate.  Immediately  add  1  gm 
of  ammonium  persulphate  and  continue  warming  until  the  pink  tint  of 
permanganic  acid  is  fully  developed,  which  will  require  about  1  minute. 
Remove  the  tubes  from  the  bath  while  oxygen  is  still  being  evolved  and 
place  in  cold  water.  When  the  solutions  are  cool  rinse  into  50-cc 
volumetric  flasks,  dilute  to  the  mark  and  mix.  If  free  carbon  is  present 
allow  it  to  settle,  then  pour  the  solution  whose  color  is  less  intense  into 
a  color  comparison  tube  (a  Nessler  or  similar  tube).  Fill  a  burette  with 
the  other  solution  and  measure  this  into  a  second  tube  until  the  color 
has  the  same  intensity,  viewed  from  above  when  placed  over  a  white 
surface  near  a  window.  From  the  relative  volumes  of  the  two  solutions 
calculate  the  percent  of  manganese  in  the  sample. 

Peter's  Method.2 — This  is  an  older  method  than  that  of  Wal- 
ters and  is  also  a  colorimetric  method,  the  manganese  being  oxi- 

1  Chem.  News,  84,  239  (1901). 

2  Ibid.,  33,  35  (1876). 


STEEL  AND  ALLOYS  467 

dized  to  permanganic  acid  by  lead  peroxide  in  presence  of  nitric 
acid.  It  is  necessary  to  remove  the  excess  of  lead  peroxide  before 
comparing  the  colors  and  this  constitutes  the  greatest  objection 
to  the  method.  The  reaction  is  expressed  as  follows : 

5Pb02+2Mn(N03)2+6HN03->5Pb(N03)2+2HMn04+2H20. 

Deshay's  Method. — Manganese  is  here  oxidized  by  lead  per- 
oxide, as  in  Peter's  Method,  followed  by  titration  of  the  perman- 
ganic acid  by  a  standard  solution  of  sodium  arsenite. 

5Na3As03+2KMnO4+6HNO3->5Na3As04+2Mn(NO3)2+ 

3H2O+2KNO3. 

Moore  and  Miller  suggested1  the  separation  of  iron  and  man- 
ganese by  precipitating  iron  as  ferric  hydroxide  by  the  addition  of 
pyridine.  The  separation  is  quantitative,  although  it  has  not 
yet  been  applied  to  iron  and  steel  analysis. 

Tungsten. — When  tungsten  is  alloyed  with  iron  in  steel  it  has 
the  effect  of  retarding  the  change  from  hard  to  annealed  steel. 
If  sufficient  tungsten  is  present  (3  or  4  percent  or  more)  the 
change  to  soft  steel  is  almost  entirely  prevented.  "Self -hard- 
ening" steels,  invented  by  Mushet  and  often  given  his  name,  con- 
tain 4  to  12  percent  of  tungsten,  2  to  4  percent  of  manganese 
and  1.50  to  2.50  percent  of  carbon.  They  are  called  self -harden- 
ing because,  when  subjected  to  the  ordinary  process  of  slow  cool- 
ing for  annealing,  they  retain  their  hardness,  even  though  they 
have  been  cooled  from  a  temperature  near  the  melting-point. 

Taylor  and  White,  in  1906,  developed  a  process  for  imparting  a 
remarkable  degree  of  toughness  to  self-hardening  steels,  so  that 
they  can  be  used  for  steel-cutting  tools  that  are  used  for  such 
rapid  cutting  that  they  become  red  hot  and  yet  do  not  lose  their 
hardness  or  toughness.  Such  steels  are  known  as  "  high-speed 
tool  steels."  The  carbon  percent  is  usually  less  than  0.75  and 
they  contain  from  5  to  25  percent  of  tungsten,  2  to  10  percent  of 
chromium  and  less  than  0.50  percent  of  manganese.  The  ther- 
mal treatment  of  "  high-speed "  steels  consists  of  heating  to  near 
the  melting-point  and  then  cooling  in  a  blast  of  air. 

The  determination  of  tungsten  in  steel  must  include  separation 
from  iron,  silicon,  carbon,  phosphorus  and  usually  chromium, 

*  J.  Am.  Chem.  Soc.,  30,  593  (1908). 


468  QUANTIT  ATIVE  ANALYSIS 

since  the  latter  metal  is  now  generally  associated  with  tungsten 
in  tool  steels.  A  gravimetric  method  is  usually  employed,  tung- 
sten being  weighed  as  tungstic  oxide,  WOs.  The  details  of  the 
following  method  are  mainly  those  given  by  Johnson.1 

Determination. — Dissolve  2  gm  of  sample  in  30  cc  of  sulphuric  acid 
(1 : 3)  in  a  250-cc  beaker  or  casserole,  heating  to  hasten  solution.  Add 
60  cc  of  nitric  acid  (specific  gravity  1.2)  and  digest  at  a  temperature 
near  the  boiling-point  until  the  residue  is  yellow  and  is  free  from  black 
particles.  Filter  and  wash  free  from  iron  by  means  of  dilute  sulphuric 
acid.  The  filter  paper  now  contains  the  main  portion  of  the  impure 
tungstic  acid.  Transfer  to  a  weighed  platinum  crucible  and  burn  the 
paper  at  a  low  temperature. 

Recover  the  tungsten  from  the  filtrate  by  precipitating  by  means 
of  cinchonine  solution  (25  gm  of  cinchonine  in  200  cc  o£  1  : 1  hydro- 
chloric acid).  Filter  and  wash  free  from  iron  by  means  of  the  cincho- 
nine solution.  Burn  the  paper  in  the  crucible  containing  the  main 
portion  of  tungstic  oxide  and  weigh,  then  add  potassium  acid  sulphate 
to  the  extent  of  about  twenty  times  the  weight  of  residue  in  the  crucible. 
Heat,  gradually  at  first,  finally  raising  the  temperature  until  the  cru- 
cible is  dull  red  and  keep  at  this  temperature  until  yellow  particles  of 
tungstic  oxide  are  dissolved.  Silica  will  remain  undissolved.  Cool 
and  place  the  crucible  in  a  250-cc  beaker  containing  100  cc  of  10-percent 
ammonium  carbonate  solution  and  warm  until  disintegration  of  the  mass 
is  complete. 

Filter  and  wash  the  residue  with  1-percent  ammonium  carbonate 
solution  until  the  washings  are  free  from  sulphates.  Ignite  the  paper 
in  the  same  crucible  as  that  used  first  and  weigh.  This  gives  the  weight 
of  ferric  oxide,  chromium  oxide  and  silica  and  this  subtracted  from  the 
weight  of  the  total  residue  gives  the  weight  of  tungstic  oxide,  WOs. 

.Calculate  the  percent  of  tungsten  in  the  sample. 

Chromium  and  Nickel. — Alloy  steels  containing  nickel  and 
chromium,  also  chromium  and  tungsten,  are  now  of  considerable 
commercial  importance.  Both  nickel  and  chromium  increase 
the  hardness,  tensile  strength  and  elastic  limit  of  steel  and  de- 
crease the  ductility  but  slightly,  if  at  all.  Nickel  also  lowers  the 
temperature  at  which  quenched  steel  is  softened  by  slow  cooling. 
In  commercial  nickel  steels  the  nickel  is  not  often  present  to  the 

1  Chemical  Analysis  of  Special  Steels,  Steel-Making  Alloys  and  Graphite, 
64. 


STEEL  AND  ALLOYS  469 

extent  of  more  than  3.50  percent,  the  carbon  content  being 
not  more  than  0.50  percent.  Chrome  steels  usually  contain  not 
more  than  3  percent  of  chromium  and  less  than  1  percent  of 
carbon.  Various  combinations  of  chromium,  nickel,  carbon,  and 
iron  produce  chrome-nickel  steels  of  great  strength  and  harden- 
ing power. 

Chromium  is  found  in  many  steels,  with  or  without  nickel, 
vanadium  or  tungsten,  and  particularly  in  high-speed  tool  steels, 
in  which  it  is  added  in  quantities  as  high  as  ten  percent.  Cain's 
method1  for  the  separation  of  chromium  and  for  the  determina- 
tion of  the  former  involves  precipitation  of  chromium  as  hydrox- 
ide, by  the  addition  of  barium  carbonate  to  a  slightly  acid  solu- 
tion in  which  all  of  the  iron  is  in  the  ferrous  condition.  Under 
these  circumstances  the  small  concentration  of  hydroxyl  ions, 
produced  by  hydrolysis  of  the  slightly  soluble  barium  carbonate, 
is  sufficient  to  exceed  the  solubility  product  with  the  chromium 
ion  but  not  with  the  ferrous  ion,  although  ferric  hydroxide  would 
precipitate  if  iron  were  allowed  to  oxidize. 

The  precipitated  chromium  hydroxide,  accompanied  by  the 
slight  excess  of  barium  carbonate,  is  oxidized  to  sodium  chromate 
by  fusion  with  sodium  carbonate  and  potassium  nitrate,  the  mass 
later  being  dissolved  in  hot  water  and  treated  with  hydrogen 
peroxide  to  complete  the  oxidation.  From  this  solution  the 
chromium  is  then  precipitated  as  lead  chromate.  This  is  re- 
moved and  dissolved  in  hydrochloric  acid  and  chromium  is 
titrated  by  standard  ferrous  ammonium  sulphate  solution. 

Determination. — In  a  covered  250-cc  Erlenmeyer  flask  dissolve  an 
amount  of  steel  that  will  give  0.06  to  0.07  gm  of  chromium,  using  about 
10  cc  of  concentrated  hydrochloric  acid  for  each  gram  of  steel.  As 
soon  as  solution  is  complete  dilute  to  100  cc  with  hot  water,  nearly 
neutralize  with  saturated  sodium  carbonate  solution  and  add  a  sus- 
pension of  barium  carbonate  in  slight  excess.  Boil  vigorously  for  15 
minutes,  adding  small  amounts  of  barium  carbonate  suspension  every 
two  or  three  minutes,  keeping  the  flask  covered  meanwhile.  About  1 
gm  excess  of  barium  carbonate  is -all  that  should  be  present  at  the  last. 
The  precipitate  will  be  green,  with  a  slight  admixture  of  white  barium 
carbonate,  but  little  or  no  brown  ferric  hydroxide  should  be  in  evidence. 

Remove  from  the  source  of  heat  and  as  soon  as  the  precipitate  has 

1J.  Ind   Eng.  Chem.,  4,  17  (1912). 


470  QUANTITATIVE  ANALYSIS 

settled,  filter  rapidly  and  wash  twice  with  hot  water.  Remove  as  much 
water  as  possible  by  suction  then  place  the  paper  and  precipitate  in  a 
platinum  crucible  and  carefully  burn  the  paper.  Add  2  gm  of  sodium 
carbonate  and  0.25  gm  of  potassium  nitrate  and  fuse  for  20  minutes. 
Support  the  inverted  and  inclined  crucible  on  a  glass  triangle  which 
has  glass  legs  about  2  cm  long  fused  to  the  corners,  place  in  a  250-cc 
beaker,  cover  with  boiling  water  and  digest  until  the  mass  has  com- 
pletely disintegrated. 

Filter  into  a  flask,  add  2  cc  of  hydrogen  peroxide  to  the  filtrate  and 
boil  for  10  minutes.  The  excess  of  hydrogen  peroxide  is  thus  expelled. 
Cool  and  transfer  to  a  250-cc  separatory  funnel.  Add  nitric  acid,  specific 
gravity  1.2,  slightly  more  than  enough  to  decompose  the  carbonate, 
allowing  the  carbon  dioxide  to  escape  by  inverting  the  separatory 
funnel  and  opening  the  stop  cock,  shaking  vigorously  at  the  last.  Trans- 
fer to  a  250-cc  beaker,  barely  neutralize  with  sodium  hydroxide  solu- 
tion, then  add  nitric  acid,  specific  gravity  1.2,  2  cc  for^each  100  cc 
of  solution.  Add  20  cc  of  20  percent  lead  acetate  solution  to  the  cold 
chromate  solution,  stirring  vigorously. 

Filter  off  the  lead  chromate  on  a  Gooch  crucible  and  wash  three  or 
four  times  with  cold  water.  Transfer  the  asbestos  and  all  of  the  pre- 
cipitate to  a  400-cc  Erlenmeyer  flask  and  add  enough  hot  dilute  hydro- 
chloric acid  to  decompose  all  of  the  precipitate.  Cool,  dilute  to  150  cc 
with  recently  boiled  and  cooled  water  and  add  from  a  pipette  exactly 
50  cc  of  approximately  tenth-normal  ferrous  ammonium  sulphate  solu- 
tion. Immediately  titrate  the  excess  of  ferrous  salt,  using  a  standard 
solution  of  potassium  dichromate  and  potassium  ferricyanide  as  an 
outside  indicator  (see  page  258)  or  standard  potassium  permanganate 
without  an  additional  indicator.  Either  a  tenth-normal  solution  or 
a  standard  solution,  1  cc  of  which  is  equivalent  to  0.002  gm  of  chromium 
is  suitable  for  this  purpose. 

At  the  same  time  make  a  blank  determination  by  titrating  25  cc 
of  the  ferrous  solution  against  the  standard  oxidizing  solution.  Sub- 
tract the  volume  of  the  latter  as  used  in  the  chromium  determination 
from  twice  the  volume  used  in  the  blank.  From  the  remainder  calculate 
the  percent  of  chromium  in  the  steel. 

Glyoxime  Method  for  Nickel.1 — This  method,  now  almost 
universally  considered  the  best  and  most  accurate  for  the  determi- 
nation of  nickel,  is  based  upon  the  extremely  slight  solubility  of 

iTschugaeff:  Ber.,  38,  2520  (1905);  Brunck:  Z.  angew.  Chem.,  20,  1844 
(1907);  Ibbotson:  Chem  News,  104,  224  (1911);  Bogoluboff:  Stahl  u. 
Eisen  (1910),  458. 


STEEL  AND  ALLOYS  471 

the  nickel  salt  of  dimethylglyoxime  in  ammoniacal  alcohol.     The 
reaction  is  represented  thus: 

H3C  v  /CH3  H3C  v  /CH3         H3CV  /CH3 

+  NiCl2-+ 
NOH  HON 


Iron,  copper,  cobalt,  chromium,  manganese,  tungsten,  vana- 
dium, etc.,  the  metals  commonly  found  in  steels,  do  not  interfere 
since  tartaric  acid  is  added  to  prevent  the  precipitation  of  their 
hydroxides. 

Determination.  —  Prepare  the  following  solutions: 

(a)  Dimethylglyoxime.  —  A  clear  solution  of  1  gm  in  100  cc  of  98 
percent  alcohol. 

(6)  Tartaric  Acid.  —  A  clear  solution  of  50  gm  in  100  cc  of  water. 

Use  1  gm  of  ordinary  nickel  steel.  If  less  than  3  percent  or  more  than 
5  percent  of  nickel  is  present  modify  the  sample  weight  accordingly. 
Dissolve  in  a  covered  casserole  in  10  cc  of  nitric  acid,  specific  gravity 
1.2.  Remove  the  cover  and  evaporate  to  dryness,  then  heat  carefully 
to  decompose  nitrates.  When  fumes  are  no  longer  evolved  cool  and 
add  20  cc  of  concentrated  hydrochloric  acid,  warming  until  solution 
is  complete.  Evaporate  to  dryness  but  do  not  heat  strongly.  Redis- 
solve  by  warming  with  5  cc  of  dilute  hydrochloric  acid,  dilute  and 
filter  to  remove  silica.  (This  precipitate  may  be  used,  if  desired,  for 
the  determination  of  silica.)  Wash  the  paper  with  hot  water  containing 
a  small  amount  of  hydrochloric  acid. 

To  the  filtrate  add  14  cc  of  tartaric  acid  solution  (b)  for  each  gram  of 
steel  used.  Make  the  solution  slightly  basic  by  the  addition  of  dilute 
ammonium  hydroxide.  About  20  to  30  cc  will  be  required.  Any 
precipitate  of  basic  ferric  tartrate  should  be  dissolved  by  adding  more 
ammonium  hydroxide.  If  the  solution  is  still  turbid  when  distinctly 
basic  more  tartaric  acid  should  be  added. 

Now  make  the  solution  slightly  acid  with  hydrochloric  acid,  heat 
nearly  to  boiling  and  add  20  cc  of  dimethylglyoxime  solution  (a)  and 
then  make  very  faintly  basic  with  dilute  ammonium  hydroxide.  The 
odor  of  ammonia  should  be  faint  after  blowing  away  the  vapors  above 
the  solution.  The  nickel  salt  precipitates  at  once  in  the  form  of  fine  red 
needles.  Allow  the  covered  solution  to  stand  for  an  hour  over  a  steam 
radiator  or  in  a  warm  place,  then  test  for  completion  of  precipitation 
by  adding  10  cc  more  of  glyoxime  solution. 

Filter  on  a  weighed  Gooch  or  alundum  crucible  which  has  been 
dried  at  120°,  wash  free  from  iron  with  hot  water  and  dry  to  constant 
weight  at  120°.  Calculate  the  percent  of  nickel  in  the  steel. 


472 


QUANTITATIVE  ANALYSIS 


Ether  Method. — The  separation  of  nickel  and  chromium 
from  iron  may  be  conveniently  made  by  the  ether  method  of 
Rothe.1  This  is  based  upon  the  fact  that  from  a  solution  in 
hydrochloric  acid  having  a  specific  gravity  between  1.100  and 
1.105  (containing  21  to  22  percent  of  acid)  ether  will  extract  all 
but  traces  of  ferric  chloride,  leaving  chlorides 
of  chromium,  nickel,  copper,  manganese, 
aluminium  and  cobalt  in  the  water  solution. 
The  apparatus  shown  in  Fig.  97  may  be  used 
for  the  separation.  The  manipulation  of  the 
apparatus  is  described  below. 

After  the  separation  of  iron  the  solution  in 
water  is  boiled  to  remove  dissolved  ether, 
after  which  several  methods  are  available 
for  the  determination  of  the  metals  in  this 
solution.  The  method  described  below  is 
probably  as  easy  of  execution  and  as  accurate 
as  any  of  these.  In  this  method  chromium  is 
precipitated  as  chromium  hydroxide  which  is 
ignited  and  weighed  as  chromium  sesquioxide, 
Cr2O3.  Nickel  is  deposited  electrolytically. 
If  copper  is  present  it  may  also  be  separated 
electrolytically  before  nickel  is  deposited. 

Determination. — The  quantity  of  steel  to  be 
taken  for  analysis  will  depend  upon  the  percents 
of  nickel  and  chromium  present.  If  1  to  3  per- 
cent of  either  metal  is  contained  in  the  steel  about 
2.5  gm  will  be  sufficient.  If  one  metal  is  present 
in  about  this  proportion  and  the  other  in  but 
traces  it  may  be  necessary  to  use  two  samples, 
making  the  determination  of  the  metal  whose 
percent  is  small  upon  a  larger  sample. 

Weigh  the  proper  quantity  of  sample  and  place  in  a  casserole.  Dis- 
solve in  30  cc  of  concentrated  hydrochloric  acid  and  10  cc  of  concen- 
trated nitric  acid,  adding  the  latter  acid  cautiously.  Evaporate  to  dry- 
ness  and  heat  for  a  short  time  then  redissolve  in  dilute  hydrochloric  acid 
and  filter  to  remove  silica.  Evaporate  until  the  liquid  thickens,  due  to 
the  separation  of  ferric  chloride  crystals.  This  gives  an  acid  of  approxi- 
mately correct  composition  for  the  ether  separation.  Transfer  the  solu- 

1Mitt.kgl.  tech.  Versuchs.  (1892),  132;  J.  Soc.  Chem.  Ind.,  11,  940  (1892). 


FIG.  97. — Appa- 
ratus for  separation 
of  iron  from  other 
metals  by  solution 
in  ether. 


STEEL  AND  ALLOYS  473 

tion  to  bulb  A  of  the  apparatus  shown  in  Fig.  97,  the  lower  cock  being 
turned  to  close  both  bulbs;  rinse  the  casserole  with  hydrochloric  acid 
having  a  specific  gravity  1.100,  until  the  bulb  is  nearly  half  full  of  solu- 
tion. Nearly  fill  the  bulb  with  ether,  close  the  upper  cock  and  mix 
gradually  by  shaking,  cooling  under  the  tap  to  avoid  rise  in  temperature, 
which  would  qause  reduction  of  ferric  chloride  by  ether.  Allow  the 
apparatus  to  stand  until  perfect  separation  of  ether  and  water  has 
occurred,  then  carefully  turn  the  lower  cock  to  establish  communica- 
tion between  the  bulbs,  allowing  the  lower  water  solution  to  run  into 
bulb  B.  Introduce  about  10  cc  more  of  hydrochloric  acid  into  A,  mix 
and  separate  as  before. 

Turn  the  lower  cock  to  allow  the  aqueous  solution  in  B  to  run  into  a 
250-cc  beaker.  Boil  the  solution  until  the  odor  of  ether  has  disappeared. 
Add  a  few  drops  of  bromine  to  oxidize  iron  and  manganese,  make 
slightly  basic  with  ammonium  hydroxide  and  boil.  The  precipitate 
consists  of  a  small  amount  of  ferric  hydroxide  together  with  chromium 
hydroxide,  aluminium  hydroxide  and  manganese  peroxide,  but  it  is  diffi- 
cult to  wash  it  free  from  the  soluble  nickel  and  copper  salts.  To  make 
the  separation  complete,  redissolve  the  precipitate  in  hydrochloric 
acid  and  reprecipitate  by  means  of  bromine  and  ammonium  hydroxide. 
Set  aside  the  solution  for  the  determination  of  nickel. 

Chromium. — Transfer  the  paper  and  precipitate  to  a  platinum  cru- 
cible and  burn  the  paper.  Add  5  gm  of  sodium  carbonate  and  0.5  gm  of 
potassium  nitrate  and  fuse  until  effervescence  occurs.  Place  the  cru- 
cible on  its  side  in  a  beaker  and  cover  with  water.  Heat  until  the  mass 
is  disintegrated,  filter  and  wash  with  1-percent  sodium  carbonate  solu- 
tion. The  residue  consists  of  ferric  oxide,  some  manganese  oxide  and 
aluminium  oxide  and  it  is  discarded.  The  solution  contains  sodium 
chromate  and  potassium  chromate  and  some  sodium  manganate. 
Evaporate  the  solution  nearly  to  dryness  after  adding  5  gm  of  potas- 
sium nitrate  and  enough  ammonium  hydroxide  to  give  a  distinct  odor. 
Dilute  to  100  cc  and  filter,  thus  removing  aluminium  hydroxide  and  man- 
ganese dioxide.  Boil  the  filtrate  to  remove  ammonia  then  add  a  slight 
excess  of  hydrochloric  acid  and  5  gm  of  sodium  acid  sulphite.  This 
reduces  sodium  chromate  to  chromium  chloride.  Boil  to  remove  all 
sulphur  dioxide,  add  a  slight  excess  of  ammonia,  boil  for  one  minute 
and  filter  the  precipitate  of  chromium  hydroxide.  Wash  with  water 
until  free  from  chlorides,  burn  the  paper,  ignite  the  precipitate  and  weigh 
as  chromium  sesquioxide,  Cr203. 

Calculate  the  percent  of  chromium  in  the  sample. 

Nickel. — The  solution  obtained  above  contains  nickel  and  copper  if 
these  were  present  in  the  steel  Add  10  cc  of  concentrated  sulphuric 
acid  and  evaporate  until  the  heavy  fumes  of  sulphuric  acid  appear, 


474  QUANTITATIVE  ANALYSIS 

thus  removing  chlorides.  Cool,  dilute  and  deposit  the  copper  and 
nickel  by  means  of  the  current,  using  the  method  described  in  connection 
with  the  analysis  of  a  nickel  coin,  page  163.  If  copper  is  known  to  be 
absent  the  solution  may  be  made  basic  with  ammonium  hydroxide  im- 
mediately after  evaporating  with  sulphuric  acid  and  diluting,  the  nickel 
then  being  deposited  at  once. 
Calculate  the  percent  of  copper  and  of  nickel  in  the  steel. 

Vanadium. — Small  amounts  of  this  element  are  added  to  many 
plain  carbon,  as  well  as  alloy  steels.  Vanadium  removes  dis- 
solved gases  from  the  steel  and  thus  promotes  the  formation 
of  sound  ingots  and  castings.  Most  of  the  vanadium  which  acts 
in  this  manner  is  removed  in  the  slag  but  small  quantities  (usu- 
ally not  more  than  1  percent)  remain  in  the  steel  and  serve  to 
raise  the  elastic  limit  and  ductility,  particularly  in  steels  contain- 
ing chromium. 

Vanadium  may  be  titrated  by  standard  potassium  permanga- 
nate solution,  with  resulting  oxidation  from  vanadyl  salts,  such 
as  vanadyl  sulphate,  VOS04,  derived  from  the  tetroxide,  to  van- 
adic  acid,  H3V04,  derived  from  the  pentoxide.  It  is  necessary 
first  to  separate  the  vanadium  from  iron,  chromium  and  manga- 
nese. The  separation  from  most  of  the  iron  can  be  accomplished 
by  a  method  analogous  to  that  used  for  separating  chromium 
from  iron,  already  described  for  the  chromium  determination, 
where  barium  carbonate  was  used  to  precipitate  chromium 
hydroxide  from  a  solution  containing  all  of  the  iron  in  the  fer- 
rous condition.  In  the  separation  of  vanadium  from  iron  cad- 
mium carbonate  is  used  instead  of  barium  carbonate,  on  account 
of  the  desirability  of  using  a  sulphuric  acid  solution  of  the  steel. 
The  precipitate  of  hydroxides  of  chromium  and  vanadium, 
together  with  the  excess  of  suspended  cadmium  carbonate,  is 
dissolved  in  sulphuric  acid  and  the  cadmium  is  precipitated  by 
hydrogen  sulphide.  From  the  solution  containing  vanadium, 
chromium  and  perhaps  a  little  iron,  the  latter  two  metals  are  pre- 
cipitated by  electrolysis,  using  a  mercury  cathode.  The  vanadium 
is  then  reduced  by  sulphur  dioxide  and  titrated  by  standard 
potassium  permanganate  solution.  The  equivalent  weight  of 
vanadium  is  calculated  from  the  equation 

5VOS04+KMn04+HH20-+5H3VO4+KHSO4+MnS04 

+  3H2S04. 


STEEL.  AND  ALLOYS 


475 


Cain's  method1  is  based  upon  the  principles  just  discussed. 
The  apparatus  used  by  Cain  for  the  electrolysis  of  the  solutions  is 
shown  in  Fig.  98.  The  bulb  is  filled  with  mercury  to  within 
2  mm  of  the  top  of  tube  A,  this  tube  being  completely  filled. 
Instead  of  this  apparatus  an  ordinary 
beaker  or  cylinder,  with  a  platinum 
wire  fused  into  the  bottom,  may  be 
used.  A  rotating  anode  is  employed. 

Determination. — Prepare  a  fiftieth- 
normal  solution  of  potassium  permanga- 
nate. Standardize  against  sodium  oxalate 
or  ferrous  ammonium  sulphate  and  calcu- 
late the  vanadium  equivalent.  Or  stan- 
dardize against  a  standard  vanadium 
steel  in  which  the  vanadium  content  is 
accurately  known,  using  the  method  for 
the  vanadium  determination  as  given 
below. 

Weigh  5  gm  of  steel  into  a  300-cc  Erlen- 
meyer  flask  and  dissolve  in  50  cc  of  10 
percent  sulphuric  acid,  keeping  the  flask 
covered.  If  insoluble  matter  remains 
filter  rapidly  into  a  flask  and  wash  with 
hot  water  two  or  three  times.  Stopper 
the  flask  to  prevent  oxidation  of  the 
ferrous  sulphate. 

Burn  the  paper  in  a  platinum  crucible, 
add  1  gm  of  potassium  pyrosulphate  or 
sodium  pyrosulphate  and  fuse  over  an 
ordinary  burner  for  5  minutes.  Cool, 
dissolve  the  fusion  in  hot  water  and  add  to  the  main  solution.  Nearly 
neutralize  with  a  saturated  solution  of  sodium  carbonate,  then  add 
finely  powdered  cadmium  carbonate  in  small  portions  at  intervals  of  4 
or  5  minutes,  boiling  between  times  and  keeping  the  flask  covered.  15 
to  20  minutes  boiling  is  usually  sufficient  and  a  gram  or  two  of  cadmium 
carbonate  should  finally  remain. 

The  precipitate  should  contain  very  little  ferric  hydroxide.  Filter 
rapidly  and  wash  the  flask  and  precipitate  twice  with  hot  water,  without 
attempting  to  remove  the  precipitate  adhering  to  the  flask.  Discard 
the  filtrate,  if  clear,  and  place  the  precipitating  flask  under  the  funnel. 

1  J.  Ind.  Eng  Chem.,  3,  476  (1911). 


FIG.  98. — Cain's  apparatus 
for  electrode  position. 


476  QUANTITATIVE  ANALYSIS 

Dissolve  the  precipitate  on  the  filter  in  the  minimum  quantity  of  nearly 
boiling  10  percent  sulphuric  acid  and  wash  the  paper  with  ho't  water. 
Boil  the  solution  in  the  flask  until  all  precipitate  is  dissolved  from  the 
sides  of  the  flask,  then  cool  and  nearly  neutralize  with  dilute  ammonium 
hydroxide.  The  faintest  excess  of  acid  should  remain  to  prevent 
precipitation  of  iron  hydroxide  by  boiling. 

Pass  a  rapid  current  of  hydrogen  sulphide  through  the  boiling  solution. 
When  the  cadmium  sulphide  is  all  precipitated,  filter  and  wash  two  or 
three  times  with  hot  water.  Concentrate,  if  necessary,  to  60  or  70  cc 
and  rinse  into  the  electrolyzing  vessel,  which  should  contain  about  200 
gm  of  mercury.  Electrolyze,  using  a  rotating  anode,  at  a  pressure  of 
6  to  7  volts.  After  15  or  20  minutes  test  a  few  drops  of  the  solution  on  a 
white  plate  with  potassium  ferricyanide  solution.  When  no  test  for 
iron  is  given  the  chromium  also  is  tolerably  certain  to  be  removed,  unless 
an  unusually  large  amount  is  present.  «, 

When  all  chromium  and  iron  have  .been  deposited,  stop  the  rotation 
of  the  anode  and  lower  it  to  within  a  very  short  distance  of  the  mercury. 
Remove  the  solution  through  the  stopcock  of  the  apparatus  shown  in 
Fig.  98  or  by  any  convenient  siphon  or  suction  arrangement  if  a 
beaker  or  cylinder  has  been  used.  When  the  surface  of  the  remaining 
solution  is  barely  above  the  anode  and  while  the  current  is  still  passing, 
rinse  down  the  sides  of  the  vessel  and  continue  to  add  water  fast  enough 
to  keep  the  anode  covered  while  the  solution  is  being  removed.  As  the 
electrolytes  become  more  diluted  the  resistance  of  the  solution  rises. 
When  the  ammeter  indicates  no  current  the  washing  may  be  considered 
as  complete. 

To  the  solution  containing  vanadyl  sulphate  add  5  cc  of  dilute  sul- 
phuric acid,  heat  to  75°  and  add  permanganate  solution  from  a  pipette 
until  a  slight  excess  is  indicated  by  the  color.  Pass  a  current  of  sulphur 
dioxide  into  the  boiling  solution  for  5  minutes,  then  pass  a  current  of 
carbon  dioxide  through  until  sulphur  dioxide  is  expelled.  Filter,  if 
necessary,  through  a  Gooch  filter  and  wash  with  hot  water.  Cool  to 
75°  and  titrate  against  a  fiftieth-normal  solution  of  potassium  permanga- 
nate. Calculate  the  percent  of  vanadium  in  the  steel. 

Oxygen. — Steel  in  the  liquid  state  dissolves  considerable 
quantities  of  oxygen  and  nitrogen  from  the  air,  as  well  as  hydro- 
gen from  dissociated  water  vapor.  Upon  cooling  these  gases  are 
released  from  the  solution  and  give  rise  to  flaws  known  as  "blow 
holes."  Oxygen,  however,  chemically  combines  with  the  iron  as 
it  cools,  forming  ferrous  and  ferric  oxides  and  it  is  in  this  way 
retained.  This  is  even  more  objectionable  than  nitrogen  or 


STEEL  AND  ALLOYS  477 

hydrogen  because  the  oxides  of  iron  render  the  steel  brittle  and 
they  also  form  gases  when  the  steel  is  heated,  by  combination 
with  carbon  of  the  steel. 

Oxygen  may  be  determined  by  Ledebur's  method.1  This 
consists  in  heating  the  finely  divided  sample  in  a  current  of  pure, 
dry  hydrogen,  absorbing  and  weighing  the  water  produced. 
Several  important  sources  of  error  must  be  avoided.  The  hydro- 
gen must  be  absolutely  free  from  water,  carbon  dioxide,  hydrogen 
sulphide,  and  oxygen.  Traces  of  the  latter  gas  are  removed  by 
passing  through  a  preliminary  heating  tube  containing  platinized 
asbestos,  then  through  a  water  absorbent.  Carbon  dioxide  and 
hydrogen  sulphide  are  removed  by  passing  through  potassium 
hydroxide  solution.  Moisture  is  also  absorbed  before  the  gas 
enters  the  tube  containing  the  sample. 

The  sample  must  be  quite  free  from  oil  and  should  be  taken 
by  very  slowly  drilling  or  milling.  If  the  metal  is  heated  by 
cutting  it  will  be  superficially  oxidized. 

The  sample  is  placed  in  a  combustion  tube  for  heating  with 
hydrogen.  A  second  furnace  may  be  used  for  the  preliminary 
heating  of  the  hydrogen  or  the  tube  containing  platinized  asbes- 
tos may  be  placed  in  the  furnace  containing  the  main  tube.  For 
the  sample  a  silica  tube  3/4  inch  by  30  inches  is  suitable.  For  the 
preliminary  heating  a  silica  tube  1/4  inch  by  12  inches  will  serve 
unless  it  is  to  be  placed  in  the  furnace  with  the  larger  tube,  in 
which  case  its  length  must  be  the  same  as  that  of  the  larger 
tube. 

The  apparatus  is  set  up  in  the  following  order:  (1)  Hydrogen 
generator  of  the  Kipp  or  similar  type,  charged  with  zinc  and 
hydrochloric  acid  (1:1).  (2)  Absorption  bottle  half  filled  with 
33-percent  potassium  hydroxide  solution.  (3)  Absorption  bottle 
half  filled  with  concentrated  sulphuric  acid.  (4)  Silica  tube, 
1/4  inch  in  diameter,  containing  platinized  asbestos  for  6  inches 
of  its  length.  (5)  U-tube  filled  with  dry  calcium  chloride.  (6) 
Silica  tube,  3/4  inch  in  diameter,  to  hold  the  boat  containing 
the  sample.  (7)  Two  U-tubes  filled  with  dry  calcium  ciiloride, 
the  first  (a)  for  absorption  of  the  water  produced  by  the  combina- 
tion of  the  oxygen  of  the  sample  with  hydrogen,  the  second  (b)  to 

^eitfaden  fur  Eisenhuttenlaboratorien,  6th  ed.,  122;  Stahl  u.  Eisen,  2, 
193  (1882). 


478  QUANTITATIVE  ANALYSIS 

act  as  a  guard  tube.  (8)  An  aspirator  similar  to  that  used  for 
carbon  dioxide  determinations.  Prepare  another  U-tube  (9) 
containing  calcium  chloride,  close  the  side. tubes  and  do  not  con- 
nect in  the  apparatus.  For  the  directions  for  filling  absorption 
tubes  and  setting  up  the  apparatus,  see  the  determination  of 
carbon  dioxide  in  carbonates,  page  129. 

Determination. — Set  up  the  apparatus  as  already  indicated  but  do 
not  connect  the  aspirator.  Pass  a  rapid  stream  of  hydrogen  through 
the  apparatus  for  30  minutes  to  displace  all  of  the  oxygen,  then  weigh 
and  insert  the  absorption  tube  (7 a)  and  heat  the  tubes  to  bright  redness. 
Continue  the  passage  of  gas  for  30  minutes  then  remove  the  tube 
(7a),  wipe  clean,  close  the  side  tubes  with  rubber  tubes  and  plugs,  leave 
in  the  balance  case  for  10  minutes,  remove  the  plugs  and  weigh.  The 
increase  in  the  weight  of  the  tube  in  the  blank  determination  should 
not  be  more  than  2  or  3  mg.  Run  a  second  blank  determination  or 
more  if  the  gain  is  not  constant  within  0.5  mg. 

Allow  the  combustion  tube  to  cool  and,  in  the  meanwhile,  weigh  20 
to  30  gm  of  finely  divided  steel  sample  and  place  in  a  boat  which  is  6 
inches  long  and  1/2  inch  wide. 

With  the  stream  of  hydrogen  still  passing  quickly  open  the  end 
which  is  nearest  the  absorption  tube  7a,  insert  the  boat,  push  it  to  the 
middle  of  the  tube,  and  quickly  insert  the  stopper.  By  opening  this 
end  of  the  tube  the  current  of  hydrogen  prevents  the  entrance  of  oxygen, 
which  would  entirely  vitiate  the  results  of  the  determination.  Insert 
the  weighed  absorption  tube,  heat  the  silica  tubes  to  redness  and  main- 
tain this  temperature  for  30  minutes  with  the  hydrogen  passing  through 
at  the  rate  of  about  six  bubbles  per  second  in  the  tube  containing  sul- 
phuric acid.  At  the  end  of  this  period  disconnect  the  absorption  tubes 
(Id)  and  (76)  from  the  silica  tube,  connect  tube  (9)  with  the  unguarded 
end  of  (7a)  and  connect  (76)  with  the  aspirator.  Draw  air  through  the 
tubes  rapidly  until  about  500  cc  has  passed,  as  measured  by  the  water 
that  has  run  out  of  the  aspirator.  Close  the  side  tubes  of  (7a)  with 
rubber  tubes  and  plugs  and  stand  in  the  balance  case.  At  the  end  of 
10  minutes  remove  the  rubber  tubes  and  plugs  and  weigh  the  absorp- 
tion tube.  From  the  weight  of  water  so  found  calculate  the  percent 
of  oxygen  in  the  sample. 

Treatment  of  Steel. — The  property  which  makes  iron  the  most 
generally  useful  of  all  metals  is  the  property  of  combining  with 
various  quantities  of  other  elements  in  such  a  way  that  its  physical 
and  mechanical  properties  are  varied  over  a  wide  range.  Scarcely 


STEEL  AND  ALLOYS  479 

second  to  this  property  is  that  of.  undergoing  important  changes 
in  character  as  the  compounds  with  carbon  are  subjected  to  dif- 
ferences in  thermal  treatment.  The  latter  property  is  possessed 
by  all  irons  containing  carbon.  These  include  the  crude  products 
of  the  blast  furnace,  "  pig  iron,"  and  the  various  grades  of  the 
more  refined  product,  known  as  "steel." 

The  fact  that  a  very  small  percent  of  carbon  gives  iron  the 
capability  of  being  hardened  by  suddenly  cooling  from  high  tem- 
peratures has  been  known  for  a  long  time.  The  beginning  of  an 
understanding  of  why  this  is  true  came  when  it  was  found 
that  certain  structural  changes  in  steel  take  place  with  thermal 
treatment.  The  entrance  of  the  microscope  into  the  field  of 
metal  testing  marked  the  beginning  of  a  new  age  for  steel,  an  age 
of  the  development  of  the  scientific  principles  underlying  thermal 
and  mechanical  treatment.  The  chemist's  analysis  is  no  longer 
expected  to  tell  the  entire  history  of  the  steel.  As  a  result  of 
the  analysis  we  know  the  composition  but  this  tells  us  only 
what  the  steel  maybe  made  to  do.  The  microscope,  following  this, 
tells  what  the  steel  has  been  made  to  do.  This  is  a  development  of 
vast  importance  to  the  user  of  steel.  When  the  manufacturer  of 
steel  articles  buys  his  bars,  sheet,  rods,  billets  or  forgings  from 
the  manufacturer  of  the  steel  itself,  he  is  chiefly  concerned  with 
the  composition,  with  respect  to  the  various  elements  that  are 
combined  with  iron  to  make  the  commercial  steel,  because  he 
knows  the  composition  that  is  necessary  for  his  particular  purpose. 
When  he  has  made  the  steel  into  the  forms  necessary  for  his 
manufactured  article  of  commerce  he  is  still  more  seriously  con- 
cerned with  the  properties  that  have  been  given,  or  are  to  be 
given,  by  the  careful  thermal  treatment  that  is  necessary.  The 
analysis  can  be  of  little  or  no  assistance  at  this  point.  The  per- 
centage composition  is  not  materially  changed  by  thermal  treat- 
ment, except  in  those  forms  of  combined  chemical  and  thermal 
treatment,  known  as  "case-hardening"  or  "cementing."  The 
microscopic  anatomy  of  the  steel  is,  on  the  contrary,  very  pro- 
foundly changed  and  these  recognizable  changes  are  so  intimately 
associated  with  changes  in  physical  and  mechanical  properties 
that  the  microscope  is,  at  this  point,  the  most  important  in- 
strument available  for  testing.  This  branch  of  testing  is  known 
as  "metallography." 


480 


QUANTITATIVE  ANALYSIS 


A  thorough  discussion  of  the  principles  of  thermal  treatment 
and  of  the  metallography  of  steel  would  require  a  volume  in  itself. 
In  the  next  following  pages  a  brief  outline  of  these  principles 
will  be  given,  with  directions  for  a  limited  number  of  experiments, 
which  will  serve  to  illustrate  the  main  points. 

Thermal  Changes. — If  steel  containing  about  0.05  percent  of 
carbon  is  allowed  to  cool  from  a  high  temperature  and  the  rate  of 
cooling  is  followed  by  means  of  a  sensitive  pyrometer,  it  will  be 
noticed  that  the  previously  uniform  rate  of  cooling  is  interrupted 
at  about  875°,  the  temperature  remaining  stationary  for  a  short 


0.2       0.4       0.6       0.8      1.0       1.2       1.4      1.6       1.8 

Percent  of  Carbon 

FIG.  99. — Recalescence  curve  for  steel. 


time  or  the  cooling  being  at  least  retarded.  At  about  750° 
another  interruption  is  noticed  and  at  690°  still  a  third.  The 
exact  location  of  these  points  depends  upon  the  composition  of 
the  steel  and  also  upon  the  rate  of  cooling,  more  rapid  cooling 
lowering  the  point  of  change  These  interruptions  in  the  cooling 
process  are  due  to  certain  internal  changes  which  involve  the 
evolution  of  heat  and  the  temperature  at  which  they  occur  are 
therefore  called  points  of  "recalescence"  or  "critical  points." 
If  a  series  of  steels  having  gradually  increasing  percents  of 
carbon  is  treated  as  just  described  it  will  be  noticed  that  the  upper 


STEEL  AND  ALLOYS  481 

critical  point  is  lowered  as  the  carbon  percent  increases,  while 
the  lower  two  critical  points  remain  practically  unaltered  until 
a  steel  containing  about  0.42  percent  of  carbon  is  reached,  when 
the  upper  two  points  merge.  As  the  carbon  percent  is  further 
increased  this  new  point  is  still  further  lowered  until  the  steel 
having  0.85  percent  carbon  is  reached  when  all  three  changes  are 
merged.  In  order  to  distinguish  the  various  points  of  recalescence 
the  lowest  is  denoted  by  Ari}  the  second  by  Ar2  and  the  highest 
by  Ar3. 

The  relations  between  carbon  percent  and  points  of  recales- 
cence are  shown  in  Fig.  99,  which  is  somewhat  idealized.  In 
steels  containing  more  carbon  than  0.85  percent,  Ari  remains  at 
the  same  temperature  while  a  new  higher  point  is  again  noticed, 
increasing  carbon  raising  the  location  of  this  point.  Sauveur 
denotes  this  by  the  symbol  Arcm. 

Allotropism  of  Iron. — The  occurrence  of  these  points  of  recal- 
escence shows  that  internal  changes  are  taking  place  while  the 
steel  is  cooling.  These  changes  might,  conceivably,  be  either 
physical  or  chemical,  or  both.  The  fact  that  Ar3  and  Ar2  are 
noticed  in  the  purest  iron  that  can  be  manufactured  indicates 
that  at  least  a  part  of  the  heat  evolution  is  due  to  allotropic 
changes.  That  such  changes  are  not  the  only  ones  occurring 
at  the  critical  points  of  steel  is  shown  by  the  fact  that  the  reca- 
lescence in  pure  iron  is  very  faint  while  in  medium  and  high  carbon 
steel  it  is  very  pronounced,  actual  glowing  being  noticed  at  Ari. 
Iron  is  now  believed  to  exist  in  at  least  three  allotropic  modifica- 
tions as  follows: 

7-iron  exists  from  the  melting-point  down  to  Ars.  It  crystal- 
lizes in  the  cubic  system,  the  prevailing  forms  of  crystals  being 
octahedra.  Its  electrical  resistance  is  about  ten  times  that  of 
a-iron  and  it  is  non-magnetic.  Its  hardness  is  somewhat  less 
than  that  of  0-iron  but  greater  than  that  of  a-iron.  It  dissolves 
iron  carbide  to  the  extent  of  25.5  percent,  corresponding  to  1.7 
percent  of  carbon. 

/3-iron  is  normally  present  between  Ars  and  Arz  until  these 
points  merge,  when  it  disappears.  It  crystallizes  in  the  cubic 
system  with  cubes  as  the  prevailing  forms.  It  is  feebly  magnetic 
and  is  very  hard.  It  has  little  or  no  solvent  power  for  carbon. 

a-iron  exists  below  Ar2.     It  also  crystallizes  in  the  cubic  system. 

31 


482  QUANTITATIVE  ANALYSIS 

Its  electrical  resistance  is  less  than  that  of  either  /3-iron  or  7-iron 
and  it  is  strongly  magnetic.  It  is  the  softest  form  of  iron. 

Influence  of  Sudden  Cooling. — The  influence  of  carbon  upon 
the  magnitude  and  location  of  the  thermal  changes  of  iron 
would  indicate  that  it  also  plays  a  part  in  the  changes  occurring 
at  the  points  of  recalescence.  This  is  confirmed  by  many  careful 
chemical  and  microscopical  examinations  of  polished  and  etched 
steel  samples.  The  question  very  naturally  arises  as  to  how  a 
microscopical  or  chemical  examination  can  be  made  of  any  but 
a-iron,  since  the  other  forms  do  not  normally  exist  at  ordinary 
temperatures.  Such  examination  is  made  possible  by  the  fact 
that  sufficiently  sudden  cooling  partially  or  entirely  arrests  the 
change  from  7-iron  to  /3-iron  and  from  /3-iron  to  a-iron,  with  the 
accompanying  changes  in  the  form  of  carbon.  This,  as  will 
later  be  shown,  is  the  property  upon  which  all  thermal' treatment 
of  steel  is  based.  If,  then,  a  sample  of  iron  is  heated  to  a  tempera- 
ture above  Ars  and  suddenly  cooled  by  quenching  in  cold  water, 
mercury,  liquid  air,  etc.,  y-iron  is  retained  as  an  abnormal 
substance  because  at  ordinary  temperatures  the  molecular 
mobility  of  iron  is  too  small  to  permit  any  changes  in  structure 
which  would  have  taken  place  at  higher  temperature  if  sufficient 
time  had  been  allowed.  Similarly  /5-iron  may  be  retained  by 
quenching  from  a  temperature  between  Ars  and  Ar2.  It  has  also 
been  shown  that  carbon  is  retained  in  the  form  which  is  nor- 
mal to  the  quenching  temperature.  This  method  of  preventing 
changes  that  normally  occur,  retaining  abnormal  structures 
and  physical  states,  opens  the  way  for  an  investigation  of  the 
condition  of  steel  at  high  temperatures. 

Proximate  Constituents  of  Slowly  Cooled  Steel. — Pure  iron  is 
never  obtained  in  industrial  practice  and  is  seldom  desired,  since 
steel  is  the  form  which  so  readily  lends  itself  to  modification  in 
its  properties  to  suit  the  most  varied  requirements. 

Ferrite. — Certain  commercial  articles  that  approach  pure  iron 
in  character  are  known  as  "ingot  iron,"  a  product  of  the  open- 
hearth  process  as  used  for  steel  making,  but  without  recarburizing 
the  iron.  If  a  small  section  of  such  iron  is  given  a  high  polish 
and  is  then  subjected  to  the  action  of  an  etching  agent,  such  as 
nitric  acid  or  tincture  of  iodine,  and  the  sample  is  then  placed 
under  a  microscope  it  will  be  found  that  the  substance  that  to  the 


STEEL  AND  ALLOYS  483 

naked  eye  appeared  quite  bright  and  homogeneous  is  really  made 
up  of  numbers  of  granules  somewhat  resembling  crystals  in  form. 
The  structure  is  really  crystalline  but  the  crystals  are  so  hampered 
in  the  process  of  formation  that  the  word  "granule"  better 
expresses  their  appearance.  The  action  of  the  etching  solution 
has  been  to  attack  the  grains  more  vigorously  at  the  lines  of 
juncture  and  therefore  to  bring  their  outlines  into  relief.  A  pho- 
tomicrograph of  such  a  section  is  shown  in  Fig.  100.  The  carbon- 
less iron  thus  appearing  as  granules  is  found  to  be  a  distinct  con- 
stituent of  all  steel  containing  less  than  0.85  percent  of  carbon. 
This  constituent  is  called  "ferrite"  and  it  is  what  has  already 
been  described  as  a-iron. 

Some  of  the  properties  of  ferrite  have  been  described  in  the 
discussion  of  a-iron.  In  addition  it  may  be  said  that  it  is  quite 
ductile,  giving  an  elongation  of  about  40  percent  in  2  inches, 
that  it  has  the  power  of  combining  with  carbon  to  form  other 
constituents  of  steel  and  that  its  tensile  strength  is  about  50,000 
pounds  per  square  inch.  It  cannot  be  hardened  by  sudden  cool- 
ing except  to  the  extent  that  is  indicated  by  the  partial  retention 
of  7-iron. 

Cementite. — If  a  piece  of  steel  containing  more  than  0.85  per- 
cent of  carbon  is  similarly  polished,  etched  and  examined  under 
somewhat  high  magnification  it  will  exhibit  certain  shaded  areas 
and  also  bright  lines,  their  extent  increasing  with  increasing 
carbon  percent.  Such  a  section  is  shown  in  Fig.  101.  The  bright 
constituent  is  iron  carbide,  FesC,  already  mentioned  in  con- 
nection with  the  analysis  of  steel.  As  a  distinct  crystalline 
constituent  of  steel,  iron  carbide  is  known  as  "cementite." 

The  properties  of  cementite  are  quite  different  from  those  of 
ferrite.  Cementite  cannot  be  obtained  in  the  pure  condition 
because,  as  its  formula  indicates,  it  contains  6.67  percent  of 
carbon,  and  steel  of  this  composition  cannot  be  prepared  without 
the  presence  of  manganese  or  other  elements  or  without  the 
separation  of  graphitic  carbon.  According  to  Sauveur1  the  ten- 
sile strength  of  cementite  is  about  5000  pounds  per  square 
inch  and  its  ductility  (represented  by  elongation)  is  practically 

1  The  Metallography  and  Heat  Treatment  of  Iron  and  Steel,  2d  edition 
138. 


484  QUANTITATIVE  ANALYSIS 

zero.  It  is  the  hardest  constituent  of  steel  and  cannot  be  made 
appreciably  harder  by  sudden  cooling. 

Pearlite. — In  the  description  of  cementite  it  was  stated  that 
steel  containing  more  than  0.85  percent  of  carbon  shows  certain 
shaded  areas  in  addition  to  the  bright  cementite,  when  it  is 
polished,  etched  and  highly  magnified.  (Fig.  101.)  These 
shaded  areas  increase  in  extent  as  the  carbon  approaches  0.85 
percent  from  either  above  or  below  this  figure.  If  the  carbon 
content  is  practically  0.85  percent  the  relatively  bright  areas  of 
cementite  and  of  ferrite  are  both  absent.  If  the  carbon  content 
is  less  than  0.85  percent  ferrite  appears  in  addition  to  the  shaded 
portion.  Both  ferrite  and  cementite  are  distinct,  homogeneous 
entities,  the  one  an  element,  the  other  a  definite  chemical  com- 
pound. The  darker  substance  is  neither  element  nor  compound 
but  is  a  composite  of  both,  so  constituted  that  it  reflects  light  in 
a  way  suggesting  mother-of-pearl.  On  this  account  it  is  known 
as  "pearlite."  The  peculiar  iridescence  of  pearlite  is  evident 
without  magnification  of  the  polished  specimen. 

If  an  unhardened  steel  is  prepared  as  already  described  and 
examined  under  high  magnification  the  structure  of  pearlite  is 
shown  quite  clearly  to  be  that  of  alternating  plates  or  laminae 
of  a  dark  and  a  light  material.  This  is  shown  in  Figs.  102  and 
103.  These  plates  have  been  shown  to  be  chemically  free  iron 
and  iron  carbide.  That  is,  they  are  the  materials  that  have  been 
described  as  ferrite  and  cementite,  although  the  latter  names  are 
generally  reserved  for  distinct  granules  of  these  materials,  not 
constituents  of  pearlite.  The  laminations  of  pearlite  show  dis- 
tinctly because  the  etching  agent  attacks  free  iron  more  readily 
than  iron  carbide,  thus  bringing  the  former  into  relative  shadow. 
The  laminae  are  usually  bent  or  contorted. 

One  of  the  most  characteristic  of  the  properties  of  pearlite 
is  its  constancy  in  composition,  the  percent  of  free  iron  being 
87.26  and  that  of  iron  carbide  12.74.  This  is  calculated  from 
the  percent  of  carbon  in  cementite  (6.67)  and  in  pearlite 
(0.85).  The  percent  of  iron  carbide  in  pearlite  is  therefore 

0  85X100 
— - cTgy —  =  12.74.     Pearlite   has   a   hardness   between   that   of 

ferrite  and  of  cementite;  its  tensile  strength  is  about  125,000 
pounds  per  square  inch  and  its  ductility  is  represented  by  an 


STEEL  AND  ALLOYS  485 


elongation  of  about  10  percent  in  2  inches.  It  is  the  only  one 
of  the  three  constituents  of  unhardened  steel  that  possesses  the 
power  of  hardening  when  suddenly  cooled  and  it  is  due  to  pearlite 
that  steel  exhibits  this  remarkable  and  useful  property. 

Relation  between  Structure  and  Carbon  Percent. — It  would 
follow  from  what  has  been  said  concerning  the  chemical  and 
physical  composition  of  ferrite,  cementite  and  pearlite  that  the 
relative  areas  of  these  substances  appearing  in  the  polished  and 
etched  section  of  slowly  cooled  steel  would  give  at  least  a  fair 
indication  of  the  percent  of  carbon.  This  is  seen  to  be  the  case 
by  an  inspection  of  Figs.  104,  105  and  106.  An  estimate  of  the 
carbon  percent  may  be  made  from  the  microscopic  appearance, 
with  an  accuracy  of  0.10  percent  for  steels  containing  less  than 
about  0.6  percent  of  carbon.  With  more  carbon  than  this  it 
becomes  more  difficult  to  judge  the  percent.  For  this  purpose 
a  magnification  of  50  to  150  diameters  is  suitable.  If  the  steel 
has  not  been  slowly  cooled  the  microscope  can  give  no  indication 
of  carbon  percent. 

Austenite. — If  a  small  piece  of  steel  is  heated  to  a  temperature 
considerably  above  Ar3  and  cooled  very  suddenly  by  quenching, 
the  microscope  does  not  reveal  the  presence  of  either  ferrite,  pearl- 
ite or  cementite.  Instead  the  mass  has  assumed  a  fairly  definite 
crystalline  appearance,  in  which  no  constituent  can  be  distin- 
guished as  different  from  another.  The  steel  has  also  been  made 
much  harder  than  formerly,  the  degree  of  hardness  depending 
upon  the  percent  of  carbon.  The  single  crystalline  substance 
composing  the  mass  of  hardened  steel  is  called  "austenite,"  after 
Roberts-Austen  the  English  metallurgist.  The  bright  portions 
of  Fig.  107  are  austenite. 

Austenite  contains  carbon  in  any  proportion  up  to  1.7  percent. 
It  is  non-magnetic  and  thus  contains  iron  in  the  7  modification. 
It  is  not  the  hardest  constituent  of  hardened  steel  but  is  harder 
than  either  ferrite  or  pearlite. 

Relation  of  Ferrite,  Cementite,  Pearlite  and  Austenite  to  the 
Critical  Points  of  Steel. — It  has  been  shown  that  slowly  cooled 
steel  contains  either  ferrite  and  pearlite,  pearlite  alone,  or  cement 
tite  and  pearlite,  according  to  whether  the  steel  contains  less 
than  0.85  percent,  exactly  0.85  percent  or  more  than  this  amount 
of  carbon,  also  that  steel  cooled  with  sufficient  rapidity  contains 


486  QUANTITATIVE  ANALYSIS 

only  austenite,  no  matter  what  the  percent  of  carbon  may  be. 
The  change  of  austenite  into  the  other  constituents  when  the 
steel  is  slowly  cooled  begins  normally  at  the  temperature  denoted 
by  the  line  ABCD  of  Fig.  99,  that  is  at  the  point  Ar3,  for  the  steel 
of  any  particular  composition.  If  less  than  0.85  percent  of  carbon 
is  contained  in  the  steel  the  formation  of  ferrite  begins  at  Ars  and 
continues  until  Ari  is  reached,  when  the  austenite  then  remaining 
changes  completely  into  pearlite.  Since  pearlite  has  a  definite 
and  constant  composition  it  will  be  seen  that  austenite  must  have 
lost  enough  iron  between  Ar3  and  Ari  to  leave  austenite  contain- 
ing 0.85  percent  of  carbon.  If  more  than  this  percent  of  carbon 
is  contained  in  the  steel  austenite  loses  iron  carbide  at  the  point 
Arcm  and  continues  to  lose  it  until  Ari  is  reached,  when  again 
pearlite  is  formed  of  what  remains. 

Steel  as  a  Solid  Solution. — This  behavior  of  slowly  cooling 
steel  suggests  the  behavior  of  ordinary  solutions  where  separation 
of  the  constituents  occurs  at  tolerably  definite  temperatures, 
depending  upon  the  percentage  composition  of  the  solution. 
For  a  solution  of  any  two  substances  in  each  other  there  exists 
a  percentage  composition  for  which  the  lowest  possible  freezing 
point  is  noticed.  If  more  of  either  constituent  is  present  there 
is  a  higher  freezing  point  and  at  this  point  the  excess  of  this 
constituent  freezes  and  separates,  leaving  a  solution  which  also 
freezes  at  or  near  the  lower  point  already  noticed.  At  the 
temperatures  of  separation  there  also  occur  thermal  changes 
which  are  usually  in  the  nature  of  evolution  of  heat.  The  solu- 
tion having  the  lowest  freezing  point  of  all  solutions  of  a  given 
pair  of  components  is  known  as  the  "eutectic"  solution. 

In  these  and  a  great  many  other  respects,  steel  resembles  liquid 
solutions.  In  fact  the  resemblance  is  so  close  that  it  can  no  longer 
be  doubted  that  austenite  is  a  solution  of  y-iron  and  iron  car- 
bide in  each  other.  To  be  sure,  austenite  is  solid  while  most 
common  solutions  are  liquid,  but  from  the  physical  standpoint 
the  mere  state  of  aggregation  is  a  point  of  secondary  importance 
in  the  consideration  of  solutions.  Austenite  possesses  the  prop- 
erty of  physical  homogeneity,  common  to  liquid  and  all  other  solu- 
tions. It  changes  completely  into  the  resolution  product,  pearlite, 
at  Ari  if  of  eutectic  composition.  If  not  of  this  composition  it 
begins  to  lose  excess  of  either  iron  or  iron  carbide,  as  ferrite  or 


STEEL  AND  ALLOYS  487 

cementite,  at  Ar3,  Ar^  or  Arcm,  just  as  the  liquid  solution  would 
lose  the  excess  of  solvent  or  solute  if  not  of  eutectic  composition. 
Coincident  with  the  separation  of  ferrite  or  of  cementite  allo- 
tropic  changes  also  occur  in  the  separated  iron. 

Martensite. — It  is  difficult  entirely  to  prevent  the  change  of 
austenite  into  the  constituents  normal  to  lower  temperatures  by 
the  process  of  sudden  cooling.'  If  the  quenching  medium  is  one 
that  will  cause  cooling  of  extreme  rapidity  the  change  may  be 
almost  entirely  arrested.  Such  media  are  ice-water,  ice  and 
salt  solution  or  liquid  air.  These  media  are  not  generally  used 
in  commercial  processes  of  hardening,  partly  because  of  intense 
mechanical  stresses  that  are  set  up  in  the  steel  by  such  sudden 
cooling.  These  stresses  will  frequently  result  in  cracking  the 
piece.  Therefore  austenite  does  not  make  up  the  mass  of  com- 
mercially treated  steels.  Instead  a  new  formation,  called  "mar- 
tensite"  (after  Martens,  German  metallurgist),  makes  its  appear- 
ance. The  true  nature  of  martensite  is  not,  even  now,  thoroughly 
understood.  It  was  formerly  thought  to  be  the  stable  solid 
solution  itself,  but  this  view  is  no  longer  held.  While  opinions 
still  differ,  martensite  is  probably  a  transition  product  between 
austenite,  on  the  one  hand,  and  pearlite  on  the  other  and  it  is  a 
solid  solution  of  iron  carbide  in  iron;  the  iron  is  not  here  in  the 
7  condition,  as  in  austenite,  but  in  either  the  /?,  the  a  or  both  ft  and 
a  conditions,  so  that  martensite  is  magnetic  and  is  even  harder  than 
austenite.  Like  austenite,  its  hardness  varies  with  the  percent  of 
carbon  in  the  steel,  i.e.,  with  the  percent  of  dissolved  iron  carbide 
in  the  martensite.  The  crystalline  appearance  of  martensite  is 
rather  characteristic.  The  dark  needles  of  Figs.  107  and  108  are 
martensite.  It  consists  of  intersecting  needles  which  usually  ar- 
range themselves  at  60°  angles,  etching  darkly  with  nitric  acid. 

Being  a  transition  substance  and  not  in  real  equilibrium  at  any 
temperature,  but  merely  a  metastable  condition  retained  by 
rapid  cooling,  martensite  is  frequently  associated  with  austenite, 
or  with  troostite  or  sorbite,  two  other  transition  constituents 
presently  to  be  described. 

Relation  between  Location  of  the  Thermal  Critical  Points 
and  the  Rate  of  Cooling  or  Heating. — Reference  has  already  been 
made  to  the  influence  of  rapid  cooling  upon  the  resolution  of 
austenite  into  its  components,  the  change  being  entirely  pre- 


488  QUANTITATIVE  ANALYSIS 

vented  by  sufficiently  rapid  cooling.  Even  when  the  rate  of 
cooling  is  comparatively  slow  a  certain  lag  is  noticed  in  the  trans- 
formation, so  that  the  critical  point  is  lowered  as  the  rate  of  cool- 
ing is  increased.  The  exact  location  of  Ari,  Ar2  and  Ar3,  there- 
fore, depends  upon  this  rate  of  cooling,  as  well  as  upon  the 
percent  of  carbon.  Similarly  the  changes  in  structure  observed 
upon  heating  steel  are  affected  by  the  rate  of  heating,  the  critical 
points  being  raised.  This  " thermal  hysteresis"  is  sometimes 
large,  as  in  the  case  of  steels  containing  nickel  or  chromium  which 
retard  the  transformation.  In  ordinary  work  the  difference 
between  the  points  observed  upon  heating  and  those  upon 
cooling  is  not  greater  than  30°.  In  order  to  distinguish  between 
the  two  points,  the  transformation  temperatures  observed  when 
the  steel  is  heated  are  denoted  by  the  symbols  Ac\,  AcZ)  Ac3  and 
Accm.  For  the  theoretical  point  where  Ac  and  Ar  would  coincide 
if  the  heating  and  cooling  were  infinitely  slow,  the  symbols  AI, 
A 2,  etc.,  are  used. 

Relation  between  Hardening  and  Annealing  of  Steel  and  the 
Constituents  Already  Described. — The  following  summary  of 
the  properties  of  the  five  constituents  of  steel  already  described 
will  form  a  basis  for  an  understanding  of  the  true  nature  of  harden- 
ing and  annealing  of  steel. 

Above  A3  steel  is  essentially  austenite,  a  solid  solution  of  iron 
carbide  in  7-iron.  Austenite  is  a  stable  substance  at  all  tempera- 
tures above  A*  and  is  very  hard.  Between  As  and  AI  austenite 
spontaneously  loses  ferrite  or  cementite,  according  to  the  percent 
of  carbon  in  the  steel,  these  substances  forming  definite  masses 
or  granules,  separate  from  the  remaining  austenite.  Upon  cool- 
ing to.Ari  the  austenite  remaining  changes  spontaneously  into 
pearlite  which  remains  mixed  with  the  granules  of  ferrite  or 
cementite  already  formed.  Ferrite  is  relatively  very  soft  and 
ductile,  cementite  is  very  hard  and  pearlite  combines  the  prop- 
erties of  ferrite  and  cementite,  being  moderately  hard  and  tough. 
Normally,  then,  steel  is  hard  above  A3  and  soft  below  AI,  the  de- 
gree of  hardness  in  both  regions  being  conditioned  by  the  per- 
cent of  carbon. 

The  latter  condition  should  be  easily  understood.  Below  AI 
low  carbon  steel  is  largely  ferrite,  the  softest  of  all  constituents 
of  steel.  Medium  carbon  steel  contains  more  pearlite  while 


STEEL  AND  ALLOYS  489 

high  carbon  steel  contains  cementite  and  no  ferrite.  The  hard- 
ness of  these  thre._  substances  increases  in  the  order  named. 
Above  As  austenite  exists  and,  being  a  solution  of  a  soft  and 
a  very  hard  substance  in  each  other,  its  hardness  will  naturally 
increase  with  the  percent  of  the  hard  constituent. 

Steel  possesses  the  properties  normal  to  temperatures  below 
A  i  only  in  case  it  has  cooled  very  slowly.  Even  ordinary 
liquid  solutions  may  be  considerably  supercooled  without  the 
normal  dissolution  taking  place.  Steel,  already  solid,  possesses 
much  less  molecular  mobility  and  requires  even  more  time  for 
such  changes  to  take  place.  If  this  time  is  denied  a  metastable 
condition  is  obtained  and  this  persists  at  ordinary  temperatures 
because  the  molecular  rigidity  of  the  mass  makes  structural 
changes  impossible.  It  is  not,  however,  possible  to  cool  austenite 
with  sufficient  rapidity  to  prevent  absolutely  its  transformation 
and  an  intermediate  product,  martensite,  always  appears  to  some 
extent.  This  is,  however,  even  harder  than  austenite  and  also 
contains  /?-  and  a-iron. 

The  effect  of  the  commercial  hardening  process  is  now  clear. 
It  consists  in  heating  steel  to  a  temperature  above  Ac$  and  sud- 
denly cooling  by  quenching,  austenite  or  martensite  or  both  being 
retained,  according  to  the  nature  of  the  quenching  medium.  The 
annealing  or  softening  process  is  simply  reheating  a  hardened 
steel  to  a  temperature  just  above  Ac$,  then  cooling  slowly  so  that 
the  softest  constituents  may  make  their  appearance.  The 
details  of  these  processes  will  naturally  vary  according  to  the 
degree  of  hardness  desired.  Some  of  these  details  are  later  dis- 
cussed along  with  tempering  and  with  quenching  media. 

Troostite.- — Since  the  transformation  of  austenite  into  its 
segregated  components  requires  an  appreciable  period  of  time  it 
is  but  natural  to  expect  that  variation  in  the  rapidity  of  cooling 
would  result  in  varying  degrees  of  imperfection  in  the  formation 
of  these  separate  components.  It  does  not  necessarily  follow  that 
these  more  or  less  imperfectly  formed  components  will  have  dis- 
tinct physical  properties,  other  than  in  the  matter  of  crystal 
formation,  but  two  fairly  distinct  transformation  products  in 
addition  to  martensite  have  been  recognized.  "Troostite" 
(after  the  French  chemist,  Troost)  is  the  stage  following  marten- 
site  in  the  transformation  of  austenite.  Its  constitution  is  not 


490  QUANTITATIVE  ANALYSIS 

known  and  much  difference  of  opinion  still  exists  with  regard  to 
it.  The  Committee  on  Nomenclature  of  the  Microscopic  Sub- 
stances and  Structures  of  Steel  and  Cast  Iron  of  the  International 
Association  for  Testing  Materials1  says  of  troostite  that  it  is 
"an  uncoagulated  conglomerate  of  the  transition  stages.  The 
degree  of  completeness  of  the  transformation  represented  by  it 
is  not  definitely  known  and  probably  varies  widely.77  Its  hard- 
ness varies  with  the  percent  of  carbon  but,  in  general,  it  lies 
between  that  of  martensite  and  that  of  sorbite  (the  next  stage  in 
the  transformation).  Its  tensile  strength  is  greater  than  that 
of  sorbite  or  pearlite,  its  ductility  less.  It  occurs  in  granular 
masses,  usually  associated  with  martensite  and  sorbite,  some- 
times also  with  pearlite.  It  is  distinguished  by  its  property  of 
coloring  more  darkly  than  either  martensite  or  sorbite;  the  lack 
of  crystalline  character  also  distinguishes  it  from  martensite. 
Troostite  is  shown  as  the  very  dark  portions  of  Fig.  108. 

Sorbite.' — The  next  stage  after  troostite,  in  the  resolution  of 
the  solid  solution  into  its  components,  is  sorbite,  so  called  in 
honor  of  Sorby.  Sorbite  is  to  be  regarded  as  imperfectly  formed 
pearlite.  The  distinctly  laminated  appearance  of  the  latter  is 
lacking,  sorbite  being  granular  and  apparently  amorphous.  It 
is  somewhat  less  ductile  than  pearlite  but  its  higher  tensile 
strength  and  elastic  limit  make  it  a  desirable  constituent  of  struc- 
tural steels.  It  is  the  principal  constituent  of  most  oil-hardened 
steels.  Fig.  109  shows  a  sorbitic  steel. 

Influence  of  Method  of  Cooling.  Quenching  Media. — If  the 
resolution  of  austenite  into  its  ultimate  transformation  products 
were  an  instantaneous  change,  requiring,  for  instance,  a  very 
small  fraction  of  a  second,  it  is  doubtful  whether  any  quenching 
medium  could  cool  it  so  suddenly  as  to  prevent  the  practically 
complete  resolution.  That  the  transformation  does  require  an 
appreciable  period  of  time  is  shown  by  the  retention  of  austenite 
in  very  suddenly  cooled  steel.  That  the  rapidity  of  cooling 
depends  largely  upon  the  nature  of  the  quenching  medium  is 
shown  by  the  appearance  of  the  transition  stages,  martensite, 
troostite  and  sorbite,  in  steel  cooled  from  above,  or  within,  the 
critical  range  by  quenching  in  such  media  as  warm  water,  oil  or 
air. 

.,  6th  Congress,  Intern.  Assoc.  Test-  Mat.,  paper  II8,  p.  18  (1912). 


STEEL  AND  ALLOYS  491 

Considering  the  gradation  of  properties  of  the  series  beginning 
with  austenite  and  ending  with  pearlite  it  will  be  understood  that 
the  properties  of  hardened  steel  may  be  varied  at  will  over  a  wide 
range  by  a  proper  selection  of  the  quenching  medium.  Thus 
heating  to  a  temperature  above  Ac$  and  quenching  in  cold  water 
will  produce  little  more  than  martensite,  with  its  high  degree  of 
hardness,  tensile  strength,  and  elastic  limit,  but  great  brittleness. 
Quenching  in  cold  water  from  a  temperature  near  Arz,  but  below 
it,  will  produce  martensite,  associated  with  more  or  less  ferrite 
or  cementite  and  also  some  troostite  or  sorbite  or  both,  the  latter 
two  products  imparting  a  higher  degree  of  toughness,  as  opposed 
to  the  brittleness  of  martensite,  but  being  also  responsible  for  a 
decreased  tenacity  and  increased  ductility.  Quenching  in  the 
same  medium  from  a  temperature  somewhat  above  ArL,  but 
nearer  to  this  point  than  to  Ar3,  will  increase  the  proportion  of 
sorbite  and  ferrite  and  decrease  that  of,  martensite  and  troostite,  re- 
sulting in  a  further  change  ;n  properties  in  the  direction  indicated. 

If  the  quenching  medium  is  oil,  slower  cooling  results  and  the 
constitution  of  the  quenched  steel  is  such  as  to  indicate  greater 
transformation  than  is  the  case  with  water.  In  other  words, 
the  tendency  is  now  toward  the  sorbite  end  of  the  series  and  away 
from  austenite. 

Cooling  in  quiet  air  or  in  an  air  blast  will  result  in  still  slower 
cooling  and  a  still  greater  percent  of  sorbite  will  now  be  formed, 
even  pearlite  appearing  if  the  piece  is  large  enough  to  cool 
slowly. 

Similar  considerations  will  apply  to  various  other  quenching 
media,  such  as  ice  water,  liquid  air,  mercury,  salt  solutions,  alco- 
hols, mixtures  of  alcohols  and  water,  etc.  The  exact  reasons  for 
the  different  rates  of  cooling  produced  by  different  media  are  not 
definitely  known.  Different  scientists  have  held  that  the  rate 
of  cooling  depends  upon  (a)  the  temperature  of  the  cooling  bath, 
(b)  its  specific  heat,  (c)  its  conductivity  for  heat  or  (d)  its  heat 
of  vaporization.  No  doubt  all  of  these  factors  are  important. 
The  number  of  substances  commercially  used  for  cooling  baths 
is  not  as  large  as  might  be  expected,  water,  oil  and  air  supplying 
most  of  the  needs. 

Tempering  by  Regulated  Cooling. — Extremely  hard  steel  is 
demanded  for  certain  purposes  but  for  most  structural  work, 


492  QUANTITATIVE  ANALYSIS 

tools,  etc.,  the  brittleness  accompanying  extreme  hardness  is 
undesirable  and  it  becomes  necessary  to  sacrifice  a  certain  degree 
of  hardness  and  tenacity  for  a  greater  degree  of  toughness.  From 
what  has  been  said  concerning  the  effect  of  varying  the  rate  of 
cooling  a  method  would  here  seem  to  be  available  for  securing 
the  desired  combination  of  properties.  To  " temper"  the  steel 
(the  property  of  hardness  is  ''tempered")  it  should  be  necessary 
to  heat  the  piece  above  Acs  and  then  to  cool  by  quenching  in 
the  medium  that  is  found  by  experience  to  permit  the  formation 
of  the  transition  constituents  that  will  give  the  desired  properties. 
This  can,  in  fact,  be  done  and  this  method  is  actually  used  to  a 
considerable  extent  as  oil-hardening. 

Tempering  by  Reheating. — A  method  of  tempering  that  is 
found  to  be  easily  controlled  is  that  of  reheating  the  thoroughly 
hardened  piece  to  a  temperature  where  the  metastable  austenite 
(or  in  commercial  practice,  usually  martensite)  is  partially  con- 
verted into  the  transition  products,  troostite  or  sorbite.  While 
the  metastable  austenite  or  martensite  may  be  retained  for  an 
indefinite  period  at  ordinary  temperatures,  it  becomes  partially 
converted  into  its  more  stable  products  long  before  the  tempera- 
ture reaches  Ac\.  Transformation  begins  as  low  as  100°  and 
takes  place  to  a  greater  extent  as  the  temperature  is  raised  toward 
Aci  which,  it  will  be  remembered,  is  about  700°.  If  it  be  sup- 
posed that  the  hardened  steel  is  never  brought  quite  to  Aci, 
the  degree  of  tempering  produced  will  vary  directly  with  the 
temperature  of  the  piece  during  the  tempering  process.  After 
the  proper  temperature  is  reached  the  steel  may  be  permanently 
fixed  in  its  tempered  condition  by  again  cooling.  In  practice 
it  is  usually  quenched  but  this  is  not  essential  to  the  tempering 
process  and  is  practised  only  for  the  purpose  of  saving  time.  The 
tempering  process  serves  also  to  relieve  internal  stresses  and 
thus  to  diminish  the  danger  of  cracking. 

The  advantages  of  this  method  of  tempering  over  the  method 
of  regulated  cooling  will  readily  be  seen.  The  rate  of  cooling, 
in  the  latter  method,  must  be  so  accurately  related  to  the  degree 
of  hardening  required  that  it  is  often  difficult  to  regulate  the 
process  with  the  required  refinement.  On  the  other  hand  there 
is  no  difficulty  in  first  completely  hardening  the  piece,  subse- 
quently raising  its  temperature  to  a  point,  experimentally  deter- 


STEEL  AND  ALLOYS  493 

mined  to  be  correct,  then  cooling.  The  method  of  tempering  by 
reheating  possesses  another  advantage,  in  that  no  ferrite,  pearlite 
or  cementite  can  be  formed  and  the  structure  and  properties  of 
the  steel  must  therefore  be  more  uniform  than  is  the  case  when 
regulated  cooling  from  above  the  critical  range  has  occurred. 
The  best  method  of  observing  the  temperature  is  by  a  good  py- 
rometer, preferably  of  the  thermoelectric  type.  In  fact,  scientific 
tempering  must  be  regulated  in  this  way.  The  method  of  "  tem- 
pering by  color"  has  been  much  practiced  in  the  past  and  con- 
tinues to  be  practiced  in  cases  where  extreme  accuracy  is  not 
required.  This  method  depends  upon  the  peculiar  progressive 
change  in  color,  noticed  upon  the  surface  of  a  previously  polished 
piece  of  steel  as  it  is  heated  to  various  temperatures  below  500°. 
The  colors  are  due  to  films  of  oxides  of  different  composition  and 
stability  at  different  temperatures. 

It  should  be  noted  that  one  of  the  purposes  of  either  method 
of  tempering  is  to  remove  stresses  always  existing  in  fully  hard- 
ened steel. 

Granulation.- — Up  to  this  point  in  the  discussion  of  thermal 
treatment  emphasis  has  been  placed  upon  the  identity  and  phys- 
ical properties  of  the  constituents  of  steel.  Scarcely  second  to 
these  in  importance  is  the  question  of  size  of  granules.  No  matter 
what  may  be  the  strength  of  the  individual  particles  composing 
a  piece  of  material,  the  strength  of  the  piece  as  a  whole  will  also 
depend  largely  upon  the  degree  of  coherence  of  the  particles.  If 
the  various  surfaces  separating  adjacent  crystals  (cleavage  planes) 
are  relatively  large  in  area  the  piece  will  suffer  permanent  rupture 
more  readily  than  if  they  are  small.  This  being  the  case,  the 
strength  of  the  piece  will  vary  inversely  as  the  size  of  the  granules, 
orientation  of  all  crystals  within  a  given  granule  being  the  same. 

The  temperature  of  steel  and  the  length  of  time  during  which  a 
given  temperature  is  maintained  determine  the  size  of  the  gran- 
ules. At  temperatures  below  Ai  any  existing  granulation  will 
remain  unchanged  for  an  indefinite  period  of  time.  If  the  tem- 
perature is  now  raised  to  Ac1}  the  destruction  of  existing  granula- 
tion is  begun  by  the  partial  formation  of  austenite.  Complete 
reformation  of  grains  does  not  take  place  at  this  point  unless  the 
steel  is  of  eutectic  composition  ("eutectoid  steel")  because  until 
Ac3  is  reached  a  certain  amount  of  free  ferrite  or  cementite  is 


494  QUANTITATIVE  ANALYSIS 

normal  to  the  steel.  For  eutectoid  steel,  complete  destruction  of 
existing  granules  occurs  and  new  granules  begin  to  form,  Ac3  and 
Aci  coinciding  for  such  steel.  For  steel  containing  less  than 
0.85  percent  of  carbon  ("hypoeutectoid  steel")  or  more  than  this 
amount  of  carbon  ("hypereutectoid  steel")  new  granulation 
of  austenite  begins  the  moment  Aci  is  reached  and  these  gran- 
ules continue  to  grow  in  size  until  Acz  is  passed,  complete  absorp- 
tion of  free  ferrite  or  cementite  being  here  accomplished.  This 
explains  the  fact  that  it  is  difficult  to  produce  thoroughly  satis- 
factory refinement  of  grain  in  steel  which  is  very  low  or  very  high 
in  carbon.  Acz  and  Aci  are  so  widely  separated  (Fig.  99)  that  the 
new  system  of  granules  has  grown  to  an  undesirable  extent  by 
the  time  Acz,  the  point  of  complete  destruction  of  old  granules, 
is  attained. 

The  growth  of  granules  increases  in  speed  at  higher  tempera- 
tures and  granules  continue  to  grow  so  long  as  the  steel  is  held 
at  a  temperature  above  A\.  From  these  considerations  the  fol- 
lowing rules  of  procedure  will  be  understood: 

1.  To  refine  the  grain  of  a  coarsely  granulated  piece  of  steel, 
reheat  the  piece  until  Ac$  is  passed,  then  cool  immediately, — 
slowly  if  the  piece  is  to  be  annealed,  suddenly  if  it  is  to  be  hard- 
ened, whether  or  not  hardening  is  to  be  followed  by  tempering. 

2.  All  thermal  treatment  for  the  purpose  of  hardening  or  an- 
nealing should  be  carried  out  at  the  lowest  temperature  that  will 
permit  the  desired  constitutional  changes  and  the  time  consumed 
in  the  treatment  should  be  as  short  as  possible,  in  order  to  avoid 
coarse  granulation  and  consequent  weakness.' 

The  effect  of  long  heating  at  high  temperatures  and  of  reheat- 
ing to  refine  the  grain  is  shown  in  Figs.  110  and  111.  These 
photomicrographs  clearly  show  that  a  good  piece  of  steel  may 
be  rendered  absolutely  unfit  by  careless  or  ignorant  treatment, 
and  also  that  many  of  such  pieces  may  be  restored  by  reheating. 

Overheating. — Serious  overheating  of  medium  or  high  carbon 
steel  produces  another  effect  in  addition  to  simple  coarsening  of 
grains.  Crystals  of  austenite,  which  is  the  solid  solution  existing 
after  Ac$  has  been  passed,  possess  a  characteristic  triangular 
arrangement.  Figure  107  will  illustrate  this.  If  austenite  grains 
have  been  permitted  to  grow  to  abnormal  sizes  this  arrangement 
is  not  entirely  broken  up  in  the  pearlite  and  ferrite  (or  cementite) 


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STEEL  AND  ALLOYS  495 

that  are  formed  on  slow  cooling.  These  abnormally  large  grains, 
separated  by  planes  instead  of  irregular,  interlocking  curved 
surfaces,  give  rise  to  additional  weakness  of  structure. 

Because  this  structure  is  produced  by  long  " soaking"  at  high 
temperatures  of  ingots  which  have  just  been  poured  from  the  steel 
ladles,  Howe  has  called  this  phenomenon  "ingotism."  Figure 
112  shows  a  steel  which  has  been  overheated  in  this  manner.  The 
cure  for  this  structure  is  to  be  found  in  reheating  until  Ac3  is 
passed  and  cooling  immediately,  as  explained  on  page  494.  If 
overheating  has  been  serious,  more  than  one  reheating  may  be 
necessary. 

Proper  Temperature  for  Thermal  Treatment  Indicated  by 
Composition. — Throughout  the  discussion  of  thermal  treatment 
reference  has  been  made  to  the  critical  points  of  steel  as  indicated 
by  Fig.  99.  On  account  of  the  pronounced  slope  of  the  graph  of 
Az  it  will  be  seen  that  a  knowledge  of  the  percent  of  carbon  is 
absolutely  essential  to  the  proper  application  of  thermal  treat- 
ment, unless  a  delicate  pyrometer  can  be  used  for  locating  the 
critical  points.  It  is  quite  easy  to  determine  the  location  of  AI 
by  the  latter  method  but  at  the  upper  critical  points  the  evolution 
of  heat  is  small  and  only  a  delicate  instrument  will  detect  it.  An 
inspection  of  the  figure  will  show  that  there  is  a  difference  of 
nearly  200°  between  the  proper  temperatures  for  hardening  or 
annealing  two  steels  of  0.05  percent  and  0.90  percent,  respectively, 
of  carbon. 

It  must  be  remembered  the  curves  of  figure  99  apply  only  to 
plain  carbon  steel.  Other  alloyed  metals  produce  important 
changes  in  the  location  of  the  transformation  points. 

Uneven  Carbon  Distribution ;  Streaks. — Aside  from  questions 
concerning  the  physical  and  chemical  condition  of  carbon  as 
affecting  the  physical  properties  of  steel,  it  is  important  that  the 
carbon  should  be  distributed  uniformly  throughout  the  piece,  in 
order  that  stresses  to  which  the  steel  may  be  subjected  in  service 
shall  not  result  in  rupture  of  the  weaker  parts.  For  instance,  it 
is  of  little  use  to  give  a  piece  thermal  treatment  to  impart  a  high 
tensile  strength  to  the  main  portion  if  here  and  there  are  masses 
of  ferrite  grains  that  have  not  responded  to  the  treatment. 
These  weaker  localities  may  start  a  failure  which  will  immediately 
result  in  rupture  of  the  piece,  merely  because  the  elastic  limit  of 


496  QUANTITATIVE  ANALYSIS 

these  spots  has  been  exceeded  by  a  stress  which  has  been  well 
below  the  elastic  limit  of  the  main  portion  of  the  mass. 

If  carbon  segregation  has  resulted  through  poor  mixing  of  the 
melted  steel,  overheating  or  other  causes,  an  irregular  distribution 
of  high  and  low  carbon  areas  will  result.  If  the  steel  is  later 
forged  or  rolled  the  ferrite  masses  become  flattened  and  the 
longitudinal  section  will  show  streaks  instead  of  irregular  masses 
(Fig.  1 1.3) .  In  neither  case  can  the  trouble  be  remedied  by  simply 
reheating  past  Acs.  It  is  true  that  at  this  point  the  ferrite  and 
carbide  have  passed  completely  into  mutual  solution  and  new 
grain  systems  are  generated.  But  the  resulting  austenite  does 
not  become  uniform  in  composition  in  a  short  time.  As  might 
be  expected  the  diffusion  of  molecules  if  iron  and  iron  carbide 
in  the  solid  solution  is  slow,  as  compared  with  diffusion  in  liquid 
solutions.  The  remedy  for  this  condition  lies  in  a  prolonged 
"soaking,"  perhaps  of  several  hours'  duration,  at  a  temperature 
above  Ac*.  Naturally,  this  also  causes  very  coarse  granulation 
but  after  the  irregular  distribution  has  been  overcome  a  reheating, 
followed  by  immediate  cooling,  will  break  up  the  coarse  system 
of  granules. 

It  should  be  noted  that  if  the  ferrite  streaks  contain  slag 
threads  it  is  difficult  or  impossible  to  restore  a  normal  structure 
by  thermal  treatment. 

Sulphur  Prints.- — In  the  discussion  of  the  occurrence  of  sul- 
phur in  steel  (page  448)  it  was  stated  that  this  element  is  found 
as  ferrous  sulphide  or  as  manganous  sulphide.  Either  of  these 
compounds  will  yield  hydrogen  sulphide  upon  treatment  with 
an  inorganic  acid.  The  distribution  of  sulphide  particles  in  a 
steel  may  then  be  shown  by  making  use  of  this  reaction.  A  sec- 
tion is  first  polished  as  though  it  were  to  be  prepared  for  etching. 
A  piece  of  ordinary  photographic  paper  is  then  soaked  in  dilute 
sulphuric  acid  and  laid,  face  down,  on  the  polished  surface  and 
carefully  smoothed  out.  The  liberated  hydrogen  sulphide  forms 
brown  silver  sulphide  in  the  paper.  After  a  suitable  exposure 
the  paper  is  lifted  from  the  steel  surface  and  placed  in  an  ordinary 
"hypo"  fixing  bath.  The  entire  operation  is  carried  out  in  a 
dim  light. 

Sulphur  prints  are  used  to  determine  roughly  the  relative 
amounts  of  sulphur  in  various  steel  samples.  A  still  more  im- 


STEEL  AND  ALLOYS 


497 


portant  use  is  in  detecting  any  segregation  of  sulphide,  since 
segregation  of  one  element  is,  in  a  general  way,  indicative  of 
segregation  of  other  elements  and  this  is  invariably  a  source  of 
weakness  in  the  steel. 

Case  Hardening. — The  treatment  of  steel  to  give  it  its  maxi- 
mum hardness  also  produces  brittleness.  This  may  be  of  small 
importance  if  the  piece  is  sufficiently  massive  or  if  it  is  to  be  sub- 
jected to  no  great  shock  or  stress  in  service.  It  often  happens, 
however,  that  small  pieces  are  required  to  resist  abrasion  on  the 
surface  and  still  must  have  sufficient  toughness  to  withstand 
shock  or  other  stresses.  Such  is  the  case  with  gears  and  other 
small  pieces  of  machinery.  No  combination  of  hardening  and 
tempering  is  suitable  for  producing  a  piece  of  steel  that  will 
comply  with  both  requirements. 

It  will  be  remembered  that  either,  hardened  or  annealed  high 
carbon  steel  is  harder  and  more  brittle  than  low  carbon  steel 
treated  in  the  same  manner.  If  a  piece  of  low  carbon  steel  could 
be  given  a  surface  or  case  containing  a  higher  percent  of  carbon 
the  problem  would  be  solved.  This  can  be  done  by  making  use 
of  the  well  known  power  of  iron  for  absorbing  carbon  when  the 
two  materials  are  at  temperatures  above  the  upper  critical  point, 
As.  This  power  is  probably  due  to  the  penetration  of  gases,  such 
as  carbon  monoxide  or  cyanogen,  derived  from  carbonaceous 
materials  with  which  the  steel  is  in  contact.  The  piece  of  "  mild  " 
steel  is  packed  in  the  carburizing  material  and  heated  to  a  temper- 
ature just  above  Acs,  the  length  of  exposure  depending  upon  the 
depth  of  case  required  and  varying  from  30  minutes  to  several 
hours.  The  percent  of  carbon  acquired  by  the  case  may  be  as 
high  as  2.5  but  it  is  not  usually  higher  than  0.90. 

The  materials  used  for  case  hardening  are  wood  charcoal, 
bone  charcoal,  ground  raw  bone,  leather  scraps,  scraps  of  horns 
and  hoofs,  etc.  Potassium  cyanide  or  ferrocyanide  is  sometimes 
used  for  quickly  producing  a  very  thin  and  hard  case. 

Thermal  Treatment  of  Case-hardened  Articles. — The  temper- 
ature necessary  for  case  hardening  depends  upon  the  percent  of 
carbon  already  contained  in  the  steel,  since  the  piece  must  be 
heated  above  Acs.  The  subsequent  thermal  treatment  is  even 
more  important.  Protracted  heating  above  the  critical  range 
has  given  both  case  and  core  a  coarsely  granulated  structure.  In 

32 


498  QUANTITATIVE  ANALYSIS 

order  to  refine  the  grain  the  piece  must  be  reheated  to  just  above 
Acz  and  then  quenched  but  this  process,  which  would  have  been 
simple  before  the  production  of  the  case,  is  now  complicated  by 
the  fact  that  Acs  for  the  core  is  considerably  higher  than  for  the 
case.  If,  for  example,  the  core  contains  0.10  percent  and  the 
case  0.90  percent  of  carbon,  reference  to  Fig.  99  will  show  that 
Ac*  for  the  core  is  more  than  100°  higher  than  for  the  case.  If 
the  piece  is  heated  to  refine  the  core  the  case,  momentarily 
refined  as  its  critical  range  is  passed,  becomes  again  coarse 
through  exposure  to  the  higher  temperature.  To  remedy  this 
defect  a  double  treatment  is  employed.  The  piece  is  first 
reheated  to  refine  the  core  and  then  quenched  in  oil  or  water. 
It  is  then  reheated  to  the  lower  temperature  for  refining  the  case 
and  again  quenched.  Any  desired  tempering  may  follow. 

Effect  of  Working. — The. mere  act  of  forging  or  rolling  steel 
into  the  required  form  does  not,  in  itself,  alter  the  constitution  of 
the  steel.  The  temperature  at  which  such  work  is  performed  has, 
on  the  other  hand,  an  important  influence  upon  the  size  and  form 
of  the  component  granules.  "Hot  working"  is  carried  out  at 
temperatures  above  the  critical  range,  "cold  working"  below  it. 
Cold  working  has  no  other  effect  upon  the  granules  than  to  flatten 
and  elongate  them.  Hot  working,  on  the  other  hand,  having  to 
do  with  steel  in  the  form  of  the  solid  solution,  has  the  effect  of 
breaking  up  existing  granulation  and  of  preventing  the  growth 
of  other  granules.  The  granulation  that  will  be  observed  in  the 
finished  piece  must  have  been  produced  after  work  was  finished 
and  this  makes  the  finishing  temperature  the  really  important 
consideration.  Reference  to  the  paragraph  on  "Granulation," 
page  493,  will  point  the  rule  that  the  finishing  temperature 
should  be  slightly  above  Arz  but  as  near  to  this  point  as  is  pos- 
sible, if  subsequent  reheating  is  not  to  be  applied.  If  the  article 
is  to  be  reheated  for  hardening,  annealing  or  refinement,  the 
finishing  temperature  is  not  particularly  important. 

Fatigue. — Under  the  influence  of  repeated  alternating  stresses, 
applied  for  long  periods,  metals  will  finally  rupture,  even  though 
the  single  stresses  may  not  have  reached  the  elastic  limit  of  the 
material  at  any  time.  Single  stresses,  if  below  the  elastic  limit 
of 'the  piece,  cause  a  minute  amount  of  slipping  between  adjacent 
crystals,  the  amorphous  enveloping  films  probably  providing 


STEEL  AND  ALLOYS  499 

the  slipping  surfaces  in  most  cases.  As  a  result  of  a  single  stress 
or  of  a  relatively  small  number  of  stresses  this  action  would  pass 
entirely  undetected.  But  as  the  application  of  the  stress  is 
repeated  many  times  the  slipping  occurs  over  and  over  again 
and  it  gradually  becomes  concentrated  in  the  parts  where  the 
stress  is  greatest  or  the  structure  weakest.  Rupture  finally 
becomes  apparent  and  the  piece  ultimately  breaks.  The 
phenomena  resulting  from  the  application  of  repeated  alternating 
stresses  are  generally  designated  by  the  term  "fatigue"  and  they 
must  be  considered  in  designing  parts  for  machinery  or  other 
uses  where  the  parts  will  be  under  severe  alternating  stresses, 
such  as  come  from  intense  and  rapid  vibration. 

Failures  due  to  fatigue  are  fairly  common  and  are  well'under- 
stood.  But  in  this  connection  a  fallacy  has  found  wide  accept- 
ance among  engineers  to  the  effect  that  vibration  under  stress 
induces  coarse  crystallization  of  the  material.  Many  supposed 
instances  of  this  may  be  found  recorded  in  the  literature.  It  is 
commonly  stated  that  bridge  structures,  for  example,  become 
coarsely  crystallized  on  account  of  the  severe  vibratory  effect  of 
vehicles  and  horses  passing  over  them  and  that  this  coarsening 
finally  results  in  failure  of  the  material.  Parts  of  machinery, 
as  of  rapidly  moving  engines,  are  sometimes  thought  to  change 
similarly. 

Belief  in  this  fallacy  is  due  to  an  imperfect  understanding 
of  the  nature  of  the  molecular  changes  occurring  in  metals, 
and  particularly  in  steel,  and  of  the  causes  underlying  these 
changes.  It  may  be  stated  positively  that  changes  in  crystal- 
lization or  granulation  cannot  take  place  in  steel  at  temperatures 
below  Acij  except  those  of  tempering  or  cold  working.  Neither 
tempering  nor  cold  work  has  occurred  in  the  cases  just  cited. 
The  effect  of  cold  work  would  be  to  distort  existing  grains, 
rather  than  to  cause  a  growth  of  grains.  Undoubtedly  many 
cases  of  failure  under  alternating  stresses  have  occurred,  which 
have  afterward  been  found  to  be  due  largely  to  coarsely  granu- 
lated material  but  no  cases  are  on  record  where  the  microscope 
has  been  used  before  as  well  as  after  the  breaking  has  occurred 
and  where  any  change  in  size  of  crystals  or  of  granules  has 
been  observed.  The  opposite  has  been  proved  many  times 
experimentally. 


500  QUANTITATIVE  ANALYSIS 

Slag. — The  occurrence  of  slag  particles  in  wrought  iron  is 
almost  universal,  as  a  complete  separation  of  iron  from  slag 
cannot  be  accomplished  by  mechanical  working.  The  modern 
commercial  form  of  nearly  pure  iron  is  found  in  ingot  iron,  a  prod- 
uct of  the  open-hearth  process  carried  out  without  subsequent 
recarburization  of  the  metal.  Iron  prepared  by  this  process 
should  be  nearly  free  from  slag  but  small,  scattered  particles 
will  be  seen,  even  in  the  best  of  such  metal  as  well  as  in  the 
various  grades  of  steel.  Slag  may  be  seen  most  readily  by 
examination  of  unetched  sections  under  the  microscope,  at  a 
magnification  of  50  to  100  diameters  (Fig.  114).  Larger  masses 
are  also  readily  visible  in  etched  sections.  Ordinary  slag  is 
composed,  for  the  most  part  of  silicate  and  sulphide  of  iron  and 
manganese.  Large  particles  will  show  a  characteristic  mottled 
effect,  like  that  of  Fig.  115. 

Apparatus  for  Work  in  Metallography. — A  detailed  description 
of  the  apparatus,  or  detailed  directions  for  manipulation  of  the 
apparatus  that  is  necessary  for  metallographic  investigation  of 
the  results  of  thermal  treatment  would  be  entirely  beyond  the 
scope  of  this  book.  In  the  following  discussion  and  the  directions 
for  a  limited  number  of  experiments  it  is  assumed  that  the  labora- 
tory is  fitted  with  the  necessary  apparatus  and  that  the  instructor 
will  provide  detailed  instructions.  An  outline  of  the  necessary 
steps  in  the  preparation  and  examination  of  samples  is  given  and 
some  experiments  illustrating  the  important  principles  of  thermal 
treatment  are  described. 

Experimental  Furnace. — A  small  furnace  which  may  be  quickly 
heated  and  cooled  and  whose  temperature  may  be  easily  con- 
trolled is  suitable  for  experimental  treatment.  Electrically 
heated  furnaces  are  most  convenient  and  the  temperature  should 
be  observed  by  means  of  a  sensitive  pyrometer  which  may  be  of 
the  recording  or  the  indicating  type.  The  furnace  chamber  need 
not  be  larger  than  2X2X3  inches. 

Cutting  and  Polishing  Machines. — It  is  not  often  possible  to 
polish  and  examine  large  specimens  and  a  hack  saw  is  almost  a 
necessity  in  the  metallographic  laboratory.  This  may  be  a  small 
hand  saw  or  one  of  the  more  expensive  mechanical  saws  which 
may  be  purchased  at  a  nominal  price. 

Before  the  microscopic  examination  can  be  made  it  is  necessary 


STEEL  AND  ALLOYS  501 

that  a  high  polish  be  given  to  the  surface  examined.  Here  again, 
polishing  may  be  done  by  hand,  but  if  much  work  is  to  be  done 
a  polishing  machine  is  almost  a  necessity.  Such  a  machine 
should  provide  at  least  four  polishing  surfaces.  The  final  polish  is 
given  by  a  very  fine  powder,  such  as  rouge,  but  it  is  quite  im- 
practicable to  polish  the  original,  rough  surface  with  such  a 
powder.  Instead,  the  piece  is  successively  polished  with  powders 
of  increasing  fineness,  each  polish  being  applied  at  a  direction 
perpendicular  to  the  next  preceding  one  and  the  application 
being  continued  until  all  scratches  made  by  the  preceding  opera- 
tion have  been  removed.  The  following  series  of  polishes  may 
be  used,  proceeding  toward  the  finest:  (1)  fine  grinding  surface 
of  alundum,  emery  or  carborundum,  (2)  fine  emery  powder  on 
broadcloth  or  canvas,  (3)  tripoli  on  broadcloth,  (4)  rouge  on 
broadcloth.  The  polishing  powders  are  kept  in  water  in  separate 
dishes  and  are  applied  to  the  polishing  cloth,  wet,  by  means  of 
brushes  while  the  machine  is  running.  It  is  highly  important 
that  the  powders  should  never  be  mixed  or  applied  to  the  wrong 
surfaces.  The  dishes  should  be  kept  covered  to  exclude  dust. 

Etching. — The  microscopic  examination  will  reveal  little  if  ap- 
plied to  the  brightly  polished  surface  of  steel,  although  graphitic 
carbon  may  easily  be  seen  in  cast  iron  by  this  means.  It  is  then 
necessary  to  bring  the  components  into  direct  relief  by  the  use 
of  some  agent  which  will  color  them  differently  or  attack  the 
grain  boundaries.  Very  many  such  etching  agents  have  been 
used,  examples  being  alcoholic  solution  of  nitric  acid  and  of 
picric  acid,  concentrated  nitric  acid  immediately  followed  by 
running  water,  tincture  of  iodine  and  others.  The  period  of 
application  will  depend  upon  the  agent  used  and  upon  the  nature 
of  the  sample.  High  carbon  steels  etch  more  rapidly  than  low 
carbon  steels  and  darken  rapidly  on  account  of  the  preponderance 
of  pearlite.  The  etching  solution  must  be  thoroughly  removed 
by  rinsing,  at  the  end  of  the  etching  process,  and  the  sample 
dried  by  rinsing  with  alcohol  and  holding  in  an  air  blast.  If  the 
sample  is  to  be  preserved  it  must  be  protected  from  oxidation 
by  a  thin  coat  of  lacquer,  this  being  removed  by  alcohol  before 
the  microscopic  examination. 

Examination. — The  microscope  that  is  to  be  used  for  metallo- 
graphic  purposes  must  be  of  a  special  type  because  reflected, 


502  QUANTITATIVE  ANALYSIS 

instead  of  transmitted,  light  must  be  used.  A  reflector  is  in- 
serted in  the  microscope,  just  above  the  objective,  and  illumina- 
tion is  produced  by  artificial  light  from  a  Welsbach  or  arc  light. 
For  the  examination  to  determine  the  approximate  carbon  per- 
cent, as  well  as  to  note  the  condition  of  granulation,  it  is  best  to 
use  the  low-power  lenses.  For  this  purpose  a  magnification  of 
50  to  150  diameters  is  convenient.  This  is  sufficient  to  show 
readily  the  separate  granules  of  pearlite  and  ferrite  or  cementite, 
or  to  determine  whether  the  steel  is  hardened,  and  to  indicate 
the  relative  area  of  pearlite  in  annealed  pieces.  For  the  closer 
examination  to  determine  the  identity,  form  of  crystallization, 
lamination,  etc.,  of  the  separate  constituents  a  magnification 
of  400  to  600  diameters  or,  in  special  cases,  even  higher  magnifi- 
cation is  desirable. 

If  a  record  of  the  results  of  the  examination  is  to  be  kept  a 
photographic  camera  should  be  attached  to  the  microscope  in 
such  a  manner  that  it  may  be  swung  into  place  after  the  visual 
examination  has 'been  made.  A  special  contrast  or  " process" 
plate  works  best  for  making  the  negative.  The  light  must  usually 
be  passed  through  ray  filters  but  the  nature  of  these  will  depend 
entirely  upon  the  nature  of  the  light  and  of  the  specimen  under 
examination. 

Other  Apparatus. — For  a  complete  study  of  the  relation  of 
physical  properties  to  metallographic  structure,  access  should  be 
had  to  physical  testing  machines  of  the  usual  sort.  A  Brinell 
hardness  tester  or  a  Shore  scleroscope,  as  well  as  a  set  for  deter- 
mining thermal  transformation  points  also  are  very  desirable. 
For  description  of  these  and  interpretation  of  the  results  obtained 
from  physical  tests  reference  must  be  made  to  special  books  on 
these  subjects.1 

Exercise:  Determination  of  Structure  with  Variation  in  Carbon 
Percent. — Select  a  series  of  annealed  simple  carbon  steels  having  ap- 
proximately the  following  percents  of  carbon:  0,  0.10,  0.30,  0.50,  0.70, 
0.90,  1.20,  or  as  many  of  these  as  may  be  obtained.  Cut  a  small  piece 
from  each  sample,  of  such  form  as  to  provide  one  plane  surface  1/2  to 

1Sauveur:  The  Metallography  and  Heat  Treatment  of  Iron  and  Steel. 
Howe:  The  Metallography  of  Steel  and  Cast  Iron. 
Rosenhain:  Physical  Metallurgy. 
Bullens:  Steel  and  Its  Heat  Treatment. 


STEEL  AND  ALLOYS  503 

3/4  inch  square,  although  the  form  of  outline  of  this  surface  is  not  im- 
portant. Grind  this  surface  until  it  is  plane,  also  slightly  round  its 
edges  to  prevent  cutting  the  polishing  cloth.  Polish  by  hand  or  on  a 
machine,  beginning  with  the  coarsest  of  the  surfaces  and  finishing  on 
the  finest,  polishing  with  each  powder  in  a  direction  perpendicular  to 
the  last  polishing  and  continuing  each  operation  until  all  scratches  left 
by  the  preceding  operation  have  disappeared.  The  polishing  heads 
must  be  kept  wet  and  gentle  pressure  used,  to  avoid  heating.  The 
appearance  of  a  film  of  oxide  on  the  polished  surface  is  an  indication 
of  too  little  water  or  too  great  pressure.  The  powder  and  water  should 
always  be  applied  in  such  a  way  that  the  sample  does  not  "drag"  on 
the  polishing  surface.  Wash  the  sample  and  the  hands  with  each  change 
to  a  finer  polishing  surface.  - 

When  the  polishing  is  finished  wash  the  sample,  but  without  rubbing 
the  polished  surface,  then  rinse  with  alcohol.  If  drops  of  water  are 
permitted  to  remain  on  the  surface,  spots  of  oxide  will  appear  in  a  few 
minutes.  Use  an  etching  solution  of  10  percent  concentrated  nitric 
acid  in  absolute  alcohol.  Pour  this  solution  into  a  shallow  dish  and 
immerse  the  specimen  in  the  solution .  with  the  polished  surface  up. 
The  length  of  exposure  necessary  will  vary  with  the  percent  of  carbon, 
3  to  10  seconds  being  required.  The  steels  containing  more  than  about 
0.50  percent  of  carbon  will  visibly  darken.  The  lower  carbon  steels 
will  simply  become  frosted  on  the  surface.  The  exposure  is  easily 
learned  by  experience  and  if  some  are  etched  too  little  they  may  be  re- 
treated. If  etched  too  deeply  the  etched  surface  is  removed  by  polish- 
ing and  a  new  application  of  the  solution  is  made. 

At  the  end  of  the  etching  process  remove  the  specimen  and  at  once 
rinse  thoroughly  with  running  water.  Rinse  off  the  water  by  alcohol 
and  dry  in  a  blast  of  clean  air.  The  piece  is  now  ready  for  examination. 

Examine  the  pieces  in  order  of  carbon  content  and,  if  possible,  make 
photomicrographs  under  the  direction  of  the  instructor.  Use  first  a 
magnification  of  about  100  diameters  and  observe  the  increase  of  the 
darker  pearlite  as  the  percent  of  carbon  increases,  f errite  decreasing  and 
finally  disappearing  when  eutectoid  steel  is  reached,  cementite  appear- 
ing in  steels  of  hypereutectoid  composition.  Under  a  magnification  of 
500  to  1000  diameters,  carefully  study  the  structure  of  the  individual 
granules  of  ferrite,  pearlite  and  cementite.  If  the  sample  of  nearly 
carbonless  iron  is  wrought  iron,  particles  of  slag  will  be  noticed.  Also 
either  wrought  iron  or  "ingot  iron"  may  show  numerous  "etching pits" 
within  the  granules  of  ferrite.  The  latter  are  due  to  the  crystalline 
character  of  ferrite  and  indicate  the  boundaries  of  smaller  crystals 
within  the  granules. 


504  QUANTITATIVE  ANALYSIS 

Hardened  Steel. — Heat  all  of  the  samples  to  temperatures  just 
above  Acs  and  immediately  quench  in  cold  water.  The  addition  of  10 
percent  of  alcohol  will  improve  the  quenching  bath.  To  determine 
the  location  of  Ac3  refer  the  carbon  content  to  Fig.  99.  The  samples 
containing  about  0.50  percent  and  0.90  percent,  respectively,  of  carbon 
are  now  to  be  examined. 

Polish,  etch,  and  examine  these  samples  as  in  the  case  of  the  un- 
hardened  pieces.  Note  the  disappearance  of  granules,  as  observed  by 
low-power  magnification,  and  the  characteristic  crystalline  appearance 
of  martensite.  Austenite  may  be  produced  by  heating  the  steel  to  a 
higher  temperature  and  cooling  in  ice  water. 

Properly  Annealed  Steel. — Reheat  the  one  of  the  hardened  pieces 
which  contains  0.50  percent  of  carbon  to  just  above  Acs  and  immediately 
cool  in  the  furnace.  When  600°  has  been  reached  the  piece  may  be 
removed  and  quenched.  Polish,  etch,  and  examine,  noting  size  of  gran- 
ules. Photograph  if  possible. 

Improperly  Annealed  Steel. — Reheat  the  annealed  sample  to  a  tem- 
perature between  900°  and  1100°  and  keep  it  at  this  temperature  from 
1  to  2  hours,  then  cool  in  the  furnace.  If  desired  it  also  may  be  quenched 
after  Ar\  is  passed.  Examine  and  note  the  excessively  coarse  granula- 
tion, cracks  frequently  appearing.  If  the  temperature  has  been  allowed 
to  reach  1400°  or  1500°  the  characteristics  of  burnt  steel  may  be  ob- 
served. Photograph,  if  possible,  and  compare  with  the  photograph  of 
the  same  piece,  properly  annealed. 

Refinement  of  Grain  of  Improperly  Annealed  Steel. — Reheat  the 
coarsely  granulated  (but  not  burnt)  steel  to  a  temperature  just  above 
Ac^  and  immediately  cool  in  the  furnace,  quenching,  if  desired,  after 
Ari  is  passed.  Examine  and  note  the  refinement  of  grain. 

Tempered  Steel. — Reheat  the  various  other  pieces  of  hardened  steel 
to  about  200°,  250°,  300°  and  350°,  respectively,  quenching  in  water  as 
soon  as  the  required  temperature  is  reached.  Polish,  etch  and  examine 
and  make  an  effort  to  identify  troostite  and  sorbite  in  these  tempered 
pieces. 

Case-hardened  Steel. — Cut  a  small  piece  of  low  carbon  steel  (about 
0.02  percent  to  0.05  percent  carbon)  and  pack  in  raw  bone,  bone  char- 
coal or  any  of  the  other  case-hardening  materials,  using  a  small  cast-iron 
box  which  may  be  covered.  Heat  to  900°  for  about  3  hours  then  cool  in 
the  furnace  in  order  to  leave  the  piece  soft  enough  to  permit  cutting  by 
the  hack  saw.  Cut  entirely  across  the  piece,  so  that  both  case  and  core 
will  appear  in  the  section,  polish,  etch  and  examine.  Note  the  differ- 
ent appearances  of  core  and  case,  indicating  more  carbon  in  the  latter. 
The  case  will  contain  more  pearlite  or  it  may  even  contain  cementite. 


STEEL  AND  ALLOYS  505 

Physical  and  Mechanical  Tests. — If  the  laboratory  is  equipped 
with  the  various  appliances  for  physical  and  mechanical  tests  of 
metals  the  work  may  be  made  even  more  interesting  and  instruc- 
tive by  comparing  and  correlating  these  properties  with  thermal 
treatment,  chemical  composition  and  microscopic  structure. 

BKASS  AND  BRONZE 

The  analysis  of  brass  and  bronze,  as  of  most  other  alloys, 
must  be  made  by  using  methods  of  different  classes  for  the  differ- 
ent constituent  metals.  It  seldom  happens  that  it  is  desirable 
to  employ  all  gravimetric  or  all  volumetric  or  electrolytic  methods 
for  the  various  metals  of  a  given  alloy.  Neither  is  it  always 
practicable  to  determine  all  of  the  metals  from  a  single  solution 
of  the  alloy  as  is  done  in  a  systematic  qualitative  analysis.  In- 
stead, the  method  that  can  be  best  adapted  to  each  individual 
metal  is  applied  and  if  the  other  metals  do  not  interfere  a  separate 
sample  is  used  in  each  case.  Thus  while  the  brasses  may  be 
considered  as  typical  of  a  fairly  large  class  of  similar  alloys  it 
should  not  be  understood  that  the  methods  here  outlined  will 
necessarily  be  the  best  for  other  alloys  in  which  the  same  metals 
are  found.  The  presence  of  additional  metals,  or  even  variation 
in  the  proportions  of  the  same  metals,  will  serve  to  make  modi- 
fications desirable. 

Analysis  of  Brass. — The  samples  may  be'  weighed  on  counterpoised 
glasses.  If  the  exact  specified  weights  are  taken  the  calculations  will 
be  simplified. 

Tin. — Weigh  samples  of  2  gm  each  into  porcelain  casseroles  or  plati- 
num dishes.  Add  25  cc  of  nitric  acid  of  specific  gravity  1.2,  cover  and 
digest  until  the  alloy  has  dissolved,  leaving  only  a  white  residue  of 
metastannic  acid,  H2Sn03.  Digest  for  30  minutes  at  nearly  boiling 
temperature,  replacing  evaporated  acid  if  necessary.  Cool,  dilute  to 
75  cc  and  filter  through  paper,  receiving  the  filtrate  in  a  casserole. 
Transfer  all  of  the  precipitate  to  the  paper  and  wash  with  hot  water, 
reserving  the  filtrate  and  washings  for  other  metal  determinations. 
Carefully  burn  the  paper  in  a  weighed  porcelain  crucible  and  ignite  the 
precipitate  for  15  minutes  over  a  Me*ker  burner.  Cool  and  weigh. 

If  the  amount  of  stannic  oxide  is  relatively  small  this  residue  may  be 
white  or  nearly  so.  It  may  then  be  calculated  directly  to  tin.  If  this 
is  not  the  case  and  the  stannic  oxide  is  contaminated  with  copper  oxide 


506  QUANTITATIVE  ANALYSIS 

and  stannic  phosphate  it  must  be  purified.  To  do  this  add  0.5  gm  each 
of  sodium  carbonate  and  sulphur,  cover  and  fuse  over  a  small  flame  until 
the  excess  of  sulphur  is  removed,  as  evidenced  by  the  absence  of  a  sul- 
phur flame  above  the  crucible.  Cool  and  digest  in  50  cc  of  boiling  water. 
Polysulphides  of  tin  and  of  the  traces  of  lead  and  copper  are  formed  by 
the  fusion  and  these  partly  dissolve  in  the  solution  of  sodium  poly- 
sulphide.  Add  a  little  powdered  sodium  sulphite  to  reduce  the  sodium 
polysulphide  to  monosulphide,  the  solution  then  becoming  faintly 
yellow.  The  monosulphides  of  copper  and  lead  now  precipitate  and  are 
removed  by  filtration,  washed,  dissolved  in  dilute  nitric  acid  and  added 
to  the  filtrate  from  the  first  tin  precipitate. 

Acidify  the  tin  solution  with  acetic  acid  and  pass  hydrogen  sulphide 
through  until  the  stannous  sulphide  is  completely  precipitated,  then 
filter  and  wash  once  or  twice.  Transfer  to  a  porcelain  crucible  and  care- 
fully burn  the  paper.  Roast  for  some  time  in  the  inclined  crucible  then 
heat  more  strongly  until  the  tin  sulphide  is  converted  completely  into 
oxide.  Weigh  as  SnC>2  and  calculate  the  tin. 

Lead. — Prepare  a  porcelain  Gooch  crucible  filter  by  washing  the  asbes- 
tos felt  with  dilute  sulphuric  acid,  then  with  hot  water  and  finally  with 
alcohol,  drying  in  the  oven  for  a  short  time,  then  heating  for  15  minutes 
at  700°,  preferably  in  an  electric  furnace  whose  temperature  is  measured 
by  a  pyrometer.  Cool  and  weigh. 

Place  the  filtrate  and  washings  from  the  metastannic  acid  in  a  casse- 
role and  add  5  cc  of  concentrated  sulphuric  acid.  Hold  in  the  hand 
over  a  flame  and  evaporate,  agitating  constantly,  until  dense  fumes  of 
sulphur  trioxide  appear.  Cool,  add  35  cc  of  water  and  boil  gently  for  1 
minute  to  dissolve  soluble  sulphates  of  copper  and  zinc.  Filter  in  the 
weighed  Gooch  crucible  and  wash  three  times  with  5-cc  portions  of  15 
percent  sulphuric  acid,  receiving  the  filtrate  and  washings  in  a  250-cc 
volumetric  flask.  Remove  the  receiving  flask  and  set  aside  with 
the  solution  for  the  copper  determination.  Wash  the  lead  sulphate  on 
the  filter  with  50  percent  alcohol  to  remove  sulphuric  acid,  discarding 
the  washings.  Dry  the  crucible  in  the  oven  for  10  minutes  then  heat 
at  700°  for  20  minutes.  Cool  and  weigh  the  lead  sulphate  and  calculate 
the  percent  of  lead  in  the  brass. 

Copper. — Since  copper  is  one  of  the  principal  elements  of  alloys  of  this 
class  the  sample  of  2  gm  is  too  large  for  this  determination.  Dilute 
the  solution  to  the  mark  on  the  volumetric  flask  and  mix  well.  Care- 
fully pipette  50  cc  into  the  beaker  in  which  electrolysis  is  to  take  place 
and  add  1  gm  of  ammonium  nitrate.  This,  by  reaction  with  sulphuric 
acid,  produces  a  small  amount  of  nitric  acid  but  without  increasing 
the  total  acidity  of  the  solution.  Connect  the  anode  and  the  weighed 
cathode  with  the  source  of  current  in  the  usual  way  and  add  water 


STEEL  AND  ALLOYS  507 

until  the  cathode  is  covered.  Mix  by  stirring,  then  conduct  the  elec- 
trolysis and  subsequent  treatment  of  copper  as  directed  on  page  156, 
keeping  the  voltage  below  2.5  to  avoid  the  deposition  of  zinc.  Calcu- 
late the  percent  of  copper  in  the  alloy. 

If  the  electrolysis  produces  a  small  deposit  of  lead  peroxide  on  the 
anode,  carefully  wash  this  and  dissolve  in  a  small  amount  of  concentrated 
hydrochloric  acid.  Determine  this  lead  as  sulphate  and  add  to  the 
percent  already  found. 

Zinc. — The  solution  from  which  the  copper  has  been  deposited  will 
represent  enough  sample  to  serve  for  the  zinc  determination  in  the  analy- 
sis of  brass  but  for  bronzes,  in  which  zinc  is  a  minor  constituent  if  it 
occurs  at  all,  it  may  be  necessary  to  use  a  larger  aliquot  portion  of  the 
filtrate  from  tin,  electrolyzing  to  remove  copper,  as  before. 

Prepare  a  10  percent  solution  of  diammonium  acid  phosphate  by 
dissolving  the  salt  in  cold  water  and  making  barely  basic  to  phenolph- 
thalein  by  adding  dilute  ammonium  hydroxide,  drop  by  drop.  The 
addition  of  ammonia  converts  any  monoammonium  salt  that  may  be 
present  into  the  diammonium  salt: 

NH4H2P04+NH40H->(NH4)2HP04+H20. 

Make  the  zinc  solution  slightly  basic  with  ammonium  hydroxide,  using 
a  drop  of  litmus  solution  as  indicator. 

Add  1  cc  of  10  percent  acetic  acid  to  the  solution  containing  zinc; 
this  should  be  enough  to  change  the  litmus  to  red.  Drop  in  '25  cc  of 
the  phosphate  solution  and  keep  nearly  boiling  for  30  minutes,  stirring 
occasionally.  Zinc  ammonium  phosphate  (ZnNH^PCX)  precipitates 
first  in  an  amorphous  condition,  changed  by  heating  and  stirring  to  the 
crystalline  modification.  Cool  and  filter  through  a  Gooch  crucible  or 
an  alundum  crucible  that  has  been  dried  at  105°  and  weighed.  Wash 
the  precipitate  with  1  percent  diammonium  phosphate  solution  until  the 
washings  test  free  from  sulphates,  then  several  times  with  50  percent 
alcohol.  Dry  at  105°  and  weigh  the  zinc  ammonium  phosphate,  from 
which  zinc  is  calculated. 

Instead  of  weighing  zinc  in  this  form  the  material  may  be  filtered 
in  an  alundum  or  Gooch  crucible  that  has  been  ignited  for  15  minutes 
(conveniently  heated  in  an  electric  furnace  at  about  800°)  and  weighed. 
The  precipitate  is  washed  as  already  directed  and  it  is  then  heated  as 
was  the  empty  crucible  for  30  minutes.  The  zinc  is  then  weighed  as  zinc 
pyrophosphate : 

2ZnNH4P04--»Zn2P207+2NH3+H20. 
The  percent  of  zinc  is  calculated  from  this  weight. 


508  QUANTITATIVE  ANALYSIS 

ANTI-FRICTION  METALS 

Soft  bearing  metals,  used  for  lining  journal  bearings,  are 
usually  alloys  containing  tin,  copper,  lead  and  antimony  in  dif- 
ferent proportions,  with  occasional  additions  of  other  elements. 
They  are  often  collectively  designated  " Babbitt  metal,"  al- 
though this  name  properly  applies  only  to  the  alloy  having  the 
approximate  composition:  tin  90,  antimony  7,  copper  3.  Many 
of  the  commercially  used  anti-friction  metals  have  the  tin  partly 
or  entirely  replaced  by  lead.  In  addition  small  amounts  of 
zinc  may  be  present,  so  that  a  complete  quantitative  analysis 
must  include  the  determination  of  these  metals,  as  well  as  of  any 
others  that  may  be  shown  by  a  qualitative  analysis. 

Tin  and  Antimony. — The  general  methods  outlined  for  brass 
and  bronze  will  apply  to  these  alloys  also.  An  exception  must  be 
made  in  the  case  of  tin,  which  cannot  be  weighed  directly  as 
oxide  on  account  of  the  presence  of  antimony.  Upon  treatment 
of  the  alloy  with  nitric  acid  the  metastannic  acid  which  is  formed 
from  the  tin  is  accompanied  by  antimonic  acid,  H3Sb04,  or  an 
indefinite  mixture  of  this  with  hydrated  antimony  trioxide.  The 
precipitation  is  not  complete  and  total  antimony  is  therefore  not 
weighed  with  the  tin. 

Volumetric  methods  are  suitable  for  these  elements.  The 
alloy  is  dissolved  in  concentrated  sulphuric  acid,  tin  forming 
stannic  sulphate  and  antimony  dissolving  as  antimonious  sul- 
phate. In  this  solution  antimony  is  titrated  by  potassium  per- 
manganate, antimonic  acid  being  produced  by  the  oxidation: 

5Sb2(S04)3+4KMn04+2H20->10H3Sb04+2K2S04 

+9H2SO4+4MnS04. 

Tin  is  then  reduced  by  heating  with  elementary  antimony 
(in  presence  of  hydrochloric  acid)  to  stannous  salts  and  is  then 
titrated  with  standard  iodine  solution: 

SnCl2+l2-»SnCl2I2 
or 

SnCl2+2HCH-I2->SnCl4+2HI. 

Analysis  of  Tin-base  Bearing  Metal.  Antimony  and  Tin. — Prepare  a 
solution  of  potassium  permanganate,  either  tenth-normal  or  of  such 
concentration  that  each  cubic  centimeter  is  equivalent  to  0.005  gm  of 


STEEL  AND  ALLOYS  509 

antimony.  Prepare  also  a  standard  iodine,  approximately  tenth-normal. 
Standardize  these  solutions  against  tin  and  antimony  of  known  purity, 
following  the  methods  that  are  now  to  be  described  for  the  determination 
of  these  elements  in  the  alloy,  using  about  0.2  gm,  accurately  weighed,  of 
each  metal  for  the  purpose. 

Weigh  about  0.4  gm  of  the  alloy  and  brush  into  a  500-cc  Pyrex  Kjel- 
dahl  digestion  flask.  Add  10  cc  of  concentrated  sulphuric  acid  and  heat 
until  all  of  the  alloy  is  dissolved  and  all  sulphur  dioxide  expelled.  Cool 
and  add  50  cc  of  water  and  10  cc  of  concentrated  hydrochloric  acid,  then 
warm  until  the  solution  is  clear  or  nearly  so.  Cool  and  add  100  cc  of 
water  and  25  cc  of  concentrated  hydrochloric  acid.  Cool  in  running 
water  and  titrate  at  once  with  standard  potassium  permanganate  solu- 
tion, adding  dropwise,  but  rapidly  and  with  constant  agitation.  The 
end  point  will  be  sharp  but  the  pink  color  will  fade  after  a  short  time, 
on  account  of  reduction  of  the  permanganate  by  hydrochloric  acid. 

Calculate  the  percent  of  antimony  in  the  alloy. 

Rinse  the  titrated  solution  into  a  500-cc  flask,  using  for  the  purpose 
50  cc  of  concentrated  hydrochloric  acid.  Add  about  1  gm  of  powdered 
tin-free  antimony  and  insert  a  rubber  stopper  carrying  two  bent  tubes. 
One  of  these  extends  beneath  the  surface  of  the  solution.  The  other 
ends  just  below  the  stopper  and  has  an  outside  connection  with  a  50 
or  100-cc  pipette.  Warm  on  the  steam  bath  for  15  minutes,  then  remove 
and  connect  the  first  named  tube  with  a  carbon  dioxide  generator  which 
will  furnish  a  rapid  stream  of  gas.  The  pipette  should  dip  into  a 
beaker  of  distilled  water.  This  excludes  outside  air  from  the  flask. 

Start  the  current  of  carbon  dioxide  then  heat  the  solution  in  the 
flask  and  boil  over  a  flame  for  5  minutes,  which  should  serve  to  reduce 
all  of  the  tin.  Remove  the  flame  and  cool  the  flask  in  cold  water  but 
keeping  carbon  dioxide  running  fast  enough  to  prevent  back  suction  of 
water  from  the  beaker. 

When  the  solution  is  cold,  carefully  remove  the  stopper  and  rinse 
down  the  tube  with  recently  boiled  water,  then  add  1  cc  of  starch  solu- 
tion and  titrate  at  once  with  standard  iodine  solution,  whirling  to  mix 
the  solutions  but  avoiding  violent  churning  with  air. 

Calculate  the  percent  of  tin  in  the  alloy. 

Lead  and  Copper. — Proceed  as  with  brass  and  bronze,  using  sample 
weights  according  to  the  approximate  percents  of  these  metals  in  the 
alloy. 


CHAPTER  XVI 
AGRICULTURAL  MATERIALS 

FERTILIZERS 

Many  elements  naturally  occurring  in  soils  are  extracted  and 
used  in  small  quantities  by  plants.  Certain  others  are  necessary 
to  the  growth  of  plant  life  and  are  demanded  in  greater  abundance. 
With  the  growth  of  the  knowledge  of  soil  chemistry  the  addition 
of  deficient  elements  to  the  soil  has  become  a  commercial  matter 
and  the  analysis  of  fertilizers  has  become  a  necessary  part  of  the 
chemist's  work,  not  only  for  the  purpose  of  placing  a  correct 
estimate  upon  the  commercial  value  of  the  fertilizer  but  also  to 
provide  a  basis  for  the  intelligent  application  of  the  fertilizer  to 
the  soil  that  lacks  it. 

Substances  added  to  the  soil  -to  promote  plant  growth  belong 
either  to  the  class  of  plant  foods  or  to  that  of  correctives.  The 
most  valuable  elements  belonging  to  the  first  class  are  nitrogen, 
phosphorus  and  potassium.  An  example  of  the  second  class  is 
calcium  carbonate,  added  to  correct  excessive  acidity  of  certain 
soils.  It  is  generally  true  that  substances  of  the  second  class 
also  provide  plant  food  but  this  is  usually  incidental  to  the  main 
purpose  for  which  they  are  added. 

The  Association  of  Official  Agricultural  Chemists  has,  for  a 
number  of  years,  carried  on  a  systematic  study  of  the  action  of 
fertilizers,  their  relative  values  as  plant  food  and  the  methods 
for  determining  the  essential  constituents.  Until  1915  the  analyt- 
ical methods  that  were  given  official  sanction  were  published  by 
the  Government  as  Bulletin  107,  Bureau  of  Chemistry,  Depart- 
ment of  Agriculture.  Since  the  date  mentioned  they  have  been 
published  as  a  part  of  the  proceedings  of  the  society.1  Practi- 
cally all  of  the  methods  given  in  the  following  pages  are  essen- 

1 J.  Assoc.  Off.  Agr.  Chem.,  Pts.  II  of  Vol.  I,  No.  4  and  of  Vol.  II,  Nos. 
1,  2  and  3. 

510 


AGRICULTURAL  MATERIALS  511 

tially  the  official  methods,  although  this  rule  is  not  universally 
followed. 

Preparation  of  Samples. — Reduce  the  gross  sample  by  rolling  and 
quartering  to  an  amount  sufficient  for  analytical  purposes.  About  100 
grams  will  usually  be  convenient.  Transfer  to  a  sieve  having  circular 
openings  1  mm  in  diameter  and  sift,  breaking  the  lumps  with  a  soft 
rubber  pestle.  Grind  in  a  mortar  the  part  remaining  on  the  sieve  until 
the  particles  will  pass  through.  Mix  thoroughly  and  preserve  the 
sample  in  tightly  stoppered  bottles.  Grind  and  sift  as  rapidly  as  pos- 
sible to  avoid  loss  or  gain  of  moisture  during  the  operation. 

Moisture. — The  moisture  of  a  fertilizer  is,  in  most  cases,  a 
substance  without  any  value  whatever  and  its  determination  is 
made  with  this  in  view. 

Determination. — Heat  2  gm  of  the  prepared  sample  for  5  hours  at 
100°.  If  the  sample  is  of  potassium  salts,  sodium  nitrate  or  ammonium 
sulphate,  heat  from  1  to  5  gm  at  about  130°  until  the  sample  ceases  to 
lose  weight.  Calculate  the  percent  of  moisture. 

Nitrogen. — Although  nitrogen  is  so  abundant  as  an  elementary 
constituent  of  the  atmosphere  it  is  one  of  the  most  costly  of  all 
the  elements  that  are  required  for  plant  growth.  This  is  because 
its  chemically  inert  character  makes  its  fixation  and  assimilation 
by  plants  a  difficult  matter.  Plants  do  not  abstract  gaseous 
nitrogen  from  the  atmosphere  although  the  roots  of  leguminous 
plants  support  certain  bacteria  whose  action  is  to  oxidize  at- 
mospheric nitrogen  to  nitric  acid,  this  being  then  fixed  in  the 
soil  by  forming  nitrates  with  such  basic  materials  as  calcium 
carbonate.  Nitrogen  may  be  added  as  an  artificial  fertilizer  in 
the  form  of  ammonium  salts,  nitrites,  nitrates  or  organic  ni- 
trogenous materials.  Crude  chloride  and  sulphate  are  the  more 
common  forms  of  ammonium  salts  used,  ammonium  sulphate 
being  obtained  during  the  process  of  gas  manufacture.  Nitrites 
are  little  used  as  commercial  fertilizers.  Nitrates  are  the  most 
common  and  probably  the  most  valuable  of  the  various  nitrog- 
enous materials.  In  the  past,  the  chief  source  of  nitrates  has 
been  the  " Chile  saltpetre"  beds  of  South  America.  The  artifi- 
cial fixation  of  atmospheric  nitrogen  has  been  successfully  ac- 
complished by  two  processes: 

(1)  When  calcium   carbide  is  heated  with  pure  nitrogen  to 


512  QUANTITATIVE  ANALYSIS 

700°-800°,  in  the  presence  of  a  small  amount  of  calcium  chloride 
or  calcium  fluoride,  calcium  cyanamide,  CaNCN,  is  formed. 
This  substance  decomposes  in  the  soil,  forming  first  cyanamine, 
then  urea  and  finally  ammonium  carbonate. 

CaNCN+C02+H2O->CaC03+H2NCN, 

H2NCN + H2O-»CO  (NH2)  2, 
CO(NH2)2+2H20-+(NH4)2CO3. 

(2)  Under  the  influence  of  an  electrical  discharge  nitrogen  and 
oxygen  combine  directly,  forming  nitric  oxide.  This  occurs  to 
some  extent  during  electrical  storms,  which  explains  the  occur- 
rence of  nitric  acid  in  rain  water.  The  reaction  is  now  used  on  a 
large  scale  for  the  production  of  nitric  acid  which  is  then  converted 
into  nitrates  and  used  for  fertilizers  and  other  purposes. 

The  most  important  nitrogenous  organic  materials  that  are 
used  for  fertilizers  are  dried  blood  and  tankage  obtained  from 
the  packing  houses,  also  fish  scraps,  guano  and  ordinary  stable 
manure.  Dried  blood  is  a  very  valuable  fertilizer  because  of 
the  large  percent  of  nitrogen  which  it  contains  (12  to  14  per- 
cent) and  because  it  is  readily  available  for  assimilation  by 
plants.  Tankage  consists  of  scraps  of  refuse  meat,  skin,  etc., 
from  which  the  oil  has  been  removed  by  steaming  and  pressing. 
Fish  scrap,  guano  and  farm  manures  are  valuable  as  nitrogenous 
fertilizers  but  are  limited  in  quantity. 

Certain  other  nitrogenous  materials  that  are  sometimes  added 
to  mixed  fertilizers  because  they  are  rich  in  nitrogen  are  useless 
on  account  of  the  fact  that  their  nitrogen  is  only  very  slowly 
available  for  assimilation  by  plants.  Such  materials  are  hair, 
horns,  hoofs,  leather  scrap,  and  peat.  When  these  are  finely 
ground  and  mixed  with  other  fertilizing  material  they  yield  rela- 
tively high  percents  of  nitrogen  in  the  analytical  process  but  are 
of  little  use  to  the  plant  life.  Their  detection  is  often  possible 
only  by  means  of  the  microscope  and  the  determination  of  their 
quantity  in  a  mixed  fertilizer  is  a  very  difficult  matter  and  for 
these  reasons  their  admixture  with  commercial  fertilizers  is 
forbidden  by  many  state  laws. 

Fertilizers  may  contain  nitrogen  in  only  one  form  or  in  a 
mixture  of  two  or  more  classes  of  compounds,  as  ammonium 
salts,  nitrates,  or  organic  compounds.  The  determination  of  the 


AGRICULTURAL  MATERIALS  513 

percent  of  nitrogen  in  each  form  is  occasionally  demanded  but 
usually  the  determination  of  total  nitrogen  is  all  that  is  required, 
it  being  understood  that  hair,  leather  scraps  and  other  such  mate- 
rials are  excluded.  The  method  of  Dumas  or  the  soda  lime" 
method  may  be  used  for  the  determination  of  total  nitrogen  but 
the  method  of  Kjeldahl,  or  one  of  the  modifications  of  this  method, 
is  better  suited  to  this  class  of  work. 

Kjeldahl's  Method. — KjeldahFs  original  process1  consists  in 
digesting  the  organic  material  with  boiling  concentrated  sul- 
phuric acid  until  complete  decomposition  has  been  effected.  The 
exact  course  of  the  reactions  cannot  be  traced  but  the  carbon  and 
hydrogen  are  completely  oxidized  and  nitrogen  is  converted  into 
ammonia,  which  immediately  combines  with  sulphuric  acid 
and  remains  as  ammonium  sulphate.  The  completion  of  decom- 
position is  insured  by  the  final  addition  of  a  small  amount  of 
potassium  permanganate.  The  solution  is  then  diluted  with 
water,  an  excess  of  sodium  hydroxide  is  added  and  the  resultant 
ammonia  is  distilled  into  a  measured  quantity  of  standard  acid 
solution.  To  dete  mine  the  excess  of  standard  acid,  potassium 
iodate  and  potassium  iodide  are  added  and  the  liberated  iodine  is 
titrated  by  a  standard  solution  of  sodium  thiosulphate.  Iodine 
is  liberated  according  to  the  following  equation: 

KIO3+5KI+3H2SO4-*3K2S04+6I+3H2O. 

Instead  of  this  method  of  determining  the  excess  of  standard 
acid  it  is  now  customary  to  titrate  the  excess  by  means  of  a 
standard  basic  solution. 

Modification  to  Include  Nitrates. — The  method  is  inapplicable 
to  the  determination  of  nitrogen  of  nitrates  because  of  the  loss 
of  nitric  acid  which  occurs  as  soon  as  the  material  is  treated 
with  sulphuric  acid.  Modifications  of  the  method  to  suit  the 
analysis  of  nitrates  will  presently  be  discussed. 

The  digestion  with  sulphuric  acid  is  best  accomplished  in  a 
pear-shaped  flask  with  a  long.necky  like  that  shown  in  Fig.  116. 
The  concentrated  sulphuric  acid  of  commerce  boils  at  tempera- 
tures ranging  from  210°  to  340°,  depending  upon  the  percent  of 
water  contained  in  it.  Such  a  temperature  is  high  enough  to 

*Z.  anal.  Chem.,  22,  366  (1883). 
33 


514 


QUANTITATIVE  ANALYSIS 


permit  condensation  of  nearly  all  of  the  vapor  without  the  use 
of  a  water  condenser,  the  long  neck  of  the  digestion  flask  serving 
for  this  purpose.  If  the  solution  is  to  be  transferred  to  a  special 
distilling  flask  the  capacity  of  the  digestion  flask  need  not  be 
greater  than  200  cc.  It  is  more  convenient,  however,  to  distill 
from  the  flask  in  which  digestion  is  accomplished,  in  which  case 
the  capacity  of  the  flask  should  be  500  cc.  The  digestion  must 
be  performed  under  a  hood  or  some  other  provision  must  be  made 


Fio.  116. — Kjeldahl  flask,  stand  and  lead  pipe  ventilator. 

for  carrying  away  the  fumes.  An  excellent  arrangement  for 
this  purpose,  is  a  lead  pipe,  6  inches  in  diameter  and  with  holes 
in  the  side  so  that  the  necks  of  a  number  of  digestion  flasks  may 
be  inserted  with  the  flask  in  an  inclined  position.  The  end  of 
the  lead  pipe  leads  to  a  chimney. 

Catalytic  Agents. — Wilfarth  showed1  that  the  addition  of 
mercuric  oxide,  -  copper  oxide  or  ferric  oxide  to  the  mixture  of 
the  organic  material  and  sulphuric  acid  considerably  accelerates 
the  reactions  that  occur  during  digestion.  The  action  is  of  a 
catalytic  nature  and  depends  upon  the  capability  of  the  metal 
of  existing  in  more  than  one  state  of  oxidation.  The  metal  ist 

*Z.  anal  Chem.,  24,  455  (1885);  Chem.  Zentr.,  [3]  16,  17  and  113  (1885) 


AGRICULTURAL  MATERIALS  515 

thus  alternately  reduced  by  organic  matter  and  oxidized  by 
sulphuric  acid,  somewhat  as  follows: 

2HgS04-*Hg2S04+S03+0, 
Hg2S04+2H2S04-+2HgS04+2H20+S02. 

The  nascent  oxygen  thus  formed  attacks  the  organic  matter. 

Of  the  three  metals  named,  mercury  serves  best  because  its 
salts  are  colorless  and  do  not  obscure  the  end  point  of  the  oxida- 
tion. It  is  necessary  in  this  case  to  precipitate  the  mercury  by 
the  addition  of  potassium  sulphide,  before  distillation,  in  order  to 
prevent  the  formation  of  mercurammonium  compounds  which 
are  not  readily  decomposed  by  sodium  hydroxide. 

Copper  sulphate  as  a  catalyst  is  often  preferred  because 
it  serves  as  an  indicator  when  sodium  hydroxide  is  added  later, 
a  deep  blue  solution  being  formed  when  the  solution  becomes 
basic. 

Prevention  of  Bumping. — During  the  distillation  of  ammonia, 
after  the  addition  of  excess  of  sodium  hydroxide,  there  is  usually 
a  tendency  toward  bumping.  In  order  to  prevent  this  the 
"official"  method  of  the  Association  of  Official  Agricultural 
Chemists  directs  the  addition  of  granulated  zinc  or  pumice  stone 
to  the  contents  of  the  flask  before  distillation.  The  reaction 
of  zinc  with  sodium  hydroxide  produces  a  continuous  evolution 
of  hydrogen  and  this  effectually  prevents  bumping.  There  is, 
however,  a  disadvantage  connected  with  the  use  of  zinc  which 
is  sometimes  serious,  in  that  the  sodium  zincate  that  is  formed  by 
the  reaction  so  increases  the  surface  tension  of  the  solution  that 
troublesome  frothing  occurs.  An  excellent  substitute  for  both 
zinc  and  pumice  is  a  small  amount  (0.5  gm)  of  crushed  porcelain 
from  which  the  dust  has  been  removed  by  sifting. 

Blank. — Sulphuric  acid  nearly  always  contains  a  small  amount 
of  ammonium  sulphate.  Distilled  water  may  also  contain  a 
small  quantity  of  ammonium  hydroxide.  In  order  to  make  the 
proper  correction  for  the  ammonia  that  will  be  derived  from  the 
reagents  a  "blank"  determination  must  be  made,  omitting  the 
sample  of  fertilizer  but  carrying  out  the  operations  exactly  as  in 
the  real  determination.  In  this  case  cane  sugar  is  added  to 
reduce  possible  traces  of  nitrates  existing  in  the  reagents,  as  they 
would  be  reduced  by  the  organic  matter  of  the  fertilizer. 


516  QUANTITATIVE  ANALYSIS 

Detection  of  Nitrates.— Mix  5  gm  of  the  fertilizer  with  25  cc  of  hot 
water  and  filter.  To  a  portion  of  this  solution  add  2  volumes  of  con- 
centrated sulphuric  acid,  free  from  nitric  acid  and  oxides  of  nitrogen, 
and  allow  the  mixture  to  cool.  Add  cautiously  a  few  drops  of  a  con- 
centrated solution  of  ferrous  sulphate  so  that  the  fluids  do  not  mix. 
If  nitrates  are  present  the  junction  shows  at  first  a  purple,  afterward  a 
brown,  color  or  if  only  a  minute  quantity  is  present,  a  reddish  color.  To 
another  portion  of  the  solution  add  1  cc  of  a  1  percent  solution  of  sodium 
nitrate  and  test  as  before  to  determine  whether  enough  sulphuric  acid 
were  added  in  the  first  test. 

Determination.  Organic  and  Ammoniacal  Nitrogen  only.  Kjeldahl 
Method. — Prepare  the  following  reagents. 

(a)  Hydrochloric  or  Sulphuric  Acid  Solution,  Half-Normal. — Stand- 
ardize against  pure  sodium  carbonate  as  directed  on  page  224,  making 
the  necessary  changes  in  weight  of  carbonate  to  account  for  tjie  different 
normality  of  the  acid. 

The  official  method  directs  the  standardization  of  these  acids  by 
weighing  silver  chloride  or  barium  sulphate  precipitated  from  measured 
volumes.  That  this  method  may  lead  to  serious  errors  is  explained  on 
page  222.  The  method  is  not  to  be  recommended. 

(6)  Sodium  Hydroxide  or  Potassium  Hydroxide  Solution,  Tenth- Normal. 
— Standardize  by  titration  against  exactly  10  cc  of  the  acid,  using 
methyl  red  as  indicator. 

(c)  Sulphuric    Acid. — The    concentrated    acid    of    the    laboratory, 
specific  gravity  1.84,  as  nearly  as  possible  free  from  nitrates  and  ammo- 
nium salts. 

(d)  Metallic  Mercury  or  Mercuric  Oxide. — Mercuric  oxide  should  be 
that  prepared  in  the  wet  way  but  not  from  mercuric  nitrate. 

(e)  Potassium  Sulphide  Solution. — Dissolve  at  the  rate  of  40  gm  for 
each  liter  of  solution.     Commercial  potassium  sulphide  is  used. 

(/)  Sodium  Hydroxide  Solution. — A  saturated  solution,  free  from 
nitrates. 

(g)  Methyl  Red  Solution. — Dissolve  1  gm  of  methyl  red  (dimethylam- 
inoazobenzeneorthocarboxylic  acid)  in  100  cc  of  95  percent  alcohol. 

If  the  approximate  percent  of  nitrogen  in  the  sample  is  known,  cal- 
culate the  weight  that  will  yield  ammonia  equivalent  to  about  35  cc 
of  the  standard  acid.  If  nothing  is  known  of  the  nitrogen  content  use 
about  1  gm  of  sample.  The  sample  must  contain  no  nitrates,  nitrites 
or  nitro-compounds.  Place  two  weighed  samples  in  500  cc  Kjeldahl 
digestion  flasks,  holding  the  latter  in  a  vertical  position  to  prevent  the 
sample  from  sticking  to  the  sides  of  the  neck,  which  should  be  dry. 
Weigh  1  gm  of  sugar  into  another  flask  and  treat  the  same  as  the  fertilizer 
sample.  Add  about  0.7  gm  of  mercuric  oxide  or  of  mercury,  or  0.3  gm  of 


AGRICULTURAL  MATERIALS  517 

copper  sulphate,  also  25  cc  of  concentrated  sulphuric  acid.  Incline  the 
flask  in  a  hood  or  with  the  neck  inserted  into  a  lead-pipe  ventilator  and 
heat  gently  until  the  violence  of  the  reactions  has  moderated,  then  grad- 
ually raise  the  temperature  until  the  acid  is  boiling.  The  flask  may  be 
heated  without  protection  by  a  gauze  if  it  is  of  Pyrex  glass  or  a  similar 
resistance  glass  and  if  it  is  placed  over  a  hole  in  a  stand  of  sheet  iron  in 
such  a  manner  that  the  flame  cannot  come  into  contact  with  the  sides 
of  the  flask  above  the  liquid.  (See  Fig.  116.) 

Digest  by  gently  boiling  until  the  solution  is  nearly  colorless.  This 
may  occur  after  a  short  time  or  the  digestion  may  require  several  hours. 
Remove  the  flame  and  at  once  drop  into  the  flask  small  quantities  of 
powdered  potassium  permanganate  until  the  solution  acquires  a  green  or 
purple  tint  which  persists  after  shaking.  Allow  the  flask  to  stand  until 
cool.  (Do  not  cool  under  a  tap.)  Carefully  add  200  cc  of  distilled 
water  and  mix  by  rotating  the  flask.  Add  about  0. 5  gm  of  crushed  porce- 
lain and  25  cc  of  4  percent  potassium  sulphide  solution,  shaking  as  the 
latter  is  added.  Have  the  connections  with  a  tin  condenser  ready  and 
have  50  cc  of  standard  acid  measured  into  a  400-cc  flask  into  which  the 
delivery  tube  (of  glass)  dips.  Most  laboratories  in  which  much  work 
of  this  kind  is  done  will  be  equipped  with  a  special  form  of  apparatus 
for  carrying  on  several  distillations  at  once.  The  flask  should  be  in  a 
vertical  position  and  some  kind  of  trap  should  be  used  to  prevent  spray 
from  being  carried  over  by  the  steam.  The  delivery  tube  should  be 
capable  of  being  detached  from  the  condenser  for  the  purpose  of  cleaning 
and  rinsing  it.  The  entire  condenser  must  be  thoroughly  rinsed  before 
each  distillation,  to  insure  freedom  from  basic  solutions. 

Pour  50  cc  of  saturated  sodium  hydroxide  solution  (which  should 
contain  but  little  carbonate)  down  the  inclined  flask  in  such  a  way  that 
mixing  does  not  occur.  Immediately  connect  with  the  condenser,  care- 
fully mix  the  contents  of  the  flask  by  shaking,  then  distill  until  about  150 
cc  of  distillate  has  been  collected.  It  sometimes  happens  that  too  much 
sulphuric  acid  has  been  added  to  hasten  a  difficult  digestion  or  that  the 
sodium  hydroxide  solution  is  not  saturated.  The  consequence  is  that 
the  solution  still  contains  an  excess  of  acid  when  ready  for  distillation. 
This  will  not  be  the  case  if  the  directions  have  been  carefully  followed 
but  the  addition  of  a  drop  of  phenolphthalein  to  the  solution  will  serve 
to  indicate  the  fact.  It  should  be  remembered,  however,  that  a  con- 
centrated solution  of  a  base  soon  decolorizes  phenolphthalein  and  this 
action  may  be  mistaken  for  an  indication  of  an  excess  of  acid.  If  copper 
sulphate  has  been  used  as  an  accelerator  a  deep  blue  color  will  indicate 
the  presence  of  sufficient  sodium  hydroxide.  In  this  case  the  addition 
of  potassium  sulphide  should  be  omitted. 

When  the  distillation  is  finished  lower  the  receiving  flask  until  the 


518  QUANTITATIVE  ANALYSIS 

delivery  tube  is  above  the  liquid,  then  remove  the  flame.  Disconnect 
the  delivery  tube  from  the  condenser  and  rinse  inside  and  outside,  allow- 
ing the  rinsings  to  run  into  the  flask.  Add  enough  methyl  red  to  tint  the 
solution,  then  titrate  with  standard  base.  Subtract  the  excess  of  acid 
thus  indicated  and  calculate  the  percent  of  nitrogen  in  the  sample, 
making  proper  correction  for  any  nitrogen  found  in  the  reagents  by 
the  blank  determination  with  sugar. 

Modifications  to  Include  the  Nitrogen  of  Nitrates.  —  It  has 
already  been  noted  that  most  of  the  nitrogen  of  nitrates  is  lost 
by  directly  heating  with  sulphuric  acid.  Asboth1  modified  the 
Kjeldahl  method  by  adding  benzoic  acid,  nitrobenzoic  acid  being 
formed  and  later  oxidized  by  potassium  permanganate.  Jodl- 
bauer2  substituted  phenolsulphonic  acid  for  benzoic  acid  and 
reduced  the  resultant  nitrophenolsulphonic  acids  to  aminophenol- 
sulphonic  acids  by  zinc  dust.  The  amino  compound  was  then 
oxidized  by  heating  with  sulphuric  acid.  The  addition  of  phos- 
phoric acid  was  also  found  to  hasten  the  oxidation.  Both  ben- 
zoic acid  and  phenolsulphonic  acid  are  now  generally  substituted 
by  salicylic  acid  and  the  reducing  agent  for  the  nitro  compound 
is  either  zinc  dust  or  sodium  thiosulphate.  The  reactions  may 
be  represented  as  follows: 

2KN03+H2S04^K2S04+2HN03, 


,  /OH 

HN03+C6H4<(  ->C6H3-COOH+H20. 

COOH  \N02 

The  nitro  compound  is  then  reduced  by  nascent  hydrogen: 

/OH  /OH 

6H+C6H3-CbOH->C6H3--COOH+2H20, 
\N02  \NH2 

or  by  sodium  thiosulphate: 

Na2S203+H2S04->Na2S04+H2S03+S, 

/OH  /OH 

6H2SO3+2C6H3-COOH+'2H2O-»6H2SO4+2C6H3-COOH. 
\N02  \NH2 

iChem.  Zentr.,  [3]  17,  161  (1886). 

2  Ibid.,  [3]  17,  433  (1886);  Z.  anal.  Chem.,  26,  92  (1887). 


AGRICULTURAL  MATERIALS  519 

The  oxidation  of  the  amino-salicylic  acid  by  sulphuric  acid  is 
not  well  enough  understood  to  be  represented  by  an  equation. 

Determination  by  the  Kjeldahl  Method,  Modified  to  Include  Nitro- 
gen of  Nitrates. — Weigh  the  sample  of  fertilizer  and  place  in  a  digestion 
flask.  The  quantity  to  be  used  is  determined  as  in  the  plain  Kjeldahl 
method,  (a)  Add  30  cc  of  concentrated  sulphuric  acid  to  which  has 
been  added  1  gm  of  salicylic  acid  and  mix  by  shaking.  After  30  minutes 
add  5  gm  of  sodium  thiosulphate  or  (b)  add  to  the  substance  30  cc 
of  concentrated  sulphuric  acid  containing  2  gm  of  salicylic  acid,  allow 
to  stand  30  minutes  and  add  2  gm  of  zinc  dust,  shaking.  Heat  gently 
until  frothing  has  ceased  then  boil  until  white  fumes  are  no  longer  found. 
Add  about  0.7  gm  of  mercury  or  of  mercuric  oxide  and  continue  the 
digestion,  distillation  and  titration  as  in  the  Kjeldahl  method.  Make 
blank  determinations  of  nitrogen  in  the  reagents.  Calculate  the  per- 
cent of  nitrogen  in  the  fertilizer. 

Gunning  Method. — It  was  observed  by  Gunning1  that  in  the 
ordinary  Kjeldahl  process  the  water  produced  by  the  oxidation 
of  organic  matter  dilutes  the  sulphuric  acid  and  retards  its 
action.  Gunning  proposed  the  addition  of  potassium  sul- 
phate which  forms  acid  sulphates  which  lose  water  much  more 
readily  than  the  hydrates  of  sulphuric  acid  so  that  the  solution 
does  not  become  diluted.  A  mixture  of  one  part  of  potassium 
sulphate  and  two  parts  of  sulphuric  acid  is  heated  together  and 
finally  allowed  to  cool.  This  mixture  is  measured  into  the  diges- 
tion flask,  where  the  digestion  is  performed  as  in  the  Kjeldahl 
process  except  that  no  mercury  is  added  and,  consequently,  no 
potassium  sulphide  is  needed  before  the  distillation.  In  the 
method  as  now  carried  out  the  required  amounts  of  potassium 
sulphate  and  sulphuric  acid  are  added  directly  to  the  flask 
without  preliminary  heating. 

Determination. — Calculate  the  weight  of  sample  required,  as  in 
the  Kjeldahl  method,  and  weigh  this  amount  into  digestion  flasks. 
Add  to  the  sample  in  the  digestion  flask  10  gm  of  powdered  potassium 
sulphate  and  15  to  25  cc  of  concentrated  sulphuric  acid.  Digest  as  in 
the  Kjeldahl  process  except  that  0.3  gm  of  copper  sulphate  is  used  in- 
stead of  mercury,  mercuric  oxide  or  potassium  permanganate.  When 
the  solution  is  colorless,  cool,  dilute  and  conduct  the  distillation  as  in  the 
Kjeldahl  process,  omitting,  however,  the  potassium  sulphide  solution. 

iZ.  anal.  Chem.,  28,  188  U889). 


520  QUANTITATIVE  ANALYSIS 

Make  a  blank  determination  as  in  the  Kjeldahl  process.     Calculate  the 
percent  of  nitrogen  in  the  sample. 

Determination  by  the  Gunning  Method,  Modified  to  Include  Nitrogen 
of  Nitrates. — To  the  weighed  sample  in  a  digestion  flask  add  30  cc  of 
concentrated  sulphuric  acid  containing  1  gm  of  salicylic  acid,  mix  and 
allow  to  stand  for  10  minutes.  Add  5  gm  of  sodium  thiosulphate  and 
heat  for  5  minutes.  Cool,  add  10  gm  of  potassium  sulphate  and  heat 
until  nearly  or  quite  colorless.  Dilute,  distill  and  titrate  as  in  the  plain 
Gunning  method.  Make  a  blank  determination  of  nitrogen  in  the 
reagents.  Calculate  the  percent  of  nitrogen  in  the  fertilizer. 

Kjeldahl-Gunning- Arnold  Method. — This  method  of  digestion 
combines  the  accelerating  action  of  mercury  salts,  potassium 
sulphate  and  cupric  sulphate.  Otherwise  the  method  is  not 
essentially  different  from  those  already  described.  *It  is  not 
applicable  to  fertilizers  containing  nitrates. 

Determination. — Digest  the  usual  amount  of  sample  with  15  to  18  gm 
of  potassium  sulphate,  1  gm  of  cupric  sulphate,  1  gm  of  mercury  or 
mercuric  oxide  and  25  cc  of  concentrated  sulphuric  acid.  Heat  gently 
until  frothing  ceases,  then  boil  the  mixture  briskly  and  continue  the 
digestion  until  the  solution  is  colorless  or  nearly  so  or  until  oxidation 
is  complete.  Cool,  dilute  with  about  200  cc  of  water,  add  50  cc  of 
potassium  sulphide  solution  and  make  basic  and  distill  as  in  the  Kjel- 
dahl method. 

Nitrogen  of  Ammonia. — The  various  methods  already  de- 
scribed for  the  determination  of  nitrogen  give  only  the  total, 
making  no  distinction  between  nitrogen  in  different  forms.  It 
is  sometimes  desirable  to  know  the  relative  amounts  of  this 
element  existing  as  ammonium  salts,  nitrates  and  organic  com- 
pounds. The  official  magnesium  oxide  method  for  determining 
nitrogen  of  ammonium  salts  follows : 

Determination. — Place  the  weighed  sample  in  a  400  or  500  cc  distilling 
flask  (the  Kjeldahl  digestion  flask  is  a  good  substitute)  with  about  200  cc 
of  water  and  5  gm  or  more  of  magnesium  oxide,  free  from  carbonates. 
Connect  with  a  tin  condenser  and  distill  100  cc  of  the  liquid  into  a 
measured  volume  of  standard  acid  (50  cc  of  half-normal  acid  is  usually 
suitable).  Titrate  the  excess  of  acid  with  standard  base  and  calculate 
the  percent  of  ammoniacal  nitrogen  in  the  sample. 

For  the  determination  of  nitrates  and  ammonium  salts  to- 
gether the  Ulsch-Street  method  is  made  official. 


AGRICULTURAL  MATERIALS  521 

Determination.  Nitric  and  Ammoniacal  Nitrogen. — Place  1  gm  of 
the  sample  in  a  500-cc  flask,  add  about  30  cc  of  water  and  2  to  3  gm  of 
iron  reduced  by  hydrogen  and,  after  standing  sufficiently  long  to  insure 
solution  of  the  soluble  nitrates  and  ammonium  salts,  add  10  cc  of  a  mix- 
ture of  strong  sulphuric  acid  with  an  equal  volume  of  water;  shake  thor- 
oughly, place  a  long  funnel  in  the  neck  of  the  flask  to  prevent  mechan- 
ical loss  and  allow  to  stand  for  a  short  time  until  the  violence  of  the 
reaction  has  somewhat  moderated.  Heat  the  solution  slowly,  boil  for 
five  minutes  and  cool.  Add  about  100  cc  of  water,  a  little  paraffin  to 
prevent  foaming  and  7  to  10  gm  of  magnesium  oxide,  free  or  nearly  so 
from  carbonates.  Connect  with  the  tin  condenser  and  boil  for  40 
minutes,  or  nearly  to  dryness,  collecting  the  ammonia  in  50  cc  of  half- 
normal  acid.  Titrate  the  excess  of  acid  and  calculate  nitrogen  of  nitrates 
and  ammonia. 

If  the  sample  consists  of  nitrates  alone,  proceed  as  above,  except  that 
25  cc  of  the  nitrate  solution,  equivalent  to  0.25  gm  of  the  sample,  is 
used  and  5  gm  of  reduced  iron.  After  boiling  add  75  cc  of  water  and 
an  excess  of  saturated  sodium  hydroxide  solution  (instead  of  magnesium 
oxide)  and  distill  as  above  described. 

Availability  of  Nitrogen. — Mention  has  already  been  made  of 
the  low  fertilizing  value  of  certain  nitrogenous  materials  because 
of  the  slow  decomposition  that  results  when  the  fertilizer  is 
added  to  the  soil.  Nitrogen  is  probably  directly  assimilated  by 
plants  only  in  the  most  highly  oxidized  form,  i.e.,  that  of  nitrates. 
Ammonium  salts  and  certain  organic  materials,  such  as  dried 
blood,  have  almost  as  great  value  because  they  readily  decom- 
pose and  oxidize  in  the  soil,  forming  nitrates.  Hoofs,  hair, 
leather  and  hide  do  not  so  decompose,  except  very  slowly  and  a 
method  of  differentiating  between  available  and  non-available 
forms  of  nitrogen  is  desirable.  The  microscope  will  detect 
ground  hair  and  other  similar  materials  but  it  can  give  only 
qualitative  results.  Fortunately  qualitative  results  are  all  that 
are  necessary  in  states  where  the  addition  of  such  materials  is 
contrary  to  law,  but  for  scientific  purposes  a  quantitative  dis- 
tinction between  available  and  non-available  nitrogen  may  be  of 
great  practical  use.  An  exact  analytical  method  for  such  a  pur- 
pose seems  to  be  impossible  because  there  is  no  sharp  distinction 
to  be  made  between  the  classes  of  fertilizer  materials.  Great 
reliance  is  placed  upon  culture  experiments,  comparing  the  effect 
of  using  different  fertilizers  with  plants  under  otherwise  identical 


522  QUANTITATIVE  ANALYSIS 

conditions.  Such  experiments  are  slow  and  have  no  value  what- 
ever for  analytical  purposes.  An  approximate  distinction  can 
be  made  by  the  use  of  potassium  permanganate  in  either  neutral 
or  basic  solution.  Readily  decomposable  materials  are  oxidized 
and  the  nitrogen  is  converted  into  ammonia.  It  has  not  yet  been 
determined  how  much  reliance  is  to  be  placed  upon  these  methods 
but  they  have  been  adopted  as  official  methods  by  the  Associa- 
tion of  Official  Agricultural  Chemists.1 

Determination.  Organic  Nitrogen  Soluble  in  Neutral  Permanga- 
nate.— Make  a  preliminary  test  as  follows : 

Place  1  gm  of  the  material  upon  an  11  cm  filter  paper  and  wash  with 
water  at  room  temperature  until  the  filtrate  measures  250  cc.  Dry 
and  determine  nitrogen  in  the  residue  by  the  Kjeldahl,  Gunning  or 
Kjeldahl-Gunning-Arnold  method,  making  a  correction  for  the  nitrogen 
of  the  filter  paper  if  necessary. 

Place  a  weighed  quantity  of  the  fertilizer,  equivalent  to  50  mg  of  the 
water-insoluble  organic  nitrogen  as  determined  above,  on  a  moistened 
11  cm  filter  paper  and  wash  with  water  at  room  temperature  until  the 
filtrate  measures  250  cc.  Transfer  the  insoluble  residue  with  25  cc  of 
tepid  water  (at  about  30°)  to  a  300  cc  low-form  beaker,  add  1  gm  of 
sodium  carbonate,  mix  and  add  100  cc  of  2  percent  potassium  permanga- 
nate solution.  Cover  with  a  glass  and  immerse  for  30  minutes  in  a  water 
or  steam  bath  so  that  the  level  of  the  liquid  in  the  beaker  is  below  that  of 
the  heating  medium.  Keep  at  100°,  stirring  twice  at  intervals  of  10 
minutes  each.  At  the  end  of  this  time  remove  from  the  bath,  add 
immediately  100  cc  of  cold  water  and  filter  through  a  heavy  15  cm 
folded  filter.  Wash  with  small  quantities  of  cold  water  until  the  filtrate 
measures  about  400  cc.  Determine  nitrogen  in  the  residue  and  filter 
by  either  of  the  three  official  methods  already  described,  correcting  for 
the  nitrogen  contained  in  the  filter.  The  nitrogen  thus  obtained  is  the 
inactive  water-insoluble  organic  nitrogen.  Subtract  this  percent  from 
that  obtained  in  the  preliminary  test.  The  remainder  is  the  percent  of 
organic  nitrogen  soluble  in  neutral  permanganate.  As  already  explained, 
this  is  an  approximate  measure  of  nitrogen  easily  available  for  plant  food. 

Determination.  Organic  Nitrogen  Soluble  in  Basic  Permanganate. — 
This  method  is  not  applicable  to  fertilizers  containing  cottonseed  meal  or 
castor  pomace.  Prepare  the  sample  as  follows: 

(a)  Mixed  Fertilizers. — Make  a  preliminary  test  as  directed  in  the 
neutral  permanganate  method.  Place  an  amount  of  material,  equiva- 

i  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  I,  No.  4,  Pt.  II,  p.  11. 


AGRICULTURAL  MATERIALS  523 

lent  to  50  mg  of  water-insoluble  organic  nitrogen,  on  a  filter  paper  and 
wash  with  water  at  room  temperature  until  the  filtrate  measures  250  cc. 

(b)  Raw  Materials. — Make  the  determination  of  water-insoluble 
organic  nitrogen  as  with  mixed  fertilizers.  Place  an  amount  of  material 
equivalent  to  50  mg  of  this  nitrogen,  in  a  small  mortar,  add  about  2  gm 
of  powdered  rock  phosphate  (to  facilitate  the  washing  process),  mix 
thoroughly  and  transfer  to  a  filter  paper.  Wash  with  water  at  room 
temperature  until  the  filtrate  measures  250  cc.  When  much  oil  or  fat 
is  present  it  is  well  to  wash  with  ether  and  allow  to  stand  until  the  odor 
of  the  latter  has  disappeared  before  washing  with  water. 

Dry  the  residue  from  either  class  of  materials  at  a  temperature  not 
exceeding  80°  and  transfer  from  the  filter  to  a  500  cc  Kjeldahl  digestion 
flask.  Add  20  cc  of  water,  about  1  gm  of  powdered  porcelain  to  prevent 
bumping  and  about  1  gm  of  paraffin  to  prevent  frothing.  A  solution 
of  potassium  permanganate  is  made  by  dissolving  25  gm  in  about  100  cc 
of  water.  Also  dissolve  150  gm  of  sodium  hydroxide  in  500  cc  of  water 
and,  after  this  has  cooled,  mix  with  the  potassium  permanganate  solu- 
tion and  dilute  the  whole  to  1000  cc.  100  cc  of  this  basic  permanganate 
solution  is  added  to  the  flask  containing  the  fertilizer  and  this  is  then 
connected  with  the  tin  condenser,  the  lower  end  of  which  dips  into  50  cc 
of  half-normal  acid. 

Digest  slowly  for  at  least  30  minutes,  below  distillation  point,  with  a 
very  low  flame,  using  coarse  wire  gauze  and  asbestos  paper  between  the 
flask  and  flame.  Gradually  raise  the  temperature  and,  after  any  danger 
of  frothing  has  passed,  distill  until  95  cc  of  the  distillate  is  obtained  and 
titrate  as  usual.  When  a  tendency  to  froth  is  noticed  lengthen  the 
digestion  period  and  no  trouble  will  be  experienced  when  the  distillation 
is  begun.  During  the  digestion  gently  rotate  the  flask  occasionally, 
particularly  if  the  material  shows  a  tendency  to  adhere  to  the  sides  of  the 
flask. 

The  nitrogen  thus  obtained  is  the  active  water-insoluble  organic 
nitrogen. 

Phosphorus. — The  most  important  sources  of  phosphorus  for 
fertilizing  purposes  are  mineral  phosphates  (chiefly  apatite,  which 
is  calcium  orthophosphate,  Ca3(PO4)2,  but  contains  some  mag- 
nesium phosphate)  ground  raw  and  steamed  bone,  slag  from 
basic  Bessemer  steel  furnaces  (known  as  " Thomas  slag")  and,  to 
a  less  extent,  fish  scrap,  oil  cake  and  tankage. 

The  normal  calcium  orthophosphate  of  mineral  deposits  has 
a  very  small  solubility  in  water  or  in  any  salt  or  acid  solutions 
commonly  occurring  in  soils.  It  is,  in  consequence,  generally 


524  QUANTITATIVE  ANALYSIS 

considered  as  a  source  of  non-available  phosphorus.  In  order 
to  change  it  to  a  soluble  form  so  that  it  may  be  used  as  a  fer- 
tilizer, the  mineral  phosphate  is  treated  with  sulphuric  acid, 
there  being  formed  calcium  acid  phosphates  and  sometimes  free 
phosphoric  acid  if  an  excess  of  sulphuric  acid  has  been  used: 

Ca3(P04)2+H2S04->CaS04+2CaHP04, 

Ca3(P04)2+2H2SO4-»2CaS04+Ca(H2P04)2, 

Ca3(P04)2+3H2S04-*3CaS04+2H3P04. 

Normal  calcium  phosphate  (tricalcium  phosphate)  is  almost 
insoluble  in  water.  Di calcium  phosphate,  CaHP04,  dissolves 
in  water  to  the  extent  of  only  0.136  gm  in  1000  cc  at  25°,  but  dis- 
solves quite  easily  in  certain  salt  solutions,  as  ammoniiim  citrate. 
Monocalcium  phosphate,  Ca(H2P04)2,  is  easily  soluble  in  water. 
Salt  solutions  existing  in  soils  dissolve  dicalcium  phosphate  in  a 
manner  similar  to  that  shown  by  ammonium  citrate  solution. 
On  this  account  the  analyst  speaks  of  "  water  soluble,"  "  citrate 
soluble,''  and  " insoluble"  phosphate.  Water  soluble  and  citrate 
soluble  forms  are  taken  together  as  "available"  phosphate  so 
that  a  distinction  between  the  two  forms  composing  this  class  is 
now  seldom  required. 

When  a  mixture  of  the  compounds  formed  by  "  acidulating" 
phosphate  rock  is  allowed  to  stand  a  reaction  occurs  between 
normal  calcium  phosphate  and  monocalcium  phosphate: 

Ca3(P04)2+Ca(H2P04)2^4CaHP04. 

The  dicalcium  phosphate  formed  in  this  way  is  known  as  "  re- 
verted" phosphate.  This  term  is  gradually  falling  into  disuse 
because  it  is  not  possible  to  distinguish  between  truly  reverted 
phosphate  and  dicalcium  phosphate  formed  by  the  action  of 
sulphuric  acid  upon  the  normal  calcium  phosphate. 

The  phosphorus  of  bones  is  in  the  form  of  normal  calcium 
phosphate  but  it  is  in  a  condition  which  makes  it  possible  for 
soil  acids  to  readily  convert  it  into  acid  phosphates.  It  also 
dissolves  in  ammonium  citrate  solution  for  the  same  reason  and 
it  is  therefore  properly  classed  as  citrate  soluble  and  as  available. 

In  reporting  the  analysis  of  fertilizers  phosphorus  is  calculated 
as  phosphorus  pentoxide,  which  is  often  improperly  called  "phos- 


AGRICULTURAL  MATERIALS  525 

phoric  acid."  It  is  therefore  customary  to  speak  of  total  "phos- 
phoric acid,"  and  of  water  soluble,  citrate  soluble  and  available 
"phosphoric  acid,"  meaning  by  these  terms  phosphorus  in  the 
various  forms  already  described,  but  calculated  as  the  pentoxide. 
These  terms  will  not,  however,  be  used  in  the  following  paragraphs. 

Total  Phosphorus. — The  various  methods  for  the  determina- 
tion of  phosphorus  have  already  been  discussed  in  connection 
with  steel  analysis,  pages  452  to  457.  The  same  methods  are 
used  in  the  analysis  of  fertilizers  but  the  preliminary  treatment 
will,  of  course,  be  quite  different  from  that  of  steel.  This  treat- 
ment must  include  (a)  destruction  of  organic  matter  and  (b) 
solution  of  the  phosphate. 

The  details  of  the  following  methods  are  essentially  those  of 
the  "official"  methods  of  the  A.  0.  A.  C.  The  choice  of  method 
for  dissolving  the  sample  will  depend  upon  the  nature  of  the 
latter. 

Preparation  of  Solution. — Treat  2.5  gm  of  the  sample  by  one  of  the 
following  methods : 

(a)  Ignite  in  a  crucible  until  organic  matter  is  removed  and  dissolve 
in  hydrochloric  acid. 

(6)  Evaporate  with  5  cc  of  magnesium  nitrate  solution  made  as 
follows:  Dissolve  320  gm  of  calcined  magnesium  oxide  in  nitric  acid, 
avoiding  an  excess  of  the  latter;  add  a  little  calcined  magnesium  oxide 
in  excess,  boil,  filter  from  the  residue  and  dilute  to  2000  cc.  After 
evaporating  the  fertilizer  and  magnesium  nitrate  solution,  ignite  until 
organic  matter  is  removed  and  dissolve  in  hydrochloric  acid. 

(c)  Boil  with  20  to  30  cc  of  concentrated  sulphuric  acid  in  a  Kjeldahl 
flask,  adding  2  to  4  gm  of  sodium  nitrate  at  the  beginning  of  the  diges- 
tion and  a  small  quantity  after  the  solution  has  become  nearly  colorless, 
or  adding  the  nitrate  in  small  portions  from  time  to  time  during  the 
digestion.     After  the  solution  is  colorless  add  150  cc  of  water  and  boil 
for  a  few  minutes. 

(d)  Digest  in  a  Kjeldahl  flask  with  concentrated  sulphuric  acid  and 
such  other  reagents  as  are  used  in  either  the  plain  or  modified  Kjeldahl 
or  Gunning  method  for  the  determination  of  nitrogen.     Do  not  add  any 
potassium  permanganate  but,  after  the  solution  has  become  colorless, 
add  about  100  cc  of  water  and  boil  for  a  few  minutes. 

(e)  Dissolve  in  30  cc  of  concentrated  nitric  acid  and  5  cc  of  concen- 
trated hydrochloric  acid  and  boil  until  organic  matter  is  destroyed. 

(/)  Add  30  cc  of  concentrated  hydrochloric  acid,  heat  and  add  cau- 


526  QUANTITATIVE  ANALYSIS 

tiously,  in  small  quantities  at  a  time,  about  0.5  gm  of  finely  pulverized 
potassium  or  sodium  chlorate  to  destroy  organic  matter. 

(g)  Dissolve  in  15  to  30  cc  of  concentrated  hydrochloric  acid  and  3  to 
10  cc  of  concentrated  nitric  acid.  This  method  is  recommended  for 
fertilizers  containing  much  iron  or  aluminium  phosphate. 

After  the  sample  of  fertilizer  has  been  brought  into  solution  by  any 
of  the  methods  described  above  cool,  dilute  to  250  cc,  mix  and  pour  into 
a  dry  filter,  discarding  the  first  10  cc  of  the  filtrate  and  allowing  the 
remainder  to  run  into  a  dry  flask  which  can  be  stoppered. 

Gravimetric  Determination. — The  following  special  reagents  are 
necessary:  Ammonium  molybdate  solution,  ammonium  nitrate  solution 
and  "magnesia  mixture,"  all  to  be  prepared  as  directed  for  the  determi- 
nation of  phosphorus  in  steel,  page  456. 

Fill  a  dry  100  cc  volumetric  flask  with  the  phosphate  solution  and 
rinse  into  a  250  cc  flask  of  resistance  glass;  or  measure  50  cc  or  25  cc, 
according  to  the  percent  of  phosphorus  present,  by  means  of  a  pipette. 
Neutralize  with  ammonium  hydroxide  and  clear  with  a  few  drops  of 
nitric  acid,  thus  dissolving  the  small  amount  of  precipitated  hydroxides 
of  iron  and  aluminium.  In  case  hydrochloric  or  sulphuric  acid  has 
been  used  as  a  solvent  for  the  fertilizer  material  add  also  15  gm  of  dry 
ammonium  nitrate. 

To  the  hot  solution  add  ammonium  molybdate  solution,  about  70  cc 
for  each  decigram  of  phosphorus  pentoxide  that  is  thought  to  be  present. 
Immerse  in  water  and  digest  at  65°  for  an  hour  and  determine  whether 
the  phosphorus  has  been  completely  precipitated  by  adding  more  moly- 
bdate solution  to  the  clear,  supernatant  liquid.  If  more  precipitate 
forms  continue  the  digestion,  followed  by  testing  as  before.  Filter 
on  paper  and  wash  with  cold  water  or,  preferably,  ammonium  nitrate 
solution.  During  the  washing  the  precipitate  that  adheres  to  the  flask 
need  not  be  completely  removed  but  it  must  be  washed. 

Place  the  flask  in  which  precipitation  was  made  under  the  filter  and 
dissolve  the  precipitate  on  the  filter  in  concentrated  ammonium  hydrox- 
ide (using  as  little  as  possible)  followed  by  hot  water,  allowing  the  solu- 
tion to  run  into  the  flask,  thus  dissolving  the  adhering  precipitate. 
Wash  the  paper  very  thoroughly  with  hot  water.  Transfer  the  entire 
solution  and  washings  to  a  250  cc  beaker  of  resistance  glass.  The  total 
volume  of  the  solution  should  not  be  greater  than  100  cc.  Nearly 
neutralize  with  hydrochloric  acid,  the  reformation  of  the  yellow  pre- 
cipitate serving  as  indicator.  Redissolve  the  precipitate  that  finally 
forms,  by  the  addition  of  a  few  drops  of  dilute  ammonium  hydroxide. 
Cool  and  add,  very  slowly  and  with  vigorous  stirring,  25  cc  of  magnesia 
mixture.  After  15  minutes  add  ammonium  hydroxide  (specific  gravity 
0.90)  equal  to  one-ninth  of  the  total  volume  of  the  solution,  stirring  as 


AGRICULTURAL  MATERIALS  527 

this  is  added.  Cover  and  allow  to  stand  for  two  hours.  Filter  and  wash 
with  dilute  ammonium  hydroxide  (the  concentrated  solution  diluted  to 
ten  times  its  original  volume)  until  practically  free  from  chlorides,  as  shown 
by  acidifying  with  nitric  acid  and  adding  silver  nitrate  solution.  Dry 
the  filter  and  precipitate  and  transfer  the  latter  to  a  porcelain  crucible, 
previously  ignited  and  weighed.  Ignite  the  filter  separately  and  trans- 
fer its  ash,  when  white,  to  the  crucible  containing  the  main  precipitate. 
Ignite  to  whiteness  or  grayish  white  over  the  blast  lamp  or  Me*ker 
burner,  weigh  and  calculate  the  percent  of  phosphorus  pentoxide. 

Volumetric  Determination — Have  the  following  solutions  ready: 

(a)  Ammonium  Molybdate. — To  100  cc  of  the  molybdate  solution  that 
was  prepared  for  the  gravimetric  determination  of  phosphorus  add  5 
cc  of  concentrated  nitric  acid.  The  solution  should  be  filtered  imme- 
diately before  using. 

(6)  Fifth-normal  Sodium  Hydroxide. — Prepared  from  boiled  water. 

(c)  Standard  Hydrochloric  or  Nitric  Acid. — This  solution  should  be 
equivalent  in  strength  to  the  standard  base.  It  should  be  made  from 
previously  boiled  and  cooled  water  and  should  be  standardized  by  titra- 
tion  against  sodium  carbonate,  using  methyl  orange  as  indicator. 

The  fertilizer  is  dissolved  by  either  of  methods  (6),  (e),  (/)  or  (0). 
Method  (e)  is  to  be  preferred  if  the  material  will  yield  to  this  treatment. 
The  solution  is  to  be  diluted  and  filtered  as  already  directed. 

In  the  case  of  fertilizers  containing  less  than  5  percent  of  phosphorus 
pentoxide,  use  an  aliquot  corresponding  to  0.5  gm  of  substance.  If  the 
percentage  is  between  5  and  20  use  an  aliquot  corresponding  to  0.1  gm  of 
substance. 

Add  5  to  10  cc  of  concentrated  nitric  acid,  the  amount  depending  upon 
whether  this  acid  has  been  used  in  making  the  solution;  or  add  ammon- 
ium nitrate  equivalent  to  this  amount  of  nitric  acid.  Nearly  neutralize 
with  ammonium  hydroxide,  precipitation  of  hydroxide  of  iron  or  alu- 
minium serving  as  indicator.  Clear  with  a  drop  of  nitric  acid,  dilute  to 
about  100  cc  and  heat  by  immersing  in  water  to  60°  to  65°.  For  phos- 
phorus pentoxide  percents  below  5  add  20  to  25  cc  of  freshly  filtered 
molybdate  solution;  for  percentages  between  5  and  20  add  30  to  35  cc 
of  molybdate  solution.  For  percentages  greater  than  20  add  sufficient 
molybdate  solution  to  insure  complete  precipitation  of  the  phosphorus. 
Stir,  allow  to  stand  in  the  bath  for  fifteen  minutes  and  filter  at  once. 
Wash  twice  with  water  by  decantation,  using  25  to  30  nc  each  time  and 
agitating  and  settling  each  time  before  decanting.  Transfer  the  precip- 
itate to  the  filter  as  thoroughly  as  can  be  done  without  the  use  of  a 
policeman  and  wash  the  flask  and  filter  with  cold  water  until  the  filtrate 
from  two  fillings  of  the  filter  yields  a  pink  color  upon  the  addition  of 
phenolphthalein  and  one  drop  of  the  standard  base. 


528  QUANTITATIVE  ANALYSIS 

Return  the  filter  paper  and  precipitate  to  the  flask  in  which  pre- 
cipitation was  made.  Add  a  measured,  small  excess  of  the  standard  base 
to  dissolve  the  yellow  precipitate,  add  a  few  drops  of  phenolphthalein 
and  titrate  the  unused  excess  of  base  with  standard  acid.  Calculate 
the  percent  of  phosphorus  pentoxide  in  the  sample. 

The  following  changes  in  the  method  just  described  are  made  optional: 

(a)  Heat  the  solution  to  only  45°  to  50°  and  allow  to  stand  in  the  bath, 
after  the  addition  of  the  molybdate  solution  for  30  minutes. 

(6)  Cool  to  room  temperature  before  adding  the  molybdate  solution. 
Add  the  latter  at  the  rate  of  75  cc  for  each  decigram  of  phosphorus 
pentoxide  present,  place  the  stoppered  flask  containing  the  solution  in  a 
shaking  apparatus  and  shake  for  30  minutes  at  room  temperature.  Fil- 
ter at  once  and  proceed  as  already  directed. 

Water  Soluble  Phosphorus. — The  phosphorus  of  untreated 
phosphate  rock  and  of  bone  and  other  organic  sources  is  in- 
soluble in  water  and  this  determination  is  omitted.  In  "acidu- 
lated "  samples  it  is  sometimes  required  although,  as  has  already 
been  stated,  citrate  insoluble  phosphorus  subtracted  from  total 
phosphorus  gives  available  phosphorus  and  there  is  little  object 
in  making  the  determination  in  any  case.  Following  is  the 
"official"  method., 

Gravimetric  Determination. — Place  2  gm  of  the  sample  on  a  9-cm 
filter,  wash  with  successive  small  portions  of  water,  allowing  each  portion 
to  pass  through  before  adding  more,  until  the  volume  of  the  filtrate  is 
about  250  cc.  Preserve  the  residue  on  the  filter  for  the  determination 
of  citrate  insoluble  phosphorus.  If  the  filtrate  is  turbid  add  nitric  acid 
until  clear.  Dilute  to  500  cc  in  a  volumetric  flask,  mix  and  determine 
the  phosphorus  in  50  or  100  cc  portions  by  the  method  above  described 
for  total  phosphorus.  Calculate  the  percent  of  water  soluble  phos- 
phorus, expressed  as  phosphorus  pentoxide. 

Volumetric  Determination. — Wash  a  2-gm  or  4-gm  sample  as  directed 
for  the  gravimetric  method  and  dilute  the  filtrate  to  500  cc.  To  50  cc 
portions  add  10  cc  of  concentrated  nitric  acid,  then  ammonium  hydrox- 
ide until  a  slight  permanent  precipitate  is  formed.  The  volume  should 
now  be  between  60  cc  and  75  cc.  Proceed  from  this  point  as  in  the  volu- 
metric determination  of  total  phosphorus. 

Citrate  Insoluble  Phosphorus. — Phosphates  that  are  insoluble 
in  neutral  ammonium  citrate  solution,  and  therefore  in  soil 
solutions,  are  so  slowly  assimilated  by  plants  that  they  are  often 
considered  to  be  of  small  value  as  fertilizers.  The  greatest  diffi- 


AGRICULTURAL  MATERIALS  529 

culty  exists,  however,  in  making  an  accurate  determination  of 
insoluble  phosphate  because  of  the  difficulty  that  is  encountered 
in  the  preparation  of  a  neutral  ammonium  citrate  solution.  In 
the  discussion  of  indicators  it  was  shown  that  no  indicator  is 
sufficiently  sensitive  to  both  acids  and  bases  as  to  indicate  accu- 
rately the  point  of  neutralization  if  both  acid  and  base  are  weakly 
ionized.  It  is  therefore  extremely  difficult  to  neutralize  exactly 
the  weak  citric  acid  by  the  weak  base,  ammonium  hydroxide, 
with  the  aid  of  any  organic  indicator.  The  action  of  ammonium 
citrate  upon  dicalcium  phosphate  is  due  to  the  presence  of 
citric  acid  of  hydrolysis: 


It  is  therefore  highly  important  that  the  concentration  of  citric 
acid  in  the  solution  should  be  the  same  in  all  cases  if  the  ana- 
lytical results  are  to  possess  any  significance,  as  the  action  of  the 
solution  is,  at  best,  but  an  arbitrary  and  approximate  imitation 
of  the  action  of  solutions  found  in  soils. 

Ammonium  Citrate  Solution.  —  Two  methods  for  the  prepara- 
tion of  neutral  ammonium  citrate  solution  are  approved  by  the 
A.  O.  A.  C.  The  first  method  is  that  of  neutralizing  a  stated 
quantity  of  citric  acid  in  solution  by  ammonium  hydroxide,  the 
indicator  being  litmus  or  azolitmin  paper.  In  the  second  method 
the  solution  is  nearly  neutralized  and  then  a  measured  volume  of 
a  solution  of  calcium  chloride  in  water  and  alcohol  is  added. 
Calcium  citrate  is  at  once  precipitated.  If  the  solution  was 
neutral,  ammonium  chloride  is  the  only  other  product  of  the 
reaction;  if  an  excess  of  citric  acid  was  present  hydrochloric 
acid  is  also  produced: 


If  the  solution  was  basic  instead  of  neutral  or  acid,  ammonium 
hydroxide  remains  after  the  precipitation.  By  testing  the  solu- 
tion with  cochineal  after  .filtration  and  then  adding  either  am- 
monium hydroxide  or  citric  acid,  as  is  indicated  as  being  neces- 
sary by  the  reaction  with  cochineal,  and  by  repeating  this  process 
as  often  as  is  necessary,  the  solution  may  finally  be  brought  to  a 
neutral  condition.  The  advantage  of  this  method  over  the  first 

34 


530  QUANTITATIVE  ANALYSIS 

official  method  is  in  the  substitution  of  an  equivalent  amount  of 
a  strong  acid  (hydrochloric  acid)  for  the  weak  citric  acid,  so 
that  an  indicator  may  now  be  chosen  of  sufficient  sensibility 
toward  ammonium  hydroxide  to  give  indication  of  real  neu- 
trality. Even  by  this  method,  however,  the  ratio  of  ammonia 
to  citric  acid  is  found  to  vary.  McCandless1  found  that  this  ratio 
for  solutions  made  by  nine  different  chemists  varied  between 
the  limits  1 : 3. 775  and  1 :4.189.  The  calculated  ratio  for  normal 
ammonium  citrate  is  1:3.766.  The  excess  of  acid  indicated  by 
the  above  ratios  would  give  the  solution  a  greater  solvent  power 
for  calcium  phosphate  and  would  give  rise  to  incorrect  results  in 
the  determination  of  insoluble  phosphorus. 

Patten  and  Marti  have  devised  a  method2  which  they  call  the 
"titration  method"  and  have  shown  that  strictly  rreutral  solu- 
tions can  easily  be  made.  The  solution  is  first  made  approxi- 
mately neutral  and  then  both  ammonia  and  citric  acid  are  deter- 
mined. The  determination  of  ammonia  is  made  by  treating 
5  cc  of  the  solution  with  magnesium  oxide  and  distilling  the 
ammonia  into  standard  acid: 

2(NH4)3C6H507+3Mg(OH)2->Mg3(C6H507)2+6NH3+6H20. 

The  excess  of  standard  acid  is  titrated  as  in  the  Kjeldahl  method 
for  the  determination  of  nitrogen.  The  determination  of  total 
citric  acid  in  the  solution  depends  upon  the  reaction  of  formalde- 
hyde with  ammonia,  either  free  or  combined  in  salts,  to  form 
hexamethylenetetramine,  a  substance  so  weakly  basic  that  it 
does  not  affect  phenolphthalein.  The  reaction  is  represented  by 
the  following  equations: 

4NH4OH+6HCHO->N(CH2NCH2)3+10H20, 
3(NH4)3C6H507+18HCH0^3N(CH2KCH2)3+18H204- 

4H3C6H507. 

The  free  citric  acid  is  then  titrated  by  a  standard  solution  of  a 
strong  base,  as  sodium  hydroxide,  in  presence  of  phenolphthalein. 
This  device  is  somewhat  similar  to  that  used  in  the  calcium 
chloride  method  where  the  weak  acid  of  the  citrate  solution  is 

1 U.  S.  Dept,  Agr.,  Bur.  Chem.,  Bull.  122,  147. 
2  J.  Ind.  Eng.  Chem.,  5,  567  (1913). 


AGRICULTURAL  MATERIALS  531 

substituted  by  a  strong  acid  and  the  question  of  neutrality  is 
qualitatively  decided  by  the  use  of  an  indicator  that  is  sensitive 
-to  weak  bases.  In  the  Patten  and  Marti  method  the  weak 
base  of  the  citrate  solution  is  destroyed  and  the  remaining  acid 
is  titrated  by  a  strong  base,  using  an  indicator  that  is  sensitive  to 
weak  acids. 

The  ratio  of  ammonia  to  citric  acid  now  having  been  quantita- 
tively determined  the  solution  is  exactly  neutralized  by  the  addi- 
tion of  the  calculated  quantity  of  either  ammonium  hydroxide 
or  citric  acid. 

Determination. — In  addition  to  the  solutions  used  for  the  determina- 
tion of  total  phosphorus,  prepare  a  neutral  solution  of  ammonium 
citrate  by  either  of  the  following  methods : 

(a)  Dissolve  370  gm  of  commercial  citric  acid  in  1500  cc  of  water  and 
nearly  neutralize  with  commercial  ammonium  hydroxide.  Cool  and 
add  dilute  ammonium  hydroxide  until  exactly  neutral,  testing  with 
litmus  or  azolitmin  paper.  Dilute  the  neutral  solution  until  the  specific 
gravity  is  1.09  at  20°  C. 

.(&)  To  370  gm  of  commercial  citric  acid  add  commercial  ammonium 
hydroxide  until  nearly  neutral.  Reduce  the  specific  gravity  by  dilution 
until  it  is  slightly  greater  than  1.09  at  20°  and  make  exactly  neutral, 
testing  as  follows :  Prepare  a  solution  of  fused  calcium  chloride  in  water, 
200  gm  to  each  liter,  and  add  4000  cc  of  95  percent  alcohol.  Make  this 
solution  neutral,  using  a  freshly  prepared  corallin  solution  as  preliminary 
indicator,  and  test  finally  by  withdrawing  a  portion,  diluting  with  an 
equal  volume  of  water  and  testing  with  cochineal  solution.  50  cc  of  this 
calcium  chloride  will  precipitate  the  citric  acid  from  10  cc  of  the  citrate 
solution.  To  10  cc  of  the  nearly  neutral  citrate  solution  add  50  cc  of  the 
alcoholic  calcium  chloride  solution,  stir  well,  filter  at  once  through  a 
folded  filter,  dilute  the  filtrate  with  an  equal  volume  of  water  and  test 
the  reaction  with  a  neutral  solution  of  cochineal.  If  the  citrate  solution 
is  shown  not  to  be  neutral,  carefully  add  ammonium  hydroxide  or  citric 
acid,  as  may  be  necessary,  to  the  solution,  mix  and  test  again.  Repeat 
this  process  until  a  neutral  reaction  is  obtained.  Add  sufficient  water 
to  reduce  the  specific  gravity  to  1.09  at  20°. 

(c)  Method  of  Patten  and  Matfi. — Dissolve  370  gm  of  commercial 
citric  acid  in  1500  cc  of  water;  add  358.3  cc  of  ammonium  hydroxide  (sp. 
gr.  0.90)  and  allow  to  cool.  Measure  50  cc  of  this  solution  into  a  250-cc 
volumetric  fl  ask,  dilute  to  the  mark  and  mix  thoroughly.  From  a  burette 
measure  5  cc  of  the  diluted  solution  into  a  beaker,  add  4  cc  of  a  neutral 
40-percent  solution  of  formaldehyde  and  titrate  with  tenth-normal 


532  QUANTITATIVE  ANALYSIS 

sodium  hydroxide  or  potassium  hydroxide  solution  in  presence  of 
phenolphthalein.  The  pink  color  should  remain  after  the  solution  is 
boiled;  if  it  does  not  the  ammonia  has  not  been  entirely  decomposed 
and  another  titration  should  be  made,  using  more  formaldehyde. 
Determine  the  total  (free  and  combined)  ammonia  in  the  solution  as 
follows:  Carefully  measure  5  cc  of  the  diluted  solution  into  a  500-cc 
Kjeldahl  digestion  flask,  add  0.5  gm  of  magnesium  oxide  and  at  once 
distill  into  50  cc  of  fifth-normal  acid.  Titrate  the  excess  of  acid 
using  cochineal  as  indicator. 

From  the  titration  of  citric  acid  and  of  ammonia  calculate  the  amount 
of  citric  acid  or  of  a  standard  solution  of  ammonium  hydroxide  that  must 
be  added  to  1450  cc  of  the  stronger  solution  of  ammonium  citrate  in 
order  to  make  an  exactly  neutral  solution.  After  neutralization  dilute 
the  solution  to  2000  cc.  The  specific  gravity  of  the  solution  as  finally 
diluted  should  be  1.09. 

Acidulated  Samples. — Place  100  cc  of  ammonium  citrate  solution  in  a 
250  cc  flask  which  is  immersed  in  warm  water.  Heat  to  65°  then  drop 
into  the  solution  the  paper  containing  the  washed  residue  from  the 
determination  of  water-soluble  phosphorus,  close  the  flask  with  a  rubber 
stopper  and  shake  vigorously  until  the  paper  is  reduced  to  a  pulp. 
(If  a  determination  of  water  soluble  phosphorus  has  not  been  required 
the  untreated  sample  may  be  used  for  the  determination  of  citrate 
insoluble  phosphorus.)  Replace  the  flask  in  the  water  bath  and  keep  at 
65°,  shaking  every  5  minutes.  At  the  expiration  of  exactly  30  minutes 
from  the  time  that  the  material  was  introduced  remove  the  flask  from 
the  bath  and  filter  the  contents  as  rapidly  as  possible.  Wash  thoroughly 
with  water  which  is  at  a  temperature  of  65°  until  the  filtrate  measures 
about  350  cc,  allowing  time  for  thorough  draining  before  adding  new 
portions  of  water;  then  determine  nitrogen  in  the  residue  by  either  of 
the  methods  used  for  total  phosphorus,  using  any  of  methods  (a),  (6), 
(c),  (d)  or  (e)  for  dissolving  the  extracted  material.  From  the  percent 
thus  obtained  for  citrate  insoluble  phosphorus  and  the  percent  of  total 
phosphorus  calculate  the  percent  of  available  phosphorus.  Also,  if  water 
soluble  phosphorus  has  been  determined,  calculate  the  percent  of  citrate 
soluble  phosphorus.  Express  all  results  as  percents  of  phosphorus 
pentoxide. 

Non-acidulated  Samples. — Treat  2  gm  of  the  sample,  without  pre- 
vious washing  with  water,  as  directed  for  acidulated  samples,  except 
when  the  material  contains  much  animal  matter,  such  as  bone,  fish,  etc., 
in  which  case  dissolve  the  residue  which  is  insoluble  in  ammonium 
citrate  by  either  of  methods  (6),  (c)  or  (d).  Determine  phosphorus  by 
gravimetric  or  volumetric  methods,  already  described. 


AGRICULTURAL  MATERIALS  533 

Potassium. — All  soils  contain  certain  minerals,  one  component 
of  which  is  potassium,  and  these  are  continually  undergoing 
decomposition  yielding  potassium  in  a  soluble  form.  The 
most  important  of  such  minerals  belong  to  the  class  of  felspars. 
Soils  are  frequently  deficient  in  potassium  and  require  fertiliza- 
tion with  some  material  containing  this  metal.  It  is  interesting 
to*  note  that  sodium,  so  nearly  allied  to  potassium  in  chemical 
properties,  is  able  to  replace  it  in  the  plant  organism  very  little, 
if  at  all. 

The  various  potassium  fertilizers  may  be  sold  singly  or  they 
may  be  components  of  mixed  fertilizers.  Organic  matter  may 
or  may  not  be  present.  If  it  is  present  it  must  be  destroyed 
by  oxidation  and  the  earth  and  alkaline  earth  metals  must  be 
separated.  The  separation  of  organic  matter  and  interfering 
metals  and  the  determination  of  potassium  are  best  accomplished 
by  the  Lindo-Gladding  method,  the  principles  of  which  have 
already  been  discussed  (page  102).  The  reagents  there  described 
are  necessary  in  this  connection.  Following  are  the  official 
methods  of  the  A.  O.  A.  C.1 

Determination. — Prepare  the  potassium  solution  from  the  fertilizer 
as  follows: 

(a)  Mixed  Fertilizers,  Wood  Ashes  and  Cotton  Hull  Ashes. — Place  2.5 
gm  of  sample  upon  a  12.5  cm  filter  paper  and  wash  with  boiling  water  until 
the  nitrate  measures  about  200  cc.  Add  to  the  filtrate  2  cc  of  concentra- 
ted hydrochloric  acid,  heat  to  boiling  and  transfer  to  a  250  cc  volumetric 
flask.  Add  to  the  hot  solution  a  slight  excess  of  ammonium  hydroxide 
and  enough  ammonium  oxalate  to  precipitate  all  of  the  calcium.  Cool 
to  20°,  dilute  to  the  mark  on  the  flask,  mix  and  pass  through  a  dry  filter, 
rejecting  the  first  25  cc  of  the  filtrate. 

(6)  Potassium  Salts,  Potassium  Magnesium  Sulphate  and  Kainite. — 
Dissolve  2.5  gm  of  sample  in  a  250  cc  volumetric  flask  and  dilute  to  the 
mark  without  the  addition  of  ammonium  hydroxide  or  ammonium 
oxalate. 

(c)  Organic  Compounds:  Cottonseed  Meal,  Tobacco  Stems,  Etc. — Satu- 
rate 10  gm  of  sample  with  concentrated  sulphuric  acid  and  evaporate 
and  ignite  at  dull  redness  to  destroy  organic  matter.  A  muffle  furnace 
will  be  found  to  be  convenient  for  this  operation.  Add  a  little  concen- 
trated hydrochloric  acid  and  warm  slightly  in  order  to  loosen  the  mass 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  I,  No.  4,  Pt.  II,  p.  12. 


534  QUANTITATIVE  ANALYSIS 

from  the  dish.     Dissolve  in  hot  water,  cool  and  dilute  to  250  cc.     Mix 
thoroughly  in  the  flask. 

For  each  class  of  materials  two  methods  for  the  determination  are 
official.  For  both  of  these  the  following  special  reagents  will  be  nec- 
essary. 

(a)  Ammonium  Chloride  Solution. — Dissolve  100  gm  of  ammonium 
chloride  in  500  cc  of  water,  add  10  gm  of  powdered  potassium  chlor- 
platinate,  warm  slightly  and  shake  at  intervals  for  6  to  8  hours.     Allow 
the  mixture  to  settle  for  several  hours  and  filter.     The  residue  of  potas- 
sium chlorplatinate  may  be  used  for  the  preparation  of  a  fresh  supply. 

(b)  Chlorplatinic  Acid  Solution. — This  solution  should  contain  1  gm 
of  platinum  in  each  10  cc. 

(c)  Alcohol. — An  80  percent  solution,  specific  gravity  0.8645  at  15°/15°. 
Denatured  alcohol,  made  according  to  formula  1,  (U.  S.  Internal  Rev., 
Reg.  No.  30,  Revised  Aug.  22,  1911,  p.  45)  and  diluted  "to  make  80 
percent  alcohol  by  volume,  may  also  be  used. 

Method  I  (Lindo-Gladding) 

(a)  Mixed  Fertilizers,  Wood  Ashes  and  Cotton  Hull  Ashes. — Evaporate 
50  cc  of  the  prepared  solution  nearly  to  dryness  in  a  dish,  add  1  cc  of 
sulphuric  acid  (1  to  1),  evaporate  to  dryness  and  ignite  at  a  full  red 
heat  until  organic  matter  is  removed  and  the  residue  is  white.  Dissolve 
the  residue  in  hot  water,  using  at  least  20  cc  for  each  decigram  of  potas- 
sium oxide  present,  add  a  few  drops  of  concentrated  hydrochloric  acid 
and  enough  chlorplatinic  acid  to  precipitate  all  of  the  potassium  and  to 
leave  about  1  cc  of  platinum  solution  in  excess.  If  the  percent  of  potas- 
sium is  approximately  known  the  quantity  of  platinum  solution  that  is 
necessary  should  be  calculated.  Contamination  with  ammonia  vapor 
must  be  avoided. 

Evaporate  the  solution  on  a  steam  bath  to  a  thick  paste  and  add  to  the 
residue  25  cc  of  80  percent  alcohol.  Stir  thoroughly  and  allow  to  stand 
for*  15  minutes.  Filter  through  a  Gooch  or  paper  filter.  If  the  filtrate 
is  not  colored  sufficient  chlorplatinic  acid  solution  is  not  present  and  the 
analysis  must  be  begun  again  with  another  portion  of  the  solution,  in- 
creasing the  amount  of  platinum  solution. 

Wash  the  precipitate  on  the  filter  with  80  percent  alcohol,  continuing 
the  washing  after  the  filtrate  has  become  colorless.  Remove  the  filtrate 
and  washings  to  the  bottle  which  has  been  provided  for  waste  platinum 
solutions  and  wash  the  precipitate  five  times  with  10  cc  portions  of  the 
ammonium  chloride  solution.  Wash  again  thoroughly  with  80  percent 
alcohol,  exercising  particular  care  to  remove  ammonium  chloride  from 
the  upper  part  of  the  filter.  Dry  the  precipitate  and  filter  for  30  min- 


AGRICULTURAL  MATERIALS  535 

utes  at  100°.  For  the  subsequent  treatment  in  case  a  paper  filter  has 
been  used,  see  page  102.  For  the  Gooch  filter  the  weight  of  potassium 
chlorplatinate  is  given  without  further  treatment. 

(6)  Commercial  Potassium  Chloride  ("Muriate  of  Potash"). — To  50  cc 
of  the  solution  already  prepared  add  a  few  drops  of  hydrochloric  acid 
and  10  cc  of  chlorplatinic  acid  solution.  Evaporate  over  a  steam  bath 
to  a  thick  paste  and  treat  the  residue  as  in  the  case  of  mixed  fertilizers. 

(c)  Potassium  Sulphate,  Potassium  Magnesium  Sulphate  and  Kainite. — 
Acidify  50  cc  of  the  solution  with  a  few  drops  of  hydrochloric  acid,  add 
15  cc  of  chlorplatinic  acid  solution  and  evaporate  on  the  steam  bath  to  a 
thick  paste.  From  this,  point  proceed  as  with  mixed  fertilizers,  except 
that  25  cc  portions  of  the  ammonium  chloride  solution  should  be  used 
in  the  washing  process. 

The  potassium  is  reported  as  percent  of  potassium  oxide  (often  called 
"potash")  instead  of  as  the  element. 

Method  II 

This  method  is  not  recommended  for  materials  containing  soluble 
sulphates,  therefore  it  is  practically  restricted  to  certain  mixed  fertilizers 
and  to  potassium  chloride  and  nitrate.  The  required  reagents  are  the 
same  as  in  method  (/). 

The  solution  of  potassium  is  prepared  as  already  directed  except  that 
in  all  cases  the  addition  of  ammonium  hydroxide  and  ammonium  oxalate 
is  omitted. 

Dilute  25  cc  (50  cc  if  the  percent  of  potassium  oxide  is  less  than  10)  to 
150  cc,  heat  to  boiling  and  add,  drop  by  drop  and  with  constant  stirring, 
a  slight  excess  of  barium  chloride  solution  until  no  further  precipitate 
of  barium  sulphate  is  produced.  Without  filtering  add  in  the  same 
manner  barium  hydroxide  (saturated  solution)  in  slight  excess.  Filter 
while  hot  and  wash  until  the  washings  test  free  from  chlorides.  Add 
to  the  filtrate  1  cc  of  concentrated  ammonium  hydroxide  and  then 
a  saturated  solution  of  ammonium  carbonate  (prepared  without  heating) 
until  the  excess  of  barium  is  precipitated.  Heat  and  add,  in  fine  powder, 
0.5  gm  of  pure  oxalic  acid  or  0.75  gm  of  ammonium  oxalate.  This 
completes  the  precipitation  of  barium.  Filter,  wash  free  from  chlorides, 
evaporate  the  filtrate  to  dry  ness  in  a  platinum  dish  and  ignite  care- 
fully below  redness  until  all  volatile  ammonium  salts  are  driven  off. 

Digest  the  residue  with  hot  water,  filter  through  a  small  filter  and 
dilute  the  filtrate,  if  necessary,  so  that  for  each  decigram  of  potassium 
oxide  there  will  be  at  least  20  cc  of  liquid.  Acidify  with  a  few  drops  of 
hydrochloric  acid  and  add  an  excess  of  chlorplatinic  acid.  Evaporate 
on  a  steam  bath  to  a  thick  paste,  cool  and  add  25  cc  of  80  percent  alcohol. 
Filter  on  a  Gooch  or  paper  filter  and  wash  with  80  percent  alcohol 


536  QUANTITATIVE  ANALYSIS 

several  times  after  the  filtrate  is  colorless.  Dry  for  30  minutes  at  100° 
and  weigh.  If  there  is  an  appearance  of  any  white  salts  in  the  precipi- 
tate, washing  with  ammonium  chloride  solution,  followed  by  80  percent 
alcohol,  may  be  necessary,  as  described  for  method  (/). 

MILK 

The  milk  of  most  mammals  has  been  analyzed  and  its  compo- 
sition determined  but,  for  practical  purposes,  the  analyst  rarely 
has  to  do  with  any  other  than  cow's  milk  and  human  milk. 
The  analysis  of  cow's  milk  may  be  made  for  purely  scientific 
purposes  as,  for  instance,  the  determination  of  the  relation 
between  the  composition  of  milk  and  the  breed  of  animal,  the 
season  of  the  year  or  the  rations  upon  which  the  animal  is  fed, 
or  the  determination  of  the  changes  that  occur  in  composition 
during  the  period  of  storage,  and  other  similar  questions. 
The  analysis  may  also  be  made  for  purposes  of  legal  control 
to  detect  sophistication.  The  analysis  of  woman's  milk  is 
usually  made  for  hygienic  purposes,  in  order  to  provide  a  basis 
for  modification  of  the  mother's  diet,  etc.,  in  cases  where  the 
infant  is  not  thriving. 

The  percentage  composition  of  milk  varies  rather  widely 
although  the  same  substances  are  found  in  practically  all  milk 
from  a  given  species  of  animal.  It  is  therefore  not  possible  to 
fix,  by  legal  enactment,  the  exact  composition  of  milk  that  is 
to  become  an  article  of  commerce,  but  certain  minimum  figures 
are  usually  established  by  law  and  any  milk  containing  a  con- 
stituent in  quantity  below  the  legal  minimum  is  considered  to 
be  adulterated. 

The  average  composition  of  cow's  milk  is  given  by  Babcock 
in  the  table  on  the  following  page. 

Adulteration  of  Milk. — Adulteration  is  practised  by  skimming 
or  watering,  or  by  both  methods.  There  is  a  rather  widely 
disseminated  popular  belief  that  the  only  part  of  milk  that  has 
much  value  is  the  cream.  Many  municipalities  control  the  com- 
position of  milk  only  by  specifying  a  minimum  for  fat  and  the 
milk  inspection  will  often  include  little  else  than  a  determina- 
tion of  the  percent  of  fat.  Milk  containing  a  high  percent  of 
butter  fat,  such  as  that  from  Jersey  cows,  could  then  be  watered 
in  such  a  way  as  to  leave  the  legal  minimum  of  fat  but  the 
solids  not  fat  would  thereby  fle  lowered. 


AGRICULTURAL  MATERIALS 


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538  QUANTITATIVE  ANALYSIS 

Proper  control  of  adulteration  can  be  secured  by  a  considera- 
tion of  the  relations  between  total  solids,  solids  not  fat  and  fat. 
Watering  has  the  effect  of  lowering  the  percent  of  total  solids, 
solids  not  fat  and  fat,  proportionately.  Skimming  lowers  the 
percent  of  fat  and  total  solids  and  slightly  raises  the  percent  of 
solids  not  fat.  If  both  watering  and  skimming  are  practised 
the  ratio  of  solids  not  fat  to  total  solids  is  slightly  increased,  while 
the  ratio  of  fats  to  total  solids  is  abnormally  low,  all  three  per- 
cents  being  lowered. 

The  methods  for  the  analysis  of  dairy  products  as  used  in  most 
laboratories  are  the  official  and  provisional  methods  of  the  A. 
O.  A.  C.1  These  are  given,  with  certain  modifications,  in  the 
following  pages. 

It  should  be  noted  that  the  fat  of  milk  quickly  rises  when  the 
milk  is  quiet  and  the  sample  must  be  mixed  thoroughly  by  pour- 
ing from  one  vessel  to  another  immediately  before  the  removal 
of  any  sample  for  a  determination.  Violent  agitation  must  be 
avoided  as  this  will  result  in  coalescence  of  the  fat  globules. 

The  analysis  should  be  completed  within  the  shortest  possible 
time  after  the  sample  is  received,  in  order  to  avoid  the  inter- 
ference of  fermentative  or  putrefactive  changes.  If  it  is  impos- 
sible to  make  all  of  the  determinations  within  a  short  time  the 
sample  for  each  determination  should  be  removed  and  weighed, 
after  which  the  work  may  be  interrupted  for  a  reasonable  period 
at  the  following  stages  in  the  various  determinations: 

Determination        j         Stage  after  which  work  may  be  interrupted 


Specific  gravity 

Total  solids 

Ash 

Total  nitrogen 

Casein  and  albumin.  . 

Lactose 

Fat  (centrifugal) 

Fat  (gravimetric) 
Formaldehyde 


Must  be  completed  at  once 

Evaporation  of  sample 

Weighing  the  sample 

Addition  cf  sulphuric  acid 

Addition  of  sulphuric  acid  to  filtered  proteids 

Addition  of  mercuric  salt  solution 

Addition  of  sulphuric  acid 

Drying  of  paper  coil  with  milk 

Test  must  be  made  at  once. 


In  case  it  is  impossible  to  begin  any  of  the  determinations  while 
the  sample  is  fresh,  add  formaldehyde  (1  part  of  40  percent  solu- 
tion to  2500  parts  of  milk)  and  place  on  ice. 
!J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Pt.  II,  p.  287. 


AGRICULTURAL  MATERIALS  539 

Specific  Gravity. — The  specific  gravity  of  milk  may  be  deter- 
mined by  means  of  a  hydrometer,  a  Westphal  balance  or  a  pyc- 
nometer,  or  by  any  method  used  for  other  liquids.  It  is  most 
convenient,  and  sufficiently  accurate,  to  use  a  hydrometer,  which 
should  be  standardized  on  account  of  the  large  errors  that  are 
frequently  made  in  graduating  these  instruments.  It  is  unfortu- 
nate that  arbitrary  scales  have  come  into  use  for  the  expression 
of  the  density  of  milk.  The  instrument  called  the  "lactometer" 
is  simply  a  hydrometer  having  an  arbitrary  scale  of  its  own, 
sometimes  also  reading  directly  in  specific  gravity.  Quevenne's 
lactometer  is  graduated  in  degrees  from  15°  to  40°,  correspond- 
ing to  specific  gravity  1.015  to  1.040.  The  New  York  Board  of 
Health  lactometer  is  graduated  also  in  arbitrary  degrees,  0° 
corresponding  to  a  specific  gravity  of  1  and  100°  to  a  specific 
gravity  of  1.029,  the  latter  figure  being  considered  as  the  specific 
gravity  of  pure  average  milk.  Degrees  on  the  New  York  lac- 
tometer would  thus  roughly  indicate  the  percent  of  whole  milk 
in  a  milk  and  water  mixture.  Such  an  indication  can  have  but 
slight  value  in  milk  testing. 

Determination. — Mix  the  sample  of  milk  by  pouring  from  one  vessel 
to  another  several  times,  avoiding  violent  agitation.  Immediately 
determine  the  specific  gravity  at  15.5°,  using  a  lactometer.  Record 
both  specific  gravity  and  lactometer  degrees. 

Total  Solids. — A  statement  of  the  chemical  nature  of  the  solids 
of  milk  has  already  been  given  in  the  table  on  page  537.  The 
fatty  constituents  are  present  in  the  form  of  an  emulsion  of 
minute  globules  which  can  be  made  to  coalesce  by  agitation, 
such  as  occurs  during  churning.  The  proteids  are  chiefly  in  the 
form  of  sols  from  which  they  are  flocculated  by  such  means  as  the 
addition  of  acids,  boiling,  etc.  The  well-known  " curdling"  of 
milk  when  it  "sours"  is  chiefly  due  to  the  development  of  lactic 
acid  by  fermentation  of  lactose.  Lactose  itself  is  present  as  a 
true  solution,  as  are  also  the .  inorganic  constituents,  although 
calcium  phosphate  combines  in  some  manner  with  casein. 

The  determination  of  total  solids  must  be  made  at  a  tempera- 
ture not  higher  than  100°  in  order  to  avoid  chemical  changes. 
Great  difficulty  is  often  experienced  in  the  attempt  to  remove 
completely  the  water,  on  account  of  the  formation  of  a  skin  of 


540  QUANTITATIVE  ANALYSIS 

flocculated  proteids.  This  effect  may  be  minimized  by  placing  a 
small  amount  of  sand  in  the  dish  and  stirring  occasionally.  The 
drying  surface  is,  in  this  way,  very  much  increased.  Aluminium 
dishes  are  suitable  for  this  determination.  They  should  be  not 
less  than  5  cm  in  diameter  and  should  be  flat  on  the  bottom. 

Determination. — Clean,  dry  and  weigh  a  dish  containing  15  to  20 
gm  of  clean  sand  and  a  short  glass  rod,  then  add  about  5  gm  of  milk. 
The  sample  may  be  weighed  in  the  dish  or  it  may  be  accurately  measured 
after  the  specific  gravity  has  been  determined.  In  either  case  it  must 
be  well  mixed  as  directed  for  the  determination  of  specific  gravity. 
Dry  at  100°  until  the  weight  is  constant,  stirring  occasionally  with  the 
glass  rod,  which  is  left  in  the  dish.  Cool  in  a  desiccator  and  weigh 
rapidly,  in  order  to  avoid  absorption  of  moisture.  Calculate  the  loss 
in  weight  as  percent  of  water. 

Ash. — The  ash  of  milk  contains  the  inorganic  constituents  in 
the  form  in  which  they  are  left  after  burning  the  solids  and  it 
does  not  represent  the  original  combinations  of  these  constituents. 

Determination. — Weigh  a  platinum  dish,  add  about  20  gm  of  milk 
and  quickly  reweigh.  Add  6  cc  of  concentrated  nitric  acid,  evaporate 
to  dryness  over  a  steam  bath  and  ignite  at  a  temperature  just  below 
redness  until  free  from  carbon.  Weigh  and  calculate  the  percent  of  ash. 

Total  Nitrogen. — The  nitrogenous  constituents  of  milk  are 
usually  calculated  as  proteids,  although  traces  of  other  nitroge- 
nous compounds  are  present.  While  the  molecular  constitu- 
tion of  the  proteids  is  not  known  the  percentage  composition  is 
established  as  varying  within  fairly  narrow  limits  for  the  different 
proteids.  The  following  table  expresses  the  approximate 
composition. 


Element 

Percent 

Carbon  

50  0  to  55  0 

Hydrogen  

6  9  to    73 

Nitrogen 

15  Oto  19  0 

Oxygen 

19  0  to  24  0 

Sulphur  

0.3  to    2.4 

Both  casein  and  albumin  contain  slightly  less  than  15.7  percent 
of  nitrogen  and,  since  these  compounds  make  up  more  than  95 
percent  of  the  total  proteids  of  milk,  the  percent  of  total  proteids 
is  found  with  sufficient  accuracy  by  multiplying  the  percent  of 

total  nitrogen  by  6.38  (  = 


AGRICULTURAL  MATERIALS  541 

Nitrogen  is  determined  by  the  Kjeldahl  or  Gunning  method, 
as  already  discussed  in  connection  with  fertilizers,  the  sample 
of  milk  being  digested  without  previous  evaporation. 

Determination. — Having  accurately  determined  the  specific  gravity 
of  the  sample,  mix  well  and  measure  5  cc  into  a  Kjeldahl  digestion 
flask,  using  a  calibrated  pipette,  and  calculate  the  weight.  Without 
evaporating  the  milk  proceed  to  determine  nitrogen  by  either  the 
Kjeldahl  or  Gunning  method,  described  on  pages  516  and  519.  Mul- 
tiply the  percent  of  nitrogen  by  6.38  and  record  the  result  as  percent 
of  total  proteids. 

Casein. — Casein  is  present  in  milk  as  a  sol  and  it  has  a  high 
degree  of  molecular  association.  It  is  flocculated  by  dilute  acids 
and  will  not  thereafter  redissolve,  so  that  it  is  to  be  classed  as  an 
irreversible  colloid.  It  dissolves  in  concentrated  acids  but  its 
chemical  nature  is  thereby  changed.  Casein  is  approximately 
separated  from  the  other  proteids  by  warming  to  40°  after  the 
addition  of  water  and  acetic  acid.  Casein  flocculates  and  is 
separated  by  filtration.  Nitrogen  in  this  residue  is  determined 
and  casein  is  calculated.  Casein  may  also  be  precipitated  by 
heating  to  40°  with  a  solution  of  potassium  aluminium  sulphate. 

Determination.  Method  I. — Weigh  a  covered  125  cc  beaker  to  milli- 
grams, add  10  cc  of  milk  and  quickly  reweigh.  Add  90  cc  of  water  which 
is  at  a  temperature  of  40°  to  42°  and  add,  at  once,  1.5  cc  of  a  10  percent 
solution  of  acetic  acid.  Stir  with  a  glass  rod  and  allow  to  stand  for  5 
minutes  longer.  Filter  and  wash  three  or  four  times  with  cold  water, 
saving  the  filtrate  and  washings,  the  total  volume  of  which  should  not 
be  greater  than  125  cc.  The  filtrate  should  be  quite  clear. 

Place  the  paper  and  casein  in  a  Kjeldahl  digestion  flask  and  determine 
nitrogen  by  the  Kjeldahl  or  the  Gunning  method.  Multiply  the  percent 
of  nitrogen  by  6.38  and  record  the  result  as  the  percent  of  casein. 

Method  II. — Weigh  a  covered  125  cc  beaker  to  milligrams,  add  10  cc  of 
milk  and  quickly  reweigh.  Add  50  cc  of  water  at  40°  then  add  2  cc 
of  alum  solution,  saturated  above  40°.  Stir  then  allow  the  precipitate 
to  settle,  transfer  to  a  filter  and  wash  with  cold  water  Transfer  the 
filter  and  casein  to  a  Kjeldahl  digestion  flask  and  determine  nitrogen  by 
the  Kjeldahl  or  Gunning  method.  Multiply  by  6.38  to  obtain  the 
percent  of  casein. 

Albumin. — Albumin  is  flocculated  and  separated  from  the  re- 
maining proteids  by  the  use  of  a  more  dilute  solution  of  acetic 


542  QUANTITATIVE  ANALYSIS 

acid  but  at  a  higher  temperature.  The  determination  of  nitrogen 
in  the  precipitate  gives  a  basis. for  the  calculation  of  the  percent 
of  albumin. 

Determination.  Method  I. — Add  a  drop  of  phenolphthalein  to  the 
nitrate  from  casein,  obtained  by  method  (I),  and  neutralize  with  an 
approximately  tenth-normal  solution  of  sodium  hydroxide  or  potassium 
hydroxide.  Add  0.3  cc  of  10  percent  acetic  acid  solution  and  heat  on  the 
steam  bath  until  the  albumin  is  completely  flocculated.  Filter,  wash 
and  determine  nitrogen  in  the  precipitate  as  in  the  determination  of 
casein.  The  percent  of  nitrogen  multiplied  by  6.38  gives  the  percent 
of  albumin. 

Method  II. — To  the  nitrate  from  the  casein  obtained  by  method  (II) 
add  0.3  cc  of  10  percent  acetic  acid  and  boil  until  albumin  is  completely 
flocculated.  Filter  and  wash  with  cold  water.  Transfer  the  paper 
and  albumin  to  a  digestion  flask  and  determine  nitrogen  by  the  Kjel- 
dahl  or  Gunning  method.  Multiply  by  6.38  and  report  the  percent 
of  albumin. 

Lactose. — The  only  carbohydrate  occurring  in  milk  in  suffi- 
cient quantity  to  be  of  any  importance  is  lactose.  It  may  be  de- 
termined by  either  optical  methods,  depending  upon  its  power  to 
rotate  the  plane  of  polarization  of  plane-polarized  light,  or  by 
reduction  methods,  based  upon  its  power  to  reduce  a  cupric  salt 
in  a  basic  solution  containing  a  tartrate. 

Optical  Methods. — With  the  exception  of  the  proteids  lac- 
tose is  the  only  optically  active  substance  in  milk  and  it  is  only 
necessary  to  remove  the  proteids  in  order  to  apply  the  polari- 
scope  to  the  determination  of  the  concentration  of  lactose.  It 
is  assumed  that  the  student  is  familiar  with  the  physical  prin- 
ciples upon  which  the  construction  of  the  various  types  of  polari- 
scopes  is  based.  The  best-known  instruments  used  for  this  pur- 
pose, are  provided  with  scales  reading  in  angular  degrees  and 
also  with  special  sugar  scales,  upon  which  each  degree  indicates 
1  percent  of  sugar  if  a  certain  specified  weight  of  material  is  con- 
tained in  100  cc  of  solution  and  polarized  in  a  200  mm  tube. 
This  specified  weight  is  the  "normal"  weight  of  a  given  instru- 
ment. The  normal  weight  of  cane  sugar  for  the  Laurent 
scale  is  16.19  gm.  For  the  Ventzke  scale  the  normal 
weight  is  26.048  gm.  These  weights  are,  however,  based  upon 


AGRICULTURAL  MATERIALS  543 

dilution  to  100  Mohr  cubic  centimeters.  (See  page  172.)  Since 
volumetric  apparatus  is  now  generally  calibrated  upon  a  basis 
of  the  true  cubic  centimeter  it  is  necessary  to  correct  these  weights 
by  multiplying  by  the  ratio  of  the  volume  of  the  true  cubic  cen- 
timeter to  that  of  the  Mohr  cubic  centimeter.  This  ratio  is: 

Density  of  water  at  17.5°     _r. 

Density  of  water  at  20°     XWt*  °f  true  CC>  m  air  at  20  •  or 

gg|X0.9972  =  0.9977. 

The  Laurent  normal  weight  for  cane  sugar  is  therefore  0.9977  X 
16.19=16.153  gm  and  the  Ventzke  normal  weight  is  0.9977X 
26.048  =  25.988  gm. 

If  lactose  is  to  be  determined  by  use  of  the  sugar  scale  the  nor- 
mal weight  for  lactose  is  calculated  by  multiplying  that  for 
sucrose  by  the  ratio  of  the  specific  rotatory  power  of  sucrose  to 
that  of  lactose.  [a]D  for  sucrose  =  66. 5  and  [a]D  for  lactose  = 
52.53.  Therefore  the  Laurent  normal  weight  for  lactose  is 
f\(\  f\ 
P2~KoX  16. 153  =  20.449  and  the  Ventzke  normal  weight  for  lac- 

f*f»     r 

tose  is  ^-53  X  25.988  =  32.899  gm.  Both  of  these  finally  ob- 
tained weights  are  based  upon  dilution  to  100  true  cubic  centi- 
meters. These  weights  of  milk  are  too  small  for  convenience 
and  it  is  customary  to  use  three  times  the  normal  weight  for  the 
Laurent,  and  twice  the  normal  weight  of  milk  for  the  Ventzke 
scale.  These  weights  then  become,  finally,  61.347  and  65.798 
gm  for  the  Laurent  and  Ventzke  scales  respectively. 

In  order  to  avoid  weighing  each  sample  of  milk  the  table  on 
page  545  is  given  for  measuring  milk  having  a  predetermined 
specific  gravity. 

For  making  these  measurements  a  burette  is  not  suitable 
because  of  the  length  of  time  that  is  thereby  required,  segrega- 
tion of  fat  taking  place.  Neither  is  the  ordinary  measuring 
pipette  with  a  cylindrical  tube4  satisfactory  as  such  a  pipette  of 
sufficient  capacity  is  too  long  to  be  used  conveniently.  A 
special  pipette  having  a  bulb  in  the  upper  part  is  better.  The 
zero  mark  is  above  the  bulb  and  the  first  mark  below  is  55. 
The  lower  part  is  a  narrow  tube  graduated  from  55  to  65  cc. 


544  QUANTITATIVE  ANALYSIS 

Precipitation  of  Proteids. — The  milk  solution  may  be  clari- 
fied, without  heating,  by  Wiley's  method,1  adding  a  solution  of 
mercuric  nitrate  containing  free  nitric  acid,  or  by  adding  a  solu- 
tion of  mercuric  iodide  containing  free  acetic  acid.  This  is  prob- 
ably a  purely  colloidal  flocculation,  the  composition  of  the  pro- 
teids  being  unchanged  by  the  action.  If  this  precipitation  is 
carried  out  in  a  100  cc  volumetric  flask  and  the  solution  is  after- 
ward diluted  to  the  mark  the  volume  of  the  solution  will  be  less 
than  100  cc,  on  account  of  the  volume  occupied  by  the  precipi- 
tated proteids.  The  concentration  of  lactose  is  consequently 
greater  than  would  be  calculated  unless  allowance  were  made 
for  the  volume  of  the  precipitate.  The  official  method  directs 
that  the  volume  of  the  precipitate  should  be  considered  as  2.4 
cc  for  the  quantity  of  milk  that  is  used  for  the  Laurent  polari- 
scope  or  2.6  cc  for  the  quantity  that  is  used  for  the  Ventzke 
polariscope.  This  is  only  a  close  approximation  but  it  is  suffi- 
ciently accurate  for  ordinary  work. 

A  more  accurate  method  for  making  allowance  for  the  volume 
of  the  precipitate  is  that  of  Wiley  and  Ewell.2  Two  flasks  are 
used,  of  100  cc  and  200  cc  capacity,  respectively.  The  same 
quantity  of  milk  is  placed  in  the  two  flasks,  the  proteids  are  pre- 
cipitated by  acid  mercuric  nitrate  solution  and  each  solution  is 
diluted  to  the  mark  on  the  flask.  The  polarization  of  the  solu- 
tion in  the  200  cc  flask  is  slightly  less  than  half  of  that  of  the  more 
concentrated  solution.  The  product  of  the  two  polarizations 
divided  by  their  difference  gives  the  corrected  reading. 

Determination. — Prepare  a  solution  of  either  mercuric  nitrate  with 
nitric  acid  or  mercuric  iodide  with  acetic  acid,  as  follows: 

Acid  Mercuric  Nitrate. — Dissolve  25  gm  of  mercury  in  50  cc  of  con- 
centrated nitric  acid,  then  add  35  cc  of  water  and  mix. 

Acid  Mercuric  Iodide. — Dissolve  33.2  gm  of  potassium  iodide,  13.5 
gm  of  mercuric  chloride  and  20  cc  of  glacial  acetic  acid  in  640  cc  of 
water. 

Determine  the  specific  gravity  of  the  milk  by  means  of  a  sensitive 
hydrometer  or  a  picnometer.  Refer  to  the  table  on  page  545  and 
measure,  at  the  temperature  at  which  the  specific  gravity  was  taken,  the 
quantity  of  milk  indicated,  the  sample  having  been  mixed  thoroughly 

1  Am.  Chem.  J.,  6,  289  (1889). 

»  J.  Am.  Chem.  Soc.,  18,  428  (1896). 


AGRICULTURAL  MATERIALS 


545 


immediately  before  making  both  measurements.    A  pipette  which  is 
capable  of  delivering  the  required  amount  in  one  portion  is  desirable. 

The  milk  is  run  into  a  volumetric  flask,  graduated  at  102.4  cc  for  the 
Laurent  or  102.6  cc  for  the  Ventzke  type  of  polarimeter.  Add  1  cc  of 
acid  mercuric  nitrate  solution  or  30  cc  of  acid  mercuric  iodide  solution, 
dilute  to  the  mark  on  the  flask,  mix  well  and  allow  the  precipitate  to 
settle.  Filter  through  a  dry  filter,  rejecting  the  first  25  cc  of  the 
filtrate,  receiving  the  remainder  in  a  dry  flask.  Polarize  in  a  200  mm 
or  400  mm  tube.  If  the  200  mm  tube  is  used  the  reading  on  the  sugar 
scale  is  to  be  divided  by  3  for  the  Laurent  or  by  2  for  the  Ventzke  scale. 
In  case  the  400  mm  tube  has  been  used  these  numbers  become  6  and  4, 
respectively.  The  quotient  in  either  case  is  the  percent  of  lactose  in 
the  milk. 


Specific  gravity 

Volume  of  milk  to  be  used 

Laurent  scale 

Ventzke  scale 

1.024 

59.90 

64.25 

1.025 

59.85 

64.20 

1.026 

59.80 

64.15 

1.027 

59.75 

64.05 

1.028 

59.70 

64.00 

1.029 

59.60 

63.95 

1.030 

59.55 

63.90 

1.031 

59.50 

63.80 

1.032 

59.45 

63.75 

1.033 

59.40 

63.70 

1.034 

59.35 

63.65 

1.035 

59.30 

63.55 

1.036 

59.20 

63.50 

Reduction  Methods.— Fehling  first  perfected1  a  method  for 
the  determination  of  reducing  sugars  by  measuring  the  amount 
of  reduction  of  a  cupric  salt  that  takes  place  in  the  presence  of 
a  strong  base  and  a  tartrate,  cuprous  oxide  being  formed.  Soxh- 
let  found,2  however,  that  the  reaction  is  not  to  be  expressed  by 
any  single  equation  and  that  the  quantity  of  cuprous  oxide  pro- 
duced varies  somewhat  with  the  conditions  of  the  experiment 
with  regard  to  the  concentration  of  the  sugar,  time  of  heating, 
amount  of  excess  of  cupric  solution  and  kind  of  sugar.  Thus, 

1  Ann.  Chem.  Pharm.,  72,  106  (1849);  106,  75  (1858). 

2  Chem.  Zentr.,  [3]  9,  218  and  236  (1878). 

35 


546  QUANTITATIVE  ANALYSIS 

sugars  having  the  same  molecular  formulas  (as  dextrose  and 
invert  sugar,  maltose  and  lactose)  do  not  reduce  the  same  weight 
of  cuprous  oxide  when  the  conditions  of  the  experiment  are  other- 
wise the  same.  The  method  has  been  so  modified  and  standard- 
ized that  reducing  sugars  may  now  be  determined  with  a  consider- 
able degree  of  accuracy  by  either  measuring  the  volume  of  copper 
solution  required  to  react  with  the  sugar,  or  by  weighing,  or  other- 
wise determining,  the  weight  of  cuprous  oxide  that  is  produced 
by  the  reaction.  If  the  latter  class  of  methods  is  used  the  weight 
of  sugar  cannot  be  directly  calculated  from  the  weight  of  cuprous 
oxide,  on  account  of  variation  in  the  products  formed  by  the 
reaction.  The  weight  of  sugar  is  found  by  the  use  of  a  table 
that  has  been  constructed  from  the  results  of  direct  experi- 
ments. Also  it  is  not  practicable  to  make  a  direct  weighing  of 
cuprous  oxide  because  of  the  oxidation  that  takes  place  during 
the  drying  process.  It  is  therefore  dissolved  in  nitric  acid  and 
the  copper  is  determined  by  one  of  the  volumetric  or  electrolytic 
processes,  or  else  the  cuprous  oxide  is  reduced  to  metallic  copper 
by  heating  in  an  atmosphere  of  hydrogen,  the  copper  being  then 
weighed. 

For  the  determination  of  lactose  in  milk  it  is  necessary  first 
to  remove  other  substances  that  would  reduce  cupric  salts  in 
basic  solution.  The  milk  proteids  are  the  only  substances  of 
this  nature  to  be  considered  and  these  may  be  removed  by  pre- 
cipitating cupric  hydroxide  in  their  presence.  For  this  purpose 
cupric  sulphate  and  sodium  hydroxide  are  added  to  the  milk. 
The  colloidal  cupric  hydroxide,  which  then  precipitates,  com- 
pletely removes  the  colloidal  proteids.  A  slight  error  is  occasioned 
by  the  fact  that  a  small  amount  of  lactose  is  also  adsorbed  by  the 
precipitate  but  this  is  usually  negligible. 

Determination. — Prepare  500  cc  of  a  half-normal  sodium  hydroxide 
solution,  also  Fehling-Soxhlet  solutions,  as  follows: 

(a)  Cupric  Sulphate  Solution. — Dissolve  34.639  gm  of  dry,  crystal- 
lized cupric  sulphate  in  water  and  dilute  to  500  cc.  If  not  clear,  filter 
through  washed  asbestos. 

(6)  Tartrate  Solution. — Dissolve  173  gm  of  potassium  sodium  tar- 
trate  ("Rochelle  salts")  and  50  gm  of  sodium  hydroxide  in  water  and 
dilute  to  500  cc.  Allow  to  stand  for  2  days  and  filter  through  washed 
asbestos. 


AGRICULTURAL  MATERIALS  547 

Measure  25  cc  of  milk  into  a  500  cc  volumetric  flask  and  calculate  its 
weight.  Add  400  cc  of  water  and  10  cc  of  copper  sulphate  solution 
(a),  mix  and  then  add  8.8  cc  of  half-normal  sodium  hydroxide  solution. 
After  the  copper  hydroxide  has  precipitated  the  solution  should  still 
contain  a  slight  excess  of  copper.  Dilute  to  the  mark  on  the  flask,  mix 
and  filter  through  a  dry  paper,  rejecting  the  first  25  cc  of  the  filtrate. 
Place  25  cc  of  solution  (a)  and  25  cc  of  solution  (b)  in  a  250  cc  beaker  of 
resistance  glass  and  add  50  cc  of  the  filtrate  from  proteids,  already 
obtained.  Heat  over  a  burner  which  is  regulated  so  that  the  boiling 
begins  in  4  minutes  and  continue  boiling  for  exactly  2  minutes  longer. 
Keep  the  beaker  covered  during  the  heating.  Filter  immediately 
through  a  Gooch  filter  and  wash  with  hot  water  but  making  no  attempt 
to  transfer  all  of  the  cuprous  oxide  to  the  filter.  Determine  the  copper 
in  this  precipitate  by  one  of  the  following  methods : 

(a)  Electrolytic  Method. — Transfer  the  asbestos  and  most  of  the 
cuprous  oxide  to  the  beaker  in  which  precipitation  was  made.  Dissolve 
the  oxide  remaining  in  the  crucible  by  means  of  2  cc  of  concentrated 
nitric  acid,  adding  the  latter  with  a  pipette  and  receiving  the  solution 
in  the  beaker  containing  the  asbestos.  Rinse  the  crucible  with  a  jet 
of  hot  water,  allowing  the  rinsings  to  flow  into  the  beaker.  Heat  until 
all  of  the  cuprous  oxide  is  dissolved,  then  filter  through  paper  and  wash 
the  paper  thoroughly  with  hot  water  to  remove  all  copper  nitrate,  re- 
ceiving the  solution  in  the  beaker  that  is  to  be  used  for  the  electrolysis. 
Electrolyze  and  determine  the  copper,  observing  all  of  the  precautions 
mentioned  in  the  discussion  on  page  156. 

(6)  Proceed  as  in  method  (a)  until  the  copper  solution  is  obtained 
and  filtered.  Determine  the  copper  in  the  solution  by  the  volumetric 
iodide  method  described  on  page  267,  beginning  with  "Boil  until  all 
bromine  is  removed  .  .  . "  in  the  third  paragraph  of  page  269. 

Conduct  a  blank  experiment,  using  50  cc  of  water  instead  of  the  milk 
solution.  If  more  than  0.5  mg  of  cuprous  oxide  is  obtained  make  a 
correction  in  the  amount  of  cuprous  oxide  found  in  the  analytical  experi- 
ment. The  alkaline  tartrate  solution  deteriorates  on  standing  and  the 
amount  of  cuprous  oxide  obtained  in  the  blank  increases  with  time. 

Find  the  weight  of  lactose  corresponding  to  the  copper  obtained,  using 
the  table  on  page  548.  Calculate  the  percent  of  lactose  in  the  milk. 

Fat. — The  composition  of  butter  fat  has  already  been  discussed 
in  connection  with  edible  oils  and  fats.  The  fat  of  milk  is  in  a 
state  of  emulsification  and  the  microscopic  globules  are  largely 
responsible  for  the  white  appearance  of  milk.  The  determina- 
tion of  fat  is  made  by  a  method  belonging  to  any  one  of  three 


548 


QUANTITATIVE  ANALYSIS 


TABLB  FOR  THE  DETERMINATION  OP  LACTOSE  (SOXHLET-WEIN) 


Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

Milli- 
grams 
of  cop- 
per 

Milli- 
grams 
of  lac- 
tose 

100 

71.6 

150 

108.8 

200 

146.9 

250 

184.8 

300 

224  .  4 

350 

263.9 

101 

72.4 

151 

109.6 

201 

147.7 

251 

185.5 

301 

225.2 

351 

264.7 

102 

73.1 

152 

110.3 

202 

148.5 

252 

186.3 

302 

225.9 

352 

265.5 

103 

73.8 

153 

111.1 

203 

149.2 

253 

187.1 

303 

226.7 

353 

266.3 

104 

74.6 

154 

111.9 

204 

150.0 

254 

187.9 

304 

227.5 

354 

267.2 

105 

75.3 

155 

112.6 

205 

150.7 

255 

188.7 

305 

228.3 

355 

268.0 

106 

76.1 

156 

113.4 

206 

151.5 

256 

189.4 

306 

229.1 

356 

268.8 

107 

76.8 

157 

114.1 

207 

152.2 

257 

190.2 

307 

229.8 

357 

269.6 

108 

77.6 

158 

114.9 

208 

153.0 

258 

191.0 

308 

230.6 

358 

270.4 

109 

78.3 

159 

115.6 

209 

153.7 

259 

191.8 

309 

231.4 

359 

271.2 

110 

79.0 

160 

116.4 

210 

154.5 

260 

192.5 

310 

232.2 

360 

272.1 

111 

79.8 

161 

117.1 

211 

155.2 

261 

193.3 

311 

232.9 

361 

272.9 

112 

80.5 

162 

117.9 

212 

156.0 

262 

194.1 

312 

233.7 

362 

273.7 

113 

81.3 

163 

118.6 

213 

156.7 

263 

194.9 

313 

234.5 

363 

274.5 

114 

82.0 

164 

119.4 

214 

157.5 

264 

195.7 

314 

325.  3^ 

364 

275.3 

115 

82.7 

165 

120.2 

215 

158.2 

265 

196.4 

315 

236.1 

365 

276.2 

116 

83.5 

166 

120.9 

216 

159.0 

266 

197.2 

316 

236.8 

366 

277.1 

117 

84.2 

167 

121.7 

217 

159.7 

.  267 

198.0 

317 

237.6 

367 

277.9 

118 

85.0 

168 

122.4 

218 

160.4 

268 

198.8 

318 

238.4 

368 

278.8 

119 

85.7 

169 

123.2 

219 

161.2 

269 

199.5 

319 

239.2 

369 

279.6 

120 

86.4 

170 

123.9 

220 

161.9 

270 

200.3 

320 

240.0 

370 

280.5 

121 

87.2 

171 

124.7 

221 

162.7 

271 

201.1 

321 

240.7 

371 

281.4 

122 

87.9 

172 

125.5 

222 

163.4 

272 

201.9 

322 

241.5 

372 

282.2 

123 

88.7 

173 

126.2 

223 

164.2 

273 

202.7 

323 

242.3 

373 

283.1 

124 

89.4 

174 

127.0 

224 

164.9 

274 

203.5 

324 

243.1 

374 

283.9 

125 

90.1 

175 

127.8 

225 

165.7 

275 

204.3 

'  325 

243.9 

375 

284.8 

126 

90.9 

176 

128.5 

226 

166.4 

276 

205.1 

326 

244.6 

376 

285.7 

127 

91.6 

177 

129.3 

227 

167.2 

277 

205.9 

327 

245.4 

377 

286.5 

128 

92.4 

178 

130.1 

228 

167.9 

278 

206.7 

328 

246.2 

378 

287.4 

129 

93.1 

179 

130.8 

229 

168.6 

279 

207.5 

329 

247.0 

379 

288.2 

130 

93.8 

180 

131.6 

230 

169.4 

280 

208.3 

330 

247.7 

380 

289.1 

131 

94.6 

181 

132.4 

231 

170.1 

281 

209.1 

331 

248.5 

381 

289.9 

132 

95.3 

182 

133.1 

232 

170.9 

282 

209.9 

332 

249.2 

382 

290.8 

133 

96.1 

183 

133.9 

233 

171.6 

283 

210.7 

333 

250.0 

383 

291.7 

134 

96.9 

184 

134.7 

234 

172.4 

284 

211.5 

334 

250.8 

384 

292.5 

135 

97.6 

185 

135.4 

235 

173.1 

285 

212.3 

335 

251.6 

385 

293.4 

136 

98.3 

186 

136.2 

236 

173.9 

286 

213.1 

336 

252.5 

386 

294.2 

137 

99.1 

187 

137.0 

237 

174.6 

287 

213.9 

337 

253.3 

387 

295.1 

138 

99.8 

188 

137.7 

238 

175.4 

288 

214.7 

338 

254.1 

388 

296.0 

139 

100.5 

189 

138.5 

239 

176.2 

289 

215.5 

339 

254.9 

389 

296.8 

140 

101.3 

190 

139.3 

240 

176.9 

290 

216.3 

340 

255.7 

390 

297.7 

141 

102.0 

191 

140.0 

241 

177.7 

291 

217.1 

341 

256.5 

391 

298.5 

142 

102.8 

192 

140.8 

242 

178.5 

292 

217.9 

342 

257.4 

392 

299.4 

143 

103.5 

193 

141.6 

243 

179.3 

293 

218.7 

343 

258.2 

393 

300.3 

144 

104.3 

194 

142.3 

244 

180.1 

294 

219.5 

344 

259.0 

394 

301.1 

145 

105  1 

195 

143.1 

245 

180.8 

295 

220.3 

345 

259.8 

395 

302.0 

146 

105.8 

196 

143.9 

246 

181.6 

296 

221.1 

346 

260.6 

396 

302.8 

147 

106.6 

197 

144.6 

247 

182.4 

297 

221.9 

347 

261.4 

397 

303.7 

148 

107.3 

198 

145.4 

248 

183.2 

298 

222.7 

348 

262.3 

398 

304.6 

149 

108.1 

199 

146.2 

249 

184.0 

299 

223.5 

349 

263.1 

399 

305.4 

AGRICULTURAL  MATERIALS 


549 


general  classes,  there  being  many  modifications  of  the  methods 
belonging  to  each  class.  These  classes  are  (1)  extraction  meth- 
ods, the  fat  being  dissolved  by  a  volatile  solvent  which  is  later 
evaporated,  leaving  the  fat  to  be  weighed,  (2)  volumetric  meth- 
ods, the  proteids  being  dissolved  by  strong  acids,  the  fat  being 
collected  and  its  volume  measured,  and  (3)  refractometric  meth- 
ods, based  upon  the  variation  of  the 
index  of  refraction  of  milk  with  varia- 
tion in  the  concentration  of  suspended 
fat  particles. 

Paper-coil  Method. — The  best-known 
extraction  method  is  that  of  Adams.1 
In  this  method  the  weighed  sample  of 
milk  is  absorbed  by  a  roll  of  porous 
paper,  which  is  then  dried  and  the  fat 
is  extracted  by  ether  or  volatile  petro- 
leum spirit.  The  removal  of  water  is 
essential  as  otherwise  other  substances, 
such  as  lactose  and  proteids,  would  also 
be  extracted.  For  the  same  reason  the 
ether  must  be  entirely  free  from  water 
and  alcohol.  These  substances  are  re- 
moved by  allowing  the  ether  to  stand 
over  metallic  sodium  until  hydrogen  is 
no  longer  evolved,  then  distilling.  Paper 
may  be  prepared  for  fat  determinations 
by  extracting  strips  of  filter  paper  with 
ether  and  drying,  but  it  is  now  possible 
to  obtain  extracted  paper  of  special 

quality  which  is  suitable  for  this  purpose.  The  apparatus  for 
continuous  extraction  is  that  devised  by  Soxhlet  or  some  other 
apparatus,  similar  in  principle.  Soxhlet's  extractor  is  shown 
in  Fig.  117.  The  volatile  solvent  is  boiled  in  the  lower  flask 
which  has  previously  been  weighed.  The  vapor  is  condensed 
above,  drops  into  the  extractor  and  there  remains  in  contact 
with  the  paper  and  milk  solids  until  it  has  filled  the  apparatus 
to  the  height  of  the  bend  in  the  siphon  tube  (s)  when  it  siphons 
back  into  the  flask  to  be  again  vaporized.  At  the  last,  the  ether 

1  Analyst,  10,  46  (1885). 


FIG.  117. — Soxhlet  ex- 
tractor with  fat  flask  and 
Hopkins  condenser. 


550  QUANTITATIVE  ANALYSIS 

is  evaporated  from  the  flask  and  the  latter  is  then  weighed  with 
the  extracted  fat. 

Determination  by  the  Paper-coil  Method. — Prepare  anhydrous  ether 
by  allowing  ordinary  ether  to  stand  over  sodium,  the  bottle  being  loosely 
stoppered,  until  hydrogen  is  no  longer  evolved.  The  sodium  is  cut 
into  small  pieces  or,  better,  is  pressed  into  wire  by  means  of  a  sodium 
press,  and  is  immediately  placed  in  the  ether.  When  gas  ceases  to  be 
evolved  distill  the  ether  from  an  electrically  heated  apparatus  or  from  a 
water  bath  from  which  the  flame  has  been  removed,  receiving  the  distill- 
ate in  a  bottle  which  is  protected  by  a  drying  tube  filled  with  calcium 
chloride.  All  flames  must  be  removed  from  the  vicinity  when  ether  is  being 
handled  or  distilled. 

Roll  a  strip  of  fat-free  porous  paper  into  a  coil  and  bind  with  a  plati- 
num or  iron  wire,  avoiding  contact  with  the  hands  as  far  as  possible. 
Special  extracted  paper  is  obtainable  in  strips  6.5  cm  by  60  cm,  already 
thoroughly  extracted  with  ether.  If  such  paper  is  not  available  cut 
heavy  filter  paper  into  strips  6.5  cm  wide  and  roll  into  a  coil  which  will 
easily  slip  into  the  continuous  extractor.  Mix  the  sample  of  milk, 
immediately  withdraw  5  cc  by  means  of  a  pipette  and  allow  the  milk  to 
run  into  the  end  of  the  coil,  by  which  it  is  absorbed.  The  weight  of 
milk  is  calculated  from  the  volume  and  specific  gravity.  Dry  the  coil  at 
100°  for  3  hours,  then  place  it  in  the  extraction  apparatus.  Clean,  dry 
and  weigh  a  low,  wide  flask  of  the  form  shown  in  Fig.  117  and  having  a 
capacity  of  100  cc.  Place  60  cc  of  anhydrous  ether  in  the  flask  and 
connect  as  shown,  using  pressed  corks  that  have  already  been  extracted 
with  ether.  Rubber  stoppers  must  not  be  used.  Heat  the  flask 
containing  the  ether  until  the  latter  boils,  using  a  water  bath  or  an  elec- 
trically heated  stove,  all  flames  being  removed.  Keep  the  ether  boiling 
and  extract  for  2  hours,  the  extractor  emptying  approximately  every 
10  minutes. 

Finally  mterrupt  the  extraction  at  a  point  just  before  the  extractor 
is  ready  to  empty  by  siphoning,  the  flask  then  containing  but  little  ether. 
Connect  the  flask  with  a  condenser  in  such  a  way  that  the  ether  may  be 
distilled  and  recovered.  When  no  more  ether  will  distill  remove  the 
condenser  and  heat  the  open  flask  over  the  bath  until  the  odor  of  ether 
nearly  disappears,  then  place  the  flask  on  its  side  in  an  oven  and  heat  at 
100°  until  the  ethereal  odor  has  been  completely  removed.  Explosions 
sometimes  occur  as  a  result  of  placing  the  flask  in  the  oven  when  too  much 
ether  remains,  the  ether-air  mixture  becoming  ignited  in  the  oven. 

Cool  the  flask  in  a  desiccator  and  weigh.  The  increase  in  weight  is 
calculated  as  fat. 


AGRICULTURAL  MATERIALS 


551 


The  paper-coil  method,  while  the  most  accurate  of  all,  has 
now  been  displaced  as  an  official  method  by  the  Rose-Gottlieb 
method1  because  of  the  greater  simplicity  of  the  latter.     Here 
the  proteids  are  dissolved  by  ammonium      ~ 
hydroxide.      The   butter   fat  is   then  ex-  r\ 
tracted  by  shaking  with  ether  and  petro-  ^  ( 
leum  ether  in  a  cylinder  which  is  provided 
with  a  side  tap  for  drawing  off  the  ethereal 
solution.      This   is   shown   in   figure    118. 
The  purpose  of  the  petroleum  ether  is  to 
diminish  the  solubility  of  lactose  in  the 
ether.     The  fat  solution  thus  obtained  is 
heated  to  vaporize  the  solvent  and  the  fat 
is  then  weighed. 

Determination  by  the  Official  Rose-Gottlieb 
Method. — Measure  10  cc  of  milk  into  a  Rohrig 
tube  or  some  similar  apparatus  and  calculate 
the  weight  from  the  specific  gravity.  Or  weigh, 
to  milligrams,  approximately  10  cc  of  milk  in  a 
small  covered  beaker,  pour  into  the  tube  and 
reweigh  the  covered  beaker.  Add  1.25  cc  of 
concentrated  ammonium  hydroxide  and  mix 
thoroughly.  (If  the  sample  is  sour  2  cc  of 
ammonium  hydroxide  should  be  used,  although 
it  should  be  remembered  that  accurate  sampling 
of  fermented  milk  is  difficult  on  account  of 
coclusion  of  fat  by  flocculated  proteids.) 

Add  10  cc  of  alcohol,  95  percent  by  volume, 
and  mix  well,  then  add  25  cc  of  washed  ether, 
stopper  and  shake  vigorously  for  30  seconds. 
Add  25  cc  of  petroleum  ether  (distilling  below 
60°)  and  shake  again  for  30  seconds.  Allow  to 
stand  for  20  minutes  or  until  the  upper  liquid  is 
practically  clear.  Draw  off  as  much  as  possible 

of  the  ether-fat  solution  (usually  0.5  to  0.8  cc    ^"^ 

...   .  .        „          .  ,         ,.         ,         FIG.  118. — Rohrig  tube 

will  be  left)  through  a  small,  quick-acting  dry     for  fat  determination. 

filter,   into   a   dry  100  cc  wide  necked  flask. 

Again  extract  the  liquid  remaining  in  the  tube,  using  15  cc  of  washed 

ether,  shaking  for  30  seconds,  adding  15  cc  of  petroleum  ether,  shaking 

1  Z.  Nahr.  Genussm.,  9,  531  (1905). 


552  QUANTITATIVE  ANALYSIS 

again  for  30  seconds  and  allowing  to  stand  for  20  minutes  for  separation. 
Draw  off  the  clear  upper  layer  through  the  same  filter  as  before  and 
into  the  same  flask.  Wash  the  tip  of  the  outlet  tube,  the  funnel  and 
the  filter  with  a  few  cubic  centimeters  of  a  mixture  of  equal  parts  of 
the  two  ethers.  For  very  accurate  work  a  third  extraction  should  be 
made.  Usually  this  will  yield  less  than  1  mg  of  fat  if  a  close  separation 
has  been  made  on  the  previous  extractions.  As  this  represents  less 
than  0.01  percent  it  may  be  ignored  in  most  work.  Evaporate  the 
ethers  slowly  on  the  steam  bath  then  dry  the  fat  to  constant  weight 
at  100°. 

Dissolve  the  fat  in  a  little  petroleum  ether  and  wash  the  flask  and  any 
solid  residue  that  may  remain  free  from  fat  with  petroleum  ether, 
leaving  the  residue  in  the  flask.  Dry  the  flask  as  before  and  reweigh 
with  the  residue.  This  weight  subtracted  from  the  first  will  .give  the 
weight  of  fat.  Blank  determinations  should  be  made  on  the  reagents, 
correcting  for  any  soluble  matter  that  may  be  obtained. 

From  the  corrected  weight  of  fat  calculate  the  percent  of  fat  in  the 
milk. 

Volumetric  Methods. — In  order  to  separate  the  fat  for  a 
volume  measurement  it  is  necessary  first  to  dissolve  the  pro- 
teids  existing  in  a  state  of  colloidal  suspension.  This  can  be 
done  by  adding  sulphuric  acid  or  both  sulphuric  and  hydro- 
chloric acids,  with  or  without  amyl  alcohol.  The  fat  is  then  col- 
lected by  whirling  in  a  centrifugal  machine.  A  special  bottle 
is  used,  having  a  graduated  neck  in  which  the  volume  of  fat  can 
be  read.  If  proper  relations  between  the  quantity  of  milk  and 
the  graduations  on  the  bottle  are  observed  the  percent  of  fat 
may  be  read  directly.  These  methods  are  sufficiently  accurate 
for  control  purposes  and  they  can  be  performed  in  a  very  short 
period  of  time.  The  chief  causes  of  inaccuracies  are  incorrect 
graduations  of  milk  bottles  and  pipettes  and  variation  in  the 
temperature  at  which  the  volume  of  fat  is  read. 

Babcock  Method. — The  best  known  of  the  volumetric  methods 
is  that  of  Babcock.1  In  this  method  sulphuric  acid  is  the  only 
proteid  solvent  used,  its  specific  gravity  lying  between  1.82  and 
1.83.  If  acid  of  lower  specific  gravity  is  used  solution  of  proteids 
is  incomplete,  while  acid  of  higher  specific  gravity  than  1.83 
causes  charring  of  the  milk  solids,  resulting  also  in  incomplete 

1  Wis.  Exp.  Sta.  Bull.,  24  (1890);  Milch.  Zeit.,  19,  746  (1890). 


AGRICULTURAL  MATERIALS 


553 


separation  of  fat.  In  order  to  avoid  all  weighing  of  samples 
a  special  pipette  is  provided,  graduated  to  deliver  17.6  cc. 
Upon  the  assumption  that  the  specific  gravity  of  milk  is  1.032 
this  volume  of  milk  would  weigh  18.2  gm.  This  assumption 
is  sufficiently  close  to  the  truth  for  ordinary 
testing  purposes.  The  graduated  bottle  has  the 
form  shown  in  Fig.  119.  The  numbered  divi- 
sions include  a  volume  of  0.20  cc,  each  division 
having  ten  subdivisions.  If  the  specific  gravity 
of  butter  fat  is  taken  as  0.91  (which  is  practically 
correct  at  60°,  the  temperature  of  measurement) 
each  numbered  division  on  the  bottle  will  hold 
0.18  gm  of  fat,  which  will  be  1  percent  of  the 
weight  of  sample.  It  will  thus  be  seen  that  the 
method  involves  several  somewhat  arbitrary 
assumptions  but,  in  spite  of  this  fact,  it 
admirably  fulfills  the  requirements  of  a  rapid 
and  fairly  accurate  method  for  the  control  of 
milk  supply. 

Official  Specifications  for  Test  Bottles  and 
Pipettes.1 — The  standard  Babcock  test  bottle 
must  conform  to  the  following  specifications: 

The  total  percent  graduation  shall  be  8.  The 
total  height  of  the  bottle  shall  be  150  to  165 
mm.  The  capacity  of  the  bulb  to  the  junction 
with  the  neck  shall  be  not  less  than  45  (true)  cc. 
(See  page  172.)  The  graduated  portion  of 
the  neck  shall  have  a  length  of  not  less  than 
63.5  mm  and  the  neck  shall  be  cylindrical  for  not  less  than  9 
mm  on  either  side  of  the  graduated  portion.  The  gradua- 
tions shall  represent  whole  percent,  halves  and  tenths  of  a 
percent.  The  capacity  of  each  percent  on  the  scale  shall  be 
0.20  true  cc. 

Standard  milk  pipettes  are  graduated  to  deliver  17.6  true  cc 
of  water  at  20°  in  5  to  8  seconds. 

Graduated  cylinders  should  be  provided  for  the  acid,  capable 
of  measuring  17.5  cc. 

The  official  method  for  calibrating  Babcock  test  bottles  is  to 

1  J.  Assoc.  Off.  Agr.  Chem.,  Vol.  II,  No.  3,  Ft.  II,  p.  289. 


FIG.  119.— 
Babcock  bottle 
for  fat  determi- 
nations. 


554  QUANTITATIVE  ANALYSIS 

fill  the  dry  bottle  to  the  zero  mark  with  pure  mercury  at  20°, 
weigh,  fill  to  the  highest  mark  and  re  weigh,  calculating  the 
bulb  and  stem  capacities  on  the  basis  of  13.5471  gm  of  dry  mercury 
for  each  cubic  centimeter  at  20°.  It  is  difficult  to  see  what  ad- 
vantage this  possesses  over  the  method  of  calibrating  by  weighing 
water  at  20°,  discussed  in  the  first  part  of  Chapter  V,  especially 
since  the  Babcock  bottle  filled  with  mercury  must  weigh  more 
than  600  gm.  Accurate  weighing  of  such  a  quantity  would 
require  a  special  balance,  as  sensitive  as  the  analytical  balance 
and  having  large  capacity. 

Milk  pipettes  and  graduates  are  calibrated  according  to  the 
official  method  by  measuring  in  a  burette  the  quantity  of  water 
delivered  by  the  instrument  at  20°.  Unless  care  has  been 
exercised  in  wetting  the  inner  surface  of  the  burette,,  using  the 
standard  method  by  which  the  burette  was  calibrated,  this 
method  will  be  subject  to  considerable  error  since  all  burettes 
are  graduated  for  delivery  and  not  for  capacity.  A  better 
method  for  calibrating  pipettes  is  described  on  page  189. 

Determination. — For  the  determination  measure  17.6  cc  of  carefully 
mixed  milk  into  each  of  at  least  two  test  bottles.  If  the  centrifuge 
carries  more  than  two  pockets  for  bottles  more  than  two  determinations 
may  be  run  at  a  time  but  in  any  case  an  even  number  of  bottles  must  be 
used  and  they  must  be  placed  opposite  each  other  in  the  machine.  Add 
to  each  bottle  17.5  cc  of  sulphuric  acid  which  has  a  specific  gravity  of 
1.82  to  1.83.  Mix  and  when  the  proteids  are  dissolved  whirl  the  bottles 
in  the  centrifuge  for  4  minutes  at  the  rate  prescribed  for  the  machine 
used .  Add  boiling  water,  filling  to  the  bottom  of  the  necks  of  the  bottles 
and  whirl  for  1  minute.  Again  add  boiling  water  to  bring  the  fat  layer 
to  about  the  middle  of  the  bottle  necks  and  whirl  again  for  1  minute. 
Immediately  place  the  bottles  in  a  glass  beaker  which  is  filled  with  water 
at  a  temperature  between  57°  and  60°.  The  water  should  surround  the 
bottle  necks  to  a  point  above  the  fat  layer.  After  1  minute  read  the 
length  of  the  fat  column,  taking  the  lowest  point  of  each  meniscus. 
Record  directly  as  the  percent  of  fat. 

Relation  between  Fat,  Total  Solids  and  Specific  Gravity.— 
On  page  536,  the  relations  of  fat,  solids  not  fat  and  total  solids  to 
each  other  were  discussed,  also  the  effect  of  watering  and  skim- 
ming upon  the  relations  existing  among  these  numbers.  There 
is  also  a  nearly  constant  relation  between  the  specific  gravity  and 


AGRICULTURAL  MATERIALS  555 

the  percents  of  fat  and  total  solids,  so  that  if  two  of  these  quanti- 
ties are  known  the  third  can  be  closely  approximated  by  the  use 
of  a  formula.  In  control  work  the  determination  of  fat  by 
Babcock's  method  is  rapid  and  easy  and  the  specific  gravity 
may  be  quickly  obtained  by  means  of  a  lactometer.  Neither 
determination  requires  a  balance  or  other  expensive  apparatus. 
On  the  other  hand  the  determination  of  total  solids  requires  the 
use  of  an  analytical  balance  and  the  necessary  evaporation, 
cooling  and  weighing  may  extend  over  a  period  of  several  hours. 
A  formula  for  calculating  this  property  is  therefore  often  desir- 
able. Richmond's  formula1  is  as  follows: 

T  =  0.25S  +  1.2F  +  0.14, 

where  T  and  F  represent  the  percents  of  total  solids  and  fat, 
respectively,  and  S  is  the  Quevenne  lactometer  reading. 

Added  Water. — Watering  may  be  detected  even  more  readily 
by  observing  certain  properties,  such  as  specific  gravity,  index  of 
refraction  or  percent  of  ash,  of  the  milk  serum  from  which  the 
curd  has  been  removed.  The  soluble  solids  in  the  serum  do  not 
vary  as  much  as  do  the  total  solids.  Proteids  and  fat  may  be 
separated  by  the  addition  of  acetic  acid  or  copper  sulphate 
or  by  allowing  the  milk  to  "sour"  spontaneously.  In  the  latter 
case  the  constants  of  the  serum  are  changed  somewhat  by  the 
conversion  of  lactose  into  lactic  acid. 

The  refractive  index  of  the  serum  may  be  determined  by  any 
of  the  standard  instruments  but  the  immersion  refractometer 
is  most  conveniently  employed.  In  this  instrument  the  position 
of  the  prism  system  in  the  tube  is  fixed,  as  in  the  butyrorefracto- 
meter,  mentioned  on  page  361.  The  chief  distinguishing  feature 
is  the  immersion  of  the  prism  in  the  liquid  to  be  tested.  The 
scale  is  an  arbitrary  one  and  reads  from  —5  to  +105,  corre- 
sponding to  indices  of  refraction  from  1.32539  to  1.36640. 

Examination  of  Acetic  Serum,  (a)  Zeiss  Immersion  Refractometer 
Reading. — To  100  cc  of  milk  at  a  temperature  of  about  20°  add  2  co 
of  25  percent  acetic  acid  (specific  -gravity  1.035)  in  a  beaker  and  heat 
the  mixture,  covered  by  a  watch  glass,  immersed  in  a  water  bath  at 
70°.  Place  the  beaker  in  ice  water  for  10  minutes  and  separate  the 
curd  by  filtering  through  a  12.5  cm  folded  filter.  Transfer  about  35  cc 


Analyst,  20,  57  (1859). 


556  QUANTITATIVE  ANALYSIS 

of  the  serum  to  one  of  the  beakers  that  accompanies  the  temperature 
control  bath  used  in  connection  with  the  Zeiss  immersion  ref ractometer ; 
take  the  refractometer  reading  at  20°,  using  a  thermometer  graduated 
to  tenths  of  degrees.  A  reading  below  39  indicates  added  water.  If  the 
reading  is  between  39  and  40  the  addition  of  water  is  not  certain  but  is 
to  be  suspected. 

(6)  Ash. — Transfer  25  cc  of  the  serum  to  a  flat  bottomed  platinum 
dish  and  evaporate  to  dryness  on  a  steam  bath,  then  heat  over  a  low 
flame  until  the  solids  are  thoroughly  charred.  Place  the  dish  in  an 
electric  muffle  furnace  and  ignite  to  a  white  ash  at  a  temperature  not 
higher  then  500°.  Cool  and  weigh.  Express  the  result  as  grams  per 
100  cc.  Ash  content  below  0.715  gm  per  100  cc  indicates  added  water. 
Multiply  by  the  factor  1.021  to  correct  for  the  addition  of  acetic  acid. 
The  result  is  the  ash  on  the  undiluted  sour  serum. 

Examination  of  Sour  Serum,  (a)  Zeiss  Immersion  Refractometer 
Reading. — Allow  the  milk  to  sour  spontaneously,  filter  and  determine 
the  immersion  refractometer  reading  of  the  clear  serum  at  20°.  A  read- 
ing below  38.3  indicates  added  water. 

(6)  Ash. — Determine  the  ash  in  25  cc  of  the  serum  obtained  from  the 
soured  milk,  using  the  method  described  above  for  acetic  serum.  Re- 
sults below  0.730  gm  per  100  cc  indicate  added  water. 

Examination  of  Copper  Serum.  Zeiss  Immersion  Refractometer 
Reading. — Use  a  solution  of  copper  sulphate  containing  72.5  gm  per 
liter,  adjusted  if  necessary  to  read  36  at  20°  on  the  scale  of  the  immersion 
refractometer.  To  one  volume  of  this  solution  add  four  volumes  of 
milk.  Shake  well  and  filter.  Determine  the  immersion  refractometer 
reading  of  the  clear  serum  at  20°.  A  reading  below  36  indicates  added 
water. 

Detection  of  Formaldehyde. — Formaldehyde  is  sometimes 
added  to  milk  in  order  to  prevent  souring,  which  is  due  to  bac- 
terial action.  Such  preservatives  as  this  are  forbid'den  in  most 
cities  and  states.  Several  qualitative  methods  will  serve  to 
detect  formaldehyde.  The  following  is  Hehner's  method.1 

Half  fill  a  test  tube  with  concentrated  sulphuric  acid,  then  carefully 
add  milk,  pouring  down  the  side  of  the  tube  in  such  a  way  as  to 
prevent  mixing  of  the  milk  and  acid.  If  formaldehyde  is  present 
a  violet  or  blue  color  will  be  developed  at  the  junction  of  the  two 
liquids. 

This  test  depends  upon  reactions  that  are  not  understood.     A 
trace  of  ferric  chloride  or  other  oxidizing  agent  was   formerly 
1  Analyst,  20,  155  (1895). 


AGRICULTURAL  MATERIALS  557 

thought  to  be  necessary  but  milk  containing  formaldehyde  will 
respond  to  the  test  without  the  addition  of  any  reagent  other  than 
sulphuric  acid. 

CREAM 

For  the  examination  of  cream  the  methods  described  for  milk  are 
used,  the  following  modifications  being  made  in  two  determina- 
tions, to  provide  for  the  differences  in  composition. 

Ash. — Use  2  to  3  gm  of  sample  instead  of  3  to  5  gm. 

Fat.  Rose-Gottlieb  Method. — Weigh  4  to  5  gm  of  the  sample  into  the 
Rohrig  tube  and  dilute  with  water  to  10.5  cc.  Proceed  as  directed  for 
milk. 

Babcock  Method. — Two  standard  cream  test  bottles  are  described  in 
the  official  methods.  One  of  these  is  6  inches  and  the  other  9  inches 
long.  The  6-inch  bottle  is  described  as  follows: 

The  total  percent  graduation  shall  be  50.  The  total  height  of  the 
bottle  shall  be  150  to  165  mm.  The  capacity  of  the  bulb  up  to  the  neck 
shall  be  not  less  than  45  cc.  The  graduated  portion  of  the  neck  shall 
have  a  length  of  not  less  than  63.5  mm  and  the  neck  shall  be  cylindrical 
for  at  least  9  mm  below  the  lowest  and  above  the  highest  graduation 
marks.  The  graduations  shall  represent  5  percent,  whole  and  halves  of 
a  percent. 

Weigh  9  of  18  gm,  depending  upon  the  fat  content  of  the  sample,  into 
a  standard  Babcock  cream  test  bottle  and  proceed  as  directed  for 
the  determination  of  fat  in  milk.  If  a  9-gm  sample  was  used  multiply 
the  reading  by  2. 

CONDENSED  MILK 

Condensed  milk  is  a  product  obtained  by  evaporating  either 
whole  milk  or  skimmed  milk,  with  or  without  the  addition  of 
cane  sugar  as  a  preservative.  The  use  of  other  preservatives  is 
forbidden  by  the  U.  S.  food  and  drugs  act  of  1906.  Unsweetened 
condensed  milk  ("evaporated  milk")  must  contain  not  less  than 
34.3  percent  of  total  solids,  including  fat,  and  not  less  than  7.8 
percent  of  fat.  It  must  contain  no  added  butter  or  butter  oil.1 
Sweetened  condensed  milk  is  usually  evaporated  skimmed  milk. 
In  the  original  package  the  solids  usually  separate  to  some 
extent  and  careful  maceration  is  necessary  in  order  to  obtain 
a  uniform  mixture  for  analysis. 

XU.  S.  Dept.  Agr.,  Food  Insp.  Decision  131  (1911). 


558  QUANTITATIVE  ANALYSIS 

The  following  methods  for  the  preparation  of  samples  and  for 
their  analysis  are  official. 

Unsweetened  Condensed  Milk. — Dilute  40  gm  of  the  homogeneous 
sample  with  60  gm  of  water  and  proceed  as  directed  for  the  analysis 
of  milk,  correcting  the  results  for  the  dilution. 

Sweetened  Condensed  Milk. — Prepare  the  sample  as  follows: 

If  cold,  place  the  can  in  water  at  30°  to  35°  until  warm.  Open, 
scrape  out  all  of  the  milk  adhering  to  the  interior  of  the  can  and  mix 
in  a  dish  sufficiently  large  to  permit  thorough  stirring  so  as  to  make  the 
mass  homogeneous.  Weigh  100  gm  into  a  500  cc  flask  and  make  up  to 
the  mark  with  water.  If  the  milk  will  not  dissolve  completely  weigh 
out  each  portion  of  the  mixed  sample  for  analysis,  without  dilution. 

Total  Solids. — Use  10  cc  of  the  sample,  prepared  as  directed  above, 
and  proceed  as  with  milk,  using  either  dry  sand  or  dry  asbestos  fiber  for 
keeping  the  mass  open. 

Ash. — Evaporate  10  cc  of  the  solution  to  dryness  on  the  steam  bath 
and  ignite  the  residue  at  a  temperature  just  below  redness  until  the  ash 
is  free  from  carbon. 

Total  Nitrogen. — Use  10  cc  of  the  solution  and  proceed  as  for  the  deter- 
mination of  total  nitrogen  in  milk.  Multiply  the  result  by  6.38  and 
report  as  protein. 

Lactose. — On  account  of  the  presence  of  sucrose,  lactose  cannot  be 
determined  in  sweetened  condensed  milk  by  a  single  polarization.  It  is 
therefore  more  convenient  to  use  a  gravimetric  method.  Measure  100 
cc  of  the  milk  solution  into  a  250  cc  volumetric  flask.  Dilute  to  about 
200  cc  and  mix.  Add  6  cc  of  copper  sulphate  solution  ((a),  page  546), 
dilute  to  the  mark  on  the  flask  and  mix.  Filter  through  a  dry  filter  and 
determine  lactose  in  the  filtrate  by  the  gravimetric  method,  described 
for  the  analysis  of  milk. 

Fat  or  Ether  Extract. — Weigh  4  to  5  gm  of  the  homogeneous,  undiluted 
sample  into  a  Rohrig  tube,  dilute  to  about  10.5  cc  and  proceed  as 
directed  for  milk. 

Sucrose  (Tentative  Method}. — From  the  percent  of  total  solids  sub- 
tract the  sum  of  the  percents  of  proteids,  lactose,  fat  and  ash.  Report 
the  difference  as  the  percent  of  sucrose. 

BUTTER  AND  SUBSTITUTES 

The  examination  of  butter  is  made  in  order  to  determine  its 
quality,  if  known  to  be  genuine,  or  to  detect  the  partial  or  total 
substitution  of  other  oils  or  of  process  butter.  The  examination 
of  butter  substitutes  is  usually  made  for  the  purpose  of  determin- 


AGRICULTURAL  MATERIALS  559 

ing  their  quality  as  edible  fats  and  also  for  the  detection  and 
determination  of  the  various  oils  and  fats  composing  them. 
Process  butter  is  a  product  obtained  by  steaming  rancid  butter, 
cooling  and  churning  with  a  small  amount  of  sweet  milk.  It  is 
distinguished  from  fresh  butter  by  the  appearance  of  fat  crystals. 
Butter  fat  that  has  not  been  melted  is  not  crystallized. 

Butter  and  butter  substitutes  contain  water,  curd,  salt  and 
small  amounts  of  milk  solids,  in  addition  to  the  fats.  These 
substances  are,  for  the  most  part,  undissolved  and  the  sample  is 
therefore  non-homogeneous,  so  that  sampling  must  be  performed 
with  unusual  care. 

Preparation  of  Sample. — If  large  quantities  of  butter  are  to  be 
sampled,  use  a  butter  "trier"  or  sampler.  Withdraw  portions  of  about 
100  to  500  gm  in  a  closed  vessel  at  as  low  a  temperature  as  possible. 
When  softened,  cool  and  at  the  same  time  shake  the  mass  violently 
until  it  is  homogeneous  and  sufficiently  solid  to  prevent  the  separation 
of  water  and  fat,  then  pour  a  portion  into  a  small  beaker,  from  which  it 
is  to  be  weighed  for  analysis.  The  sample  should  nearly  or  completely 
fill  the  beaker  and  should  be  kept  in  a  cold  place  until  analyzed. 

Moisture. — After  butter  is  churned  it  is  washed  to  remove 
most  of  the  buttermilk  and  it  is  then  worked  to  remove  the  water. 
A  determination  of  moisture  will  serve  to  show  the  efficiency  of 
the  working  process.  Moisture  will  vary  between  the  approxi- 
mate limits  of  4  percent  and  35  percent.  About  15  percent 
should  be  regarded  as  the  normal  water  content. 

Determination. — Place  1.5  to  2.5  gm  of  butter  in  a  weighed  dish  with 
a  flat  bottom  which  has  a  surface  of  at  least  20  sq  cm  and  dry  over  a 
water  or  steam  bath  until  the  weight  is  constant,  each  drying  to  be  for 
one  hour.  The  use  of  clean  dry  sand  is  admissible  but  must  be  omitted 
if  this  dried  sample  is  to  be  used  for  the  indirect  determination  of  fat. 
Calculate  the  percent  of  moisture  in  the  sample. 

Fat  or  Ether  Extract. — Fat  is  determined  indirectly,  weighing 
the  solids  left  after  extraction  with  ether  or  gasoline,  or  directly 
by  the  methods  already  described  for  the  determination  of  fat 
in  milk.  There  is  little  to  be  gained  by  making  the  determina- 
tion, since  fat  necessarily  comprises  the  major  portion  of  the 
butter. 


560  QUANTITATIVE  ANALYSIS 

Determination  by  the  Indirect  Method. — Dissolve  in  the  dish  with 
anhydrous,  alcohol-free  ether  or  gasoline  the  dry  butter  obtained  in  the 
moisture  determination  in  which  sand  was  not  used,  transfer  to  a  weighed 
Gooch  crucible  by  the  acid  of  a  wash  bottle  filled  with  the  solvent  and 
wash  until  the  residue  is  free  from  fat.  Dry  the  crucible  and  contents 
at  100°  for  1-hour  periods  until  the  weight  is  constant  and  calculate  the 
percent  of  fat.  Preserve  the  contents  of  the  crucible  for  the  determina- 
tion of  casein. 

Determination  by  the  Direct  Method. — From  the  dry  butter  obtained 
in  the  determination  of  moisture,  either  with  or  without  the  use  of  sand, 
wash  the  fat  with  anhydrous,  alcohol-free  ether,  receiving  the  solution 
in  a  weighed  flask.  Distill  the  ether  and  dry  the  extract  at  100°  until  it 
ceases  to  lose  weight,  the  dryings  not  to  exceed  one  hour  in  duration. 
Calculate  the  percent  of  fat  in  the  sample. 

Casein. — Buttermilk  which  is  not  removed  by  washing  adds  a 
flavor  to  the  butter  and  this  is  considered  desirable  by  many 
users.  It  also  injures  the  keeping  qualities  of  the  butter  because 
of  its  tendency  toward  the  formation  of  substances  of  unpleasant 
taste  and  odor  by  subsequent  fermentation.  The  presence  of 
buttermilk  is  made  evident  by  proteids,  which  are  usually  re- 
ported as  casein.  The  percent  of  proteids  in  good  butter  is  less 
than  1. 

Determination. — Cover  the  -crucible  containing  the  residue  from  the 
determination  of  fat  by  the  indirect  method  and  heat,  gently  at  first, 
gradually  raising  the  temperature  to  just  below  redness.  The  cover 
may  then  be  removed  and  the  ignition  continued  until  the  residue  is 
white.  Weigh  and  calculate  the  loss  in  weight  as  casein. 

Salt.— Good,  marketable  butter  usually  contains  between  1 
and  6  percent  of  added  salt.  Its  determination  is  made  by  ex- 
traction with  water,  followed  by  titration  or  precipitating  and 
weighing  as  silver  chloride. 

Determination. — Weigh  5  to  10  gm  of  butter  in  a  counterpoised 
beaker,  using  portions  of  about  1  gm  from  different  parts  of  the  sample. 
Add  about  20  cc  of  hot  water  and,  after  the  butter  is  melted,  transfer 
the  whole  to  a  50  cc  separatory  funnel.  Insert  the  stopper  and  shake  for 
a  few  moments.  Let  stand  until  the  fat  has  all  collected  on  the  top  of 
the  water,  then  draw  off  the  latter  into  a  flask,  being  careful  to  let  none 
of  the  fat  globules  pass.  Again  add  hot  water  to  the  beaker  and  repeat 
the  extraction  from  ten  to  fifteen  times,  using  each  time  from  10  to  15 


AGRICULTURAL  MATERIALS  561 

cc  of  water.  The  washings  will  contain  all  but  a  trace  of  the  sodium 
chloride  originally  present  in  the  butter.  Determine  its  amount  in  the 
whole  or  in  an  aliquot  part  of  the  liquid  by  titration  with  standard 
silver  nitrate  solution,  using  potassium  chromate  as  indicator.  This 
method  is  described  on  page  397. 

Examination  of  the  Fat. — The  question  as  to  the  nature  of  the 
oils  and  fats  composing  butter  and  butter  substitutes  can  be 
answered  only  by  a  more  thorough  examination  than  that  already 
described.  The  composition  of  butter  fat  and  of  oleomargerine 
has  already  been  discussed  in  connection  with  the  analysis  of 
edible  fats  and  oils.  The  examination  of  the  fat  will  be  made  by 
the  same  methods  as  are  there  described.  It  is  sufficient  to 
recall  the  following  approximate  figures  as  the  most  character- 
istic differences  between  butter  fat  and  the  mixture  known  as 
"oleomargerine." 


Butter  fat 

Oleomargerine 

Butyro-refractometer.                        .    . 

42 

48 

Iodine  value  

36 

53 

Hehner  value     

87 

93 

Soluble  acids 

4  5 

0  7 

Reichert-Meissl  value  
Jean'a  eolubilitv  test.  . 

24 
63 

1 
30 

If  cocoanut  oil  is  used  in  oleomargerine  the  Reichert-Meissl 
value  is  raised  but  this  oil  is  easily  detected  by  noting  the 
Polenske  value.  (See  page  375.) 

For  the  examination  of  the  fat  the  water,  curd  and  salt  must 
be  removed  and  the  clear  fat  obtained. 

Preparation  of  Sample. — Melt  the  butter  and  keep  it  in  a  dry  place 
for  two  or  three  hours,  the  temperature  being  held  at  about  60°,  until 
the  water  and  curd  have  entirely  separated.  Pour  off  the  clear,  super- 
natant fat  and  filter  through  a  dry  paper  in  a  funnel  heated  by  boiling 
water,  or  in  an  oven  at  about  60°.  Should  the  filtered  liquid  fat  not  be 
perfectly  clear  it  must  be  filtered  again. 

Determination. — Determine  the  .constants  mentioned  in  the  above 
table,  using  the  methods  earlier  described  for  the  analysis  of  all  edible 
fats  and  oils. 

Qualitative  Tests :  Microscopic  Examination.— Place  a  small  portion 
of  the  fresh,  unmelted  sample,  taken  from  the  interior  of  the  mass,  on  a 
glass  slide,  add  a  drop  of  pure  olive  oil,  gently  press  over  it  a  cover  glass 

36 


562  QUANTITATIVE  ANALYSIS 

and  examine  for  crystals  of  lard,  etc.,  using  a  magnification  of  120  to 
150  diameters.  Examine  the  same  specimen  with  polarized  light  and  a 
selenite  plate  without  the  use  of  olive  oil.  Pure  fresh  butter  will  show 
neither  crystals  nor  a  parti-colored  field  with  selenite.  Other  fats, 
melted  and  cooled  and  mixed  with  butter,  will  usually  show  crystals  and 
variegated  colors  with  the  selenite  plate. 

For  further  microscopic  study  dissolve  about  4  cc  of  the  fat  in  15  cc 
of  ether  in  a  test  tube.  Close  the  tube  with  a  loose  plug  of  cotton  and 
allow  to  stand  12  hours  at  about  20°.  When  crystals  form  at  the  bottom 
of  the  tube  remove  them  with  a  pipette,  glass  rod  or  tube,  place  on  a 
slide,  cover  and  examine.  The  crystals  formed  by  later  deposits  may 
be  examined  in  a  similar  way. 

Foam  Test. — Heat  2  or  3  gm  of  the  sample  in  a  spoon  or  dish  over  a 
free  flame.  Genuine,  fresh  butter  will  foam  abundantly,  whereas 
process  butter  will  bump  and  sputter  without  foaming.  Oleomargerine 
behaves  like  process  butter  but  chemical  tests  will  determine  whether 
the  sample  is  oleomargerine  or  process  butter. 

Detection  of  Annatto  and  Saffron. — Dissolve  5  gm  of  the  fat  in  50  cc 
of  ether  in  a  wide  tube  and  shake  the  solution  vigorously  with  12  to  15 
cc  of  a  very  dilute  solution  of  potassium  hydroxide,  which  must  still  be 
basic  after  it  separates  from  the  ether  solution.  Allow  to  stand  for  a 
few  hours,  then  draw  off  the  aqueous  layer,  evaporate  to  dryness  and 
test  with  sulphuric  acid  which  in  the  presence  of  annatto  gives  first  a 
blue  or  violet  blue,  changing  quickly  to  green  and  finally  to  brown. 

Saffron,  which  would  be  extracted  at  the  same  time,  acts  differently 
when  treated  with  sulphuric  acid,  not  giving  the  green  coloration. 

The  aqueous  solutions,  if  not  clear  enough  for  the  tests,  must  not  be 
filtered,  as  the  filter  paper  will  take  up  large  quantities  of  the  coloring 
matter.  They  can  be  clarified  by  shaking  with  fresh  portions  of  ether 
or  carbon  disulphide. 

Uncolored  butter,  treated  in  this  way,  gives  only  a  very  slight  trace 
of  coloring  matter. 

Geissler's  Test  for  Azo  Colors. — Spread  a  few  drops  of  the  clarified 
fat  upon  a  porcelain  surface  and  add  a  pinch  of  fuller's  earth.  In  the 
presence  of  the  various  azo  coloring  matters  a  pink  to  violet  red  colora- 
tion will  appear  in  a  few  minutes.  Some  varieties  of  fuller's  earth  react 
much  more  readily  with  azo  colors  than  do  others.  The  earth  should 
therefore  be  previously  tested  with  dilute  solutions  of  known  azo  dyes. 

Low's  Test  for  Azo  Colors. — Melt  a  small  amount  of  the  fat  in  a  test 
tube,  add  an  equal  volume  of  a  mixture  of  one  part  of  concentrated  sul- 
phuric acid  and  four  parts  of  glacial  acetic  acid  and  heat  nearly  to  the 
boiling  point,  the  liquids  being  thoroughly  mixed  by  shaking.  Set 
aside  and  after  the  acid  solution  has  settled  out  it  will  be  colored  wine 


AGRICULTURAL  MATERIALS  563 

red  in  the  presence  of  azo  colors,  while  with  pure  butter  fat  little  or  no 
color  will  be  produced. 

Acid  and  Base  Tests  for  Added  Colors. — Pour  into  each  of  two  test 
tubes  about  2  gm  of  the  filtered  fat  dissolved  in  ether.  Into  one  of 
the  tubes  pour  1  or  2  cc  of  hydrochloric  acid  and  into  the  other  about 
the  same  volume  of  2  percent  potassium  hydroxide  solution.  Shake  the 
tubes  and  allow  to  stand.  In  the  presence  of  azo  dye  the  test  tube  to 
which  the  acid  has  been  added  will  show  a  pink  to  wine  red  coloration, 
while  the  basic  solution  in  the  other  tube  will  show  no  color.  If,  on  the 
other  hand,  annatto  or  other  vegetable  color  has  been  used  the  basic 
solution  will  be  colored  yellow  while  no  color  will  be  apparent  in  the 
acid  solution. 


CHAPTER  XVII 
THE  FIRE   ASSAY 

GOLD  AND  SILVER  ORES 

The  determination  of  gold  and  silver  in  ores  is  not  conveniently 
made  by  ordinary  gravimetric  or  volumetric  methods.  This  is 
because  of  (1)  the  relatively  small  quantity  of  these  metals  often 
contained  in  ores  that  are  commercially  valuable,  (2)  the  diffi- 
culties involved  in  dissolving  the  relatively  large  quantities  of 
ore  that  must  therefore  be  used  for  analysis  and  (3)  the  commer- 
cial demand  for  methods  that  will  not  involve  a  large  expenditure 
of  time.  The  method  that  is  commonly  used  for  this  purpose 
involves  the  fusion  of  the  ore  with  relatively  large  quantities  of 
the  various  fluxes  and  other  reagents,  the  gold  and  silver  being 
finally  obtained  in  the  elementary  condition,  in  which  form  they 
are  weighed.  Dependence  is  placed  upon  the  "noble"  character 
of  gold  and  silver,  oxidation  by  heating  in  air  being  impossible, 
upon  the  readiness  with  which  these  metals  alloy  with  lead  and 
upon  the  "base"  character  of  the  latter  metal,  separation  by 
oxidation  being  therefore  possible.  Lead  or  a  lead  compound 
is  made  a  part  of  the  fusion  mixture  and  it  completely  removes 
the  gold  and  silver  from  the  fusion.  From  this  alloy  lead  is  later 
oxidized  and  removed,  leaving  gold  and  silver  free  from  other 
metals  and  from  the  gangue  of  the  ore. 

The  term  "assay"  was  formerly  used  to  designate  all  such 
work  as  is  now  included  by  the  term  "analysis."  In  order  to 
distinguish  the  analysis  of  ores  by  fusion  processes  such  an  analy- 
sis was  called  a  "fire  assay."  "Assay"  now  usually  applies 
only  to  the  latter  class  of  methods. 

Sampling. — Gold  commonly  occurs  in  the  elementary  condi- 
tion in  ores.  It  is  sometimes  in  the  form  of  nuggets  of  varying 
size  but  the  greater  part  of  the  world's  supply  of  gold  comes  from 
ores  in  which  the  metal  is  not  visible,  it  being  in  very  small 
flattened  plates  or  scales  which  are  brown  instead  of  showing  the 

564 


THE  FIRE  ASSAY  565 

brassy  yellow  color  of  massive  gold.  These  scales  may  be  found 
associated  with  almost  any  other  mineral.  Quartz,  hematite, 
pyrite,  chalcopyrite,  slate,  syenite  and  sand  are  common  gold- 
bearing  minerals  but  the  metal  is  found  in  certain  localities  in  a 
great  many  other  minerals.  Gold  also  occurs  in  silver  ores  and 
in  combination  with  tellurium  as  sylvanite,  calaverite,  and 
petzite,  tellurides  of  gold  and  silver. 

Silver  also  occurs  native,  but  more  commonly  combined  with 
other  elements,  common  ores  being  argentite,  Ag2S,  pyrargerite, 
AgsSbSs,  and  cerargerite  or  horn  silver,  AgCl.  Silver  is  also 
nearly  always  found  in  lead  ores,  so  that  both  lead  and  silver 
(and  frequently  gold)  can  be  extracted  from  a  single  ore  with 
profit. 

The  general  discussion  of  sampling,  found  in  an  earlier  section 
of  this  book,  will  apply  to  the  sampling  of  gold  and  silver  ores, 
with  the  additional  note  that  all  operations  must  be  performed 
with  even  greater  care  than  in  most  other  cases.  The  compara- 
tively large  variation  in  the  commercial  value  of  a  gold  ore,  as 
indicated  by  a  small  variation  in  analytical  results,  makes  a 
high  degree  of  accuracy  absolutely  essential.  The  finally  ob- 
tained sample  should,  if  possible,  be  at  least  as  large  as  100  gm 
and  it  may  be  500  gm.  It  must  be  ground  to  pass  a  sieve  having 
100  meshes  to  the  linear  inch  and  it  is  often  ground  to  pass  a 
200-mesh  sieve.  Thorough  mixing  and  careful  quartering 
are  necessary  for  each  stage  in  the  division  of  the  large  sample. 

Weighing. — A  quantity  of  ore  varying  from  about  3  gm  to 
about  60  gm  will  be  used  for  the  assay,  the  weight  taken  depend- 
ing upon  the  richness  of  the  ore.  For  weighing  the  ground 
ore  or  "pulp"  the  analytical  balance  may  be  used  but  a  balance 
of  somewhat  less  sensibility  is  convenient  on  account  of  the 
greater  speed  possible  in  weighing  the  relatively  large  samples. 
The  ordinary  "pulp  balance"  is  enclosed  in  a  glass  case,  as  are 
more  sensitive  balances.  Its  sensibility  need  not  be  greater  than 
1,  this  making  a  somewhat  stronger  and  heavier  balance  possible. 
The  pans  should  be  detachable  so  that  they  may  be  lifted  by 
forceps  when  brushing  off  the  ore,  and  they  should  be  about  8  cm 
in  diameter. 

On  account  of  the  confusion  of  systems  of  weights  in  use  in 
the  mining  industries  it  is  not  convenient  to  use  the  metric 


566  QUANTITATIVE  ANALYSIS 

weights  for  weighing  the  sample  of  gold  or  silver  ore.  Ores  are 
always  estimated  in  avoirdupois  tons  of  2000  pounds.  On  the 
other  hand,  gold  and  silver  are  weighed  in  the  Troy  system  so  that 
the  content  of  the  ore  must  be  stated  in  Troy  ounces  per  avoirdu- 
pois ton.  In  the  work  of  assaying,  the  metals  as  finally  obtained 
from  the  ores  are  weighed  on  a  very  sensitive  balance,  using 
milligram  weights  and  fractions.  The  "assay  ton"  (abbre- 
viated to  A.  T.)  has  been  invented  as  a  unit  for  weighing  the 
sample  for  assaying,  this  unit  bearing  the  same  relation  to  the 
milligram  as  that  of  the  avoirdupois  ton  to  the  Troy  ounce. 

1  avoirdupois  ton  =2000X7000=14,000,000  grains; 
1  Troy  ounce  =  480  grains,  therefore 

1  avoirdupois  ton  =~'L~AQQ =29,166+  Troy  ounces. 

The  assay  ton  is  therefore  made  to  weigh  29166  mg  or  29.166  gm. 
If  an  assay  ton  of  ore  is  taken  for  assaying,  the  number  of  milli- 
grams and  fractions  of  gold  and  silver  obtained  will  indicate 
directly  the  "ounces -per  ton"  of  these  metals  in  the  ore.  Simple 
fractions  or  multiples  of  the  assay  ton  may  be  used  with  equal 
convenience. 

The  value  of  gold  is  fixed  by  statute  at  $20.67  per  ounce  Troy 
and  the  value,  per  ton,  of  a  gold  ore  may  be  calculated  from  this. 
The  commercial  value  of  silver  fluctuates,  although  the  legal 
coinage  ratio  of  silver  to  gold  in  the  United  States  is  -fa,  so  that 
1  ounce  Troy  of  silver  has  a  coinage  value  of  $1.29. 

For  the  final  weighing  of  the  gold  and  silver  a  balance  of  high 
sensibility  and  precision  is  necessary.  Differences  as  small  as 
0.01  mg  should  be  perceptible.  If  the  sample  used  were  as  small 
as  0.1  A.  T.,  as  is  frequently  the  case,  even  this  amount  of  error 
would  involve  a  difference  of  0.1  ounce  per  ton  and  this,  in  a  gold 
ore,  would  involve  an  uncertainty  in  value  of  $2.06  per  ton  of  ore. 
In  order  to  produce  a  balance  of  such  sensibility  the  knife  edges 
must  be  very  finely  and  accurately  ground  and  the  moving  parts 
must  be  very  light.  The  pans  are  only  2  to  3  cm  in  diameter  and 
instead  of  the  usual  supporting  bows  the  pans  are  suspended 
from  a  single  wire.  This  type  of  assayer's  balance  is  called  a 
"button  balance"  because  it  is  used  only  for  weighing  the 
"buttons"  of  metal. 


THE  FIRE  ASSAY 


567 


Decomposition  of  the  ore  and  extraction  of  the  gold  and  silver 
may  be  accomplished  by  either  the  crucible  process  or  the  scorifi- 
cation  process. 

Crucible  Process. — In  this  process  the  finely  ground  ore  is 
intimately  mixed  with  fluxes  and  lead  oxide  and  a  small  amount 
of  an  organic  reducing  agent  is  also  added  unless  the  ore  contains 
enough  material  of  a  reducing  character.  Three  essential 
changes  take  place  when  the  mix- 
ture is  heated  in  a  crucible: 

(1)  The   ore    combines    with    the 
fluxes  and  a  fused  slag  is  produced, 

(2)  the  lead  oxide  is  reduced  to 
finely    divided   lead,   and    (3)   the 
particles  of  reduced  lead,  settling 
in  a  fine  spray  through  the  whole 
mass  of  slag,  alloy  with  the  parti- 
cles  of   mechanically   freed   silver 
and  gold,   later  collecting   at   the 
bottom  of  the  crucible. 

For  the  fusion  of  the  mixture  of 
ore  and  fluxes  there  is  required  a 
large  crucible  of  fire  clay,  of  the 
form  shown  in  Fig.  120.  This  is 
placed  in  a  furnace  large  enough  to 
heat  two,  four,  six  or  even  more 
crucibles  at  a  time.  Such  a  furnace 
is  found  in  all  assaying  laboratories. 
It  may  be  heated  by  coke,  gas, 
gasoline,  kerosene  or  crude  oil.  A  detailed  description  of  such 
furnaces  will  be  found  in  any  of  the  special  books  dealing  with 
assaying. 

The  reagents  that  are  used  in  the  crucible  process  may  be 
classified  as  fluxes,  reducing  agents,  oxidizing  agents  and  lead 
compounds,  although  some  reagents  fall  in  more  than  one  of 
these  classes. 

Fluxes. — Fluxes  must  be  chosen  with  regard  to  the  nature  of 
the  ore  that  is  being  decomposed.  The  formation  of  a  fusible 
slag  is  due  to  a  chemical  combination  of  acid  and  basic  materials 
to  form  a  salt  or  mixture  of  salts.  If  the  gangue  of  the  ore  is  of  an 


FIG.  120. — Assay  crucible. 


568  QUANTITATIVE  ANALYSIS 

acid  nature,  as  is  the  case  with  quartz  and  many  silicates,  a 
basic  flux  will  be  required.  If,  on  the  other  hand,  the  ore  contains 
much  iron  oxide,  limestone  or  similar  metallic  oxides  or 
carbonates  it  will  require  a  flux  having  an  acid  character.  Many 
materials,  either  ore  refractories  or  fluxes,  are  classed  as  acid  or 
basic  when  they  have  the  composition  of  salts.  This  is  because 
they  have  the  power  of  combining  with  other  substances  to  form 
salts  of  lower  fusing  points,  or  because  they  change  in  such  a  way 
as  to  become  acid  or  basic  when  heated.  Thus  clay,  a  silicate 
of  aluminium,  is  classed  as  an  acid  refractory  because  it  will 
combine  with  calcium  oxide,  sodium  oxide,  etc.,  (added  as  car- 
bonates and  changed  by  heating)  forming  double  silicates,  fusing 
at  lower  temperatures.  Carbonates  of  the  alkali  and  alkaline 
earth  metals,  while  they  have  the  composition  of  normal  salts,  lose 
carbon  dioxide  when  heated  with  acid  materials  and  thus  act  the 
same  as  though  the  oxides  had  been  originally  added.  They  are 
therefore  classed  as  basic  fluxes.  The  principal  fluxes  that  are 
used  in  assaying  are  as  follows: 

Acid  Fluxes. — Silica,  borax  and  borax  glass  are  the  common 
members  of  this  class  although  a  great  many  other  compounds 
might  be  used.  Silica  combines  readily  with  metallic  oxides 
or  carbonates  forming  silicates  of  various  melting  points,  the 
latter  depending  upon  the  proportions  of  flux  and  metallic  oxide. 
Borax  or  sodium  pyroborate,  Na2B407.10H2O,  and  borax  glass, 
which  is  the  anhydrous  salt,  Na2B407,  act  as  acid  fluxes  because 
of  their  power  of  combining  with  metallic  oxides  to  form  ortho- 
borates  and  metaborates: 

Na2B407+5CaO->2NaCaB03+Ca3(B03)2. 

This  action  may  be  more  easily  understood  if  oxide  formulas  are 
used. 

Na2O.2B203+5CaO^Na20.2CaO.B2O3+3CaO.B203. 

The  reactions  of  crystallized  and  anhydrous  sodium  pyroborate 
are  identical  but  the  swelling  of  borax  which  accompanies  the 
loss  of  water  of  crystallization  often  causes  a  loss  of  a  part  of  the 
fusion  mixture.  On  this  account  the  borax  should  first  be  fused, 
cooled  and  powdered.  This  gives  the  product  known  as  borax 
glass. 


THE  FIRE  ASSAY  569 

Basic  Fluxes. — Fluxes  which  are  basic  or  which  become  basic 
when  heated  are  sodium  carbonate,  sodium  bicarbonate,  potas- 
sium carbonate,  potassium  bicarbonate,  calcium  carbonate, 
calcium  oxide,  lead  oxide,  ferric  oxide  and  argols.  The  alkali 
carbonates  and  bicarbonates  become  oxides  when  heated,  espe- 
cially when  an  acid  substance  is  present  to  combine  with  the 
oxides  as  formed.  The  bicarbonates  are  somewhat  cheaper  than 
the  normal  carbonates  but  they  evolve  twice  as  much  carbon 
dioxide  for  a  given  amount  of  metallic  oxide  formed  and  the 
normal  salts  are  therefore  preferable.  This  difference  is  shown 
by  the  following  equations: 

Na2C03+Si02->Na2Si03+CO2; 
2NaHC03+SiO2-»Na2Si03+2C02+H2O. 

For  similar  reasons  calcium  oxide  is  preferable  to  calcium  carbon- 
ate, although  neither  of  these  fluxes  is  extensively  used  in  assay- 
ing. Lead  oxide  is  an  excellent  flux  for  silica  and  silicates  as 
most  lead  silicates  fuse  easily  and  become  more  fluid  than  many 
other  slags.  On  account  of  its  relatively  high  cost  it  is  substi- 
tuted, as  far  as  possible,  by  some  of  the  cheaper  fluxes  already 
named.  It  is,  however,  always  added  to  the  mixture  to  serve  as 
a  source  of  finely  divided  lead  and  it  therefore  always  acts  to  some 
extent  as  a  basic  flux.  The  amount  which  will  actually  enter  the 
slag  is  diminished  by  increasing  the  proportion  of  other  basic 
fluxes,  such  as  sodium  carbonate.  Argols  or  crude  potassium 
bitartrate,  KHCiH^Oe,  is  not  added  to  serve  as  a  flux  but  to 
reduce  lead  oxide.  However,  it  acts  as  a  basic  flux  by  virtue  of 
the  potassium  which  it  contains,  heating  causing  decomposition: 

2KHC4H406->K20+5H20+6CO+2C. 

Carbon  monoxide  and  carbon  thus  formed  reduce  lead  oxide. 
Reducing  Agents. — Metallic  lead  must  be  very  intimately 
mixed  with  the  ore  after  fusion  has  taken  place,  in  order  that  it 
may  alloy  with  all  particles  of  gold  and  silver.  It  is  not  practica- 
ble to  mix  elementary  lead  with  the  ore  before  fusion  because  of 
the  difficulty  involved  in  reducing  the  metal  to  a  sufficiently  fine 
state  of  division  and  because  lead  so  mixed  would  settle  out  of 
the  mixture  before  complete  decomposition  and  fusion  of  the  ore. 
Instead,  it  is  better  to  mix  finely  powdered  lead  oxide  with  the 


570  QUANTITATIVE  ANALYSIS 

charge,  providing  a  reducing  agent  that  will  act  upon  the  oxide 
somewhat  slowly,  in  this  way  providing  intimate  contact  of  the 
minute  particles  of  lead  with  every  portion  of  the  ore.  Reducing 
agents  may  constitute  a  part  or  all  of  the  gangue  of  the  ore.  The 
most  important  of  such  reducing  agents  occurring  in  ores  are 
sulphides  and  arsenides.  Reactions  like  the  following  may  occur : 

5PbO+FeS2->5Pb+FeO+2S02. 

2PbO+PbS^3Pb+SO2. 
3PbO+ZnS-»ZnO-fSOs+3Pb. 

Many  ores,  on  the  other  hand,  contain  no  reducing  agents  or 
even  contain  oxidizing  agents,  such  as  ferric  oxide  or  cupric 
carbonate.  In  such  cases  its  becomes  necessary  to  add  a  reducing 
agent  to  the  crucible  charge.  Reducing  agents  used  for  this 
purpose  are  argols,  charcoal,  flour  and  starch.  Many  other 
organic  compounds  might  be  used  with  equal  success  but  the 
cost  would  generally  be  higher  and  no  advantage  would  be  gained. 
The  exact  reducing  power  of  none  of  the  common  reducing  agents 
can  be  calculated  because  they  are  not  usually  uniform  in  com- 
position. A  preliminary  fusion  with  lead  oxide  and  suitable 
fluxes  will  establish  this  value.  1  gm  of  argols  of  average  purity 
will  reduce  about  10  gm  of  lead,  under  the  average  conditions 
that  obtain  in  the  crucible  fusion. 

Oxidizing  Agents. — It  has  already  been  stated  that  some  ores, 
particularly  those  containing  sulphides,  reduce  lead  oxide  with 
the  production  of  metallic  lead.  They  may  even  reduce  more 
oxide  than  is  desirable,  in  which  case  the  quantity  of  reduced 
lead  must  be  diminished  by  the  addition  of  an  oxidizing  agent. 
Whether  this  acts  upon  the  sulphides  or  upon  the  lead  after 
reduction  is  immaterial.  Both  actions  occur  to  some  extent 
but  the  ultimate  effect  of  either  is  to  cause  less  lead  to  be  finally 
obtained. 

Instead  of  adding  an  oxidizing  agent  the  ore  may  be  subjected 
to  a  preliminary  roasting  or  heating  in  air,  oxidation  of  sulphides 
and  arsenic  leaving  the  ore  almost  entirely  free  from  reducing 
power,  but  this  is  an  operation  that  requires  additional  time  and 
it  is  usually  simpler  to  add  an  oxidizing  agent  to  the  mixture 
which  is  to  be  heated  in  the  crucible. 

The  only  oxidizing  agent  that  is  commonly  used  in  assaying 


THE  FIRE  ASSAY  571 

is  potassium  nitrate.  Sodium  nitrate  would  serve  equally  well 
but  its  tendency  to  deliquesce  in  moist  air  hinders  proper  mixing 
with  the  other  constituents  of  the  crucible  charge.  As  a  substi- 
tute for  an  oxidizing  agent  metallic  iron  is  often  used,  usually 
in  the  form  of  nails  that  are  long  enough  to  reach  through  the 
entire  mixture  in  the  crucible  and  to  protrude  above.  Iron  acts 
as  a  "desulphurizer,"  by  forming  ferrous  sulphide,  a  compound 
which  does  not  reduce  lead  oxide: 


PbS+Fe-*Pb+FeS. 

This  last  reaction  produces  lead  but  not  as  much  as  would  be 
formed  if  lead  sulphide  (of  galena)  were  allowed  to  react  with 
lead  oxide: 

PbS+2PbO->3Pb+S02. 

Lead  Compounds.  —  Lead  oxide  has  already  been  mentioned 
in  connection  with  fluxes.  In  the  form  of  litharge  this  is  the  only 
lead  compound  that  is  ever  used  for  this  purpose  in  the  crucible 
process  for  decomposing  ores.  On  account  of  the  almost  uni- 
versal association  of  lead  with  silver  in  nature  it  is  difficult  to 
obtain  litharge  that  is  entirely  free  from  silver.  It  is  often 
furnished  by  manufacturers  under  the  label  "silver  free"  but 
this  is,  by  no  means,  to  be  taken  as  an  assurance  that  it  contains 
no  silver.  A  preliminary  assay  of  litharge  should  be  made  with 
each  lot  purchased  and  the  proper  correction  made  in  the  crucible 
assays  of  ores.  This  corresponds  to  the  "blank"  determinations 
often  made  in  connection  with  other  analytical  processes. 

Crucible  Charge.  —  The  correct  charge  for  the  crucible  fusion 
can  be  made  only  upon  the  basis  of  a  knowledge  of  the  composi- 
tion of  the  ore.  The  experienced  assayer  can  usually  decide  from 
the  general  appearance  of  the  ore  as  to  the  approximate  mixture 
that  is  necessary  for  the  proper  fusion  and  extraction  of  the  gold 
and  silver.  This  is  true,  however,  only  in  case  the  unground  ore 
can  be  inspected,  since  powdering  destroys  the  characteristic 
appearance  of  most  minerals.  Fortunately  a  certain  latitude 
in  the  proportion  of  the  ingredients  is  permissible.  Small  varia- 
tions from  the  composition  of  the  best  possible  mixture  will 
often  work  no  more  serious  harm  than  production  of  a  slag 


572  QUANTITATIVE  ANALYSIS 

requiring  a  somewhat  higher  temperature  for  complete  lique- 
faction or  the  production  of  a  lead  button  that  is  somewhat 
larger  than  is  desirable.  The  fact  that  some  variation  is  per- 
missible and  also  that  experienced  assayers  do  not  often  make  pre- 
liminary analyses  of  ores  often  leads  the  student  to  the  conclusion 
that  the  mixing  of  charges  is  largely  a  matter  of  guess  work  in 
any  case.  This  is  very  far  from  being  true. 

To  enter  into  a  detailed  discussion  of  all  classes  of  ores  is  not 
possible  in  this  brief  treatment  of  the  subject  of  assaying.  Cer- 
tain typical  ores  will  be  considered  and  the  approximate  crucible 
charge  for  each  class  stated.  A  knowledge  of  the  fundamental 
principles  of  assaying  will  thereby  be  gained  and  the  proper 
treatment  of  other  ores  will  be  known  as  a  result  of  more  extensive 
work  in  the  assaying  laboratory. 

Preliminary  Assay. — In  order  to  determine  the  exact  propor- 
tions of  the  various  components  of  the  crucible  charge  necessary 
for  the  best  results,  a  preliminary  assay  may  be  made,  using 
smaller  quantities  of  the  materials  than  will  be  used  in  the  final 
assay.  This  serves  the  purpose  of  a  preliminary  analysis  and  the 
degree  of  fluidity  of  the  slag  and  the  size  of  the  lead  button 
obtained  will  indicate  the  necessary  modification  of  the  charge 
for  the  final  assay.  The  preliminary  assay  is  usually  omitted 
unless  the  available  sample  of  ore  is  small  or  unless  the  ore  is 
already  powdered  so  that  its  chemical  nature  cannot  be  deter- 
mined by  inspection.  It  is  usually  preferable  to  use  the  full 
quantities  of  ore  and  reagents.  If  the  assay  is  successful  it  need 
not  be  repeated.  If  it  is  unsuccessful  it  serves  all  of  the  purposes 
of  a  preliminary  and  a  new  assay  may  then  be  made.  The  slag 
should  be  perfectly  fluid  and  the  lead  button  obtained  should 
weigh  25  to  30  gm. 

Silicious  Ores  Containing  No  Reducing  or  Oxidizing  Com- 
pounds.— The  chief  gangue  of  this  class  of  ores  is  quartz  or  sili- 
cates such  as  clay  or  the  felspars.  The  crucible  charge  must 
contain  the  proper  fluxes  for  the  production  of  a  liquid  slag,  also 
litharge  and  a  reducing  agent.  The  only  fluxes  required  are 
sodium  carbonate  and  litharge.  Argols  is  probably  the  best 
reducing  agent.  After  the  materials  are  thoroughly  mixed  they 
are  placed  in  the  crucible  and  covered  with  a  layer  of  sodium 
chloride  about  1/4  inch  thick.  The  salt  fuses  at  a  comparatively 


THE  FIRE  ASSAY  573 

low  temperature  (776°)  and  the  liquid  cover  thus  formed  prevents 
the  loss  of  powdered  ore  by  action  of  escaping  gases. 

Silicious  Ores. — A  typical  charge  for  a  silicious  ore  would  be 
as  follows: 

Ore 1  A.  T. 

Litharge 50  gm 

Sodium  carbonate 50  gm 

Borax  glass 2  gm 

Argols 2 . 5  gm 

Salt cover 

Borax  glass  is  here  added  to  increase  the  fluidity  of  the  slag  by 
the  formation  of  lead  borate.  If  crystallized  borax  is  used, 
nearly  twice  as  much  will  be  required,  as  will  be  seen  from  the 
molecular  weights  of  the  two  substances  (202  and  382). 

In  this,  as  in  all  other  charges  suggested  in  the  following  pages, 
the  stated  proportions  of  the  components  are  merely  average 
proportions  for  ores  of  the  general  characters  indicated.  Varia- 
tion in  the  charges  will  often  be  necessary  but  these  will  generally 
be  made  upon  the  advice  of  the  instructor.  No  condensed  state- 
ment of  this  kind  could  meet  all  of  the  conditions  brought  about 
by  small  variations  in  the  composition  of  ores.  It  is  well  to 
remember  that  increasing  the  proportion  of  sodium  carbonate 
somewhat  increases  the  size  of  the  lead  button,  also  that  too  much 
borax  gives  a  thick  and  viscous  slag.  The  variation  in  the  size 
of  the  lead  button  is  due  to  the  fact  that  with  small  quantities 
of  sodium  carbonate  present  more  litharge  enters  the  slag  as  a 
basic  flux,  leaving  the  argols  to  be  oxidized  to  some  extent  by 
the  air. 

Silicious  and  Carbonate  Ores. — For  an  ore  in  which. limestone 
occurs  with  the  silicious  gangue  the  following  mixture  will  serve: 

Ore 1  A.  T. 

Litharge 35  gm 

Sodium  carbonate 35  gm 

Borax  glass 5  gm 

Silica 5  gm 

Argols 2.5  gm 

Salt cover 

Oxidizing  Ores. — Oxidizing  ores  contain  certain  reducible 
oxides  or  carbonates  such  as  those  of  iron  or  manganese,  ferric 


574  QUANTITATIVE  ANALYSIS 

oxide  being  the  most  common.  If  the  main  portion  of  the  gangue 
is  still  silicious  the  charge  will  be  similar  to  that  stated  above 
for  a  silicious  ore,  but  modified  by  increasing  the  amount  of 
argols  and  by  the  addition  of  more  borax  glass,  borax  or  silica  to 
serve  as  a  flux  for  the  metallic  oxides. 

The  following  charge  may  be  used  for  a  silicious  ore  containing 
about  50  percent  of  hematite: 

Ore 1  A.  T. 

Litharge 40  gm 

Sodium  carbonate 30  gm 

Borax  glass 10  gm 

Silica 5  gm 

Argols 7  gm 

Salt cover 

If  the  slag  is  sticky  instead  of  fluid  increase  the  proportion  of 
sodium  carbonate.  If  the  lead  button  is  too  small  increase  the 
amount  of  argols,  calculating  the  amount  to  be  added  from  the 
previously  determined  reducing  power  of  the  argols.  If  the  ore 
contains  more  than  50  percent  of  hematite  increase  the  borax 
glass,  silica  and  argols  accordingly.  If  it  contains  less  than  this 
quantity  decrease  the  amount  of  these  substances. 

Reducing  Ores. — Ores  containing  sulphides  or  other  reducing 
agents  capable  of  reducing  lead  oxide  must  first  be  roasted  or  else 
an  oxidizing  agent  or  a  desulphurizer  must  be  added.  The 
addition  of  potassium  nitrate  is  recommended.  The  production 
of  nitrogen  oxides  and  sulphur  dioxide  causes  some  disturbance 
and  the  preliminary  heating  must  be  moderated  accordingly. 
The  addition  of  iron  as  a  desulphurizer  is  not  suitable  for  ores 
containing  arsenic  on  account  of  the  formation  of  a  "speiss" 
or  arsenide  of  iron.  This  speiss  separates  from  both  slag  and 
lead  and  it  usually  carries  gold  if  this  metal  is  present  in  the  ore. 
If  iron  nails  are  used  they  must  be  removed  from  the  crucible 
before  pouring  and  they  sometimes  cause  trouble  on  account 
of  the  adherence  of  small  globules  of  lead,  this  causing  a  loss  of 
gold  and  silver. 

For  a  pyritic  ore  the  following  charges  are  suggested  for  trial 
assays.  Unless  the  reducing  power  of  the  ore  is  exactly  known 
it  is  impossible  to  predict  the  weight  of  lead  that  will  be  obtained 
but  the  charge  may  be  modified,  if  necessary,  after  the  first  trial. 


THE  FIRE  ASSAY 


575 


Nitrate  Method    Iron  Method 


Ore..  0.5  A.  T. 


70  gm 

25  gm 

Borax  glass 2.5  gm 


Litharge 

Sodium  carbonate. 


Silica 

Potassium  nitrate. . 
Nails,  cut,  20d .... 
Salt. .  . 


2  gm 
5  gm 


0.5  A.  T. 

30  gm 

50  gm 

5  gm 

2  gm 


4 
cover 


If  the  lead  button  obtained  by  the  nitrate  method  is  too  small 
decrease  the  amount  of  potassium  nitrate;  if  it  is  too  large  increase 
the  potassium  nitrate.  If  the  button  from  the  iron  method  is 
too  small  add  a  calculated  weight  of  argols. 

Ores  Containing  Copper,  Arsenic  or  Antimony. — If  ores  con- 
taining compounds  of  copper,  arsenic  or  antimony  are  treated 
by  the  methods  already  described,  without  modification,  these 
metals  will  be  reduced  and  will  enter  the  lead  button  as  constitu- 
ents of  the  alloy.  The  button  will  thereby  be  rendered  brittle 
and  difficult  to  free  from  slag.  This  action  must  not  be  prevented 
by  the  addition  of  potassium  nitrate  because  this  will  prevent 
also  the  formation  of  a  lead  button  of  sufficient  size.  It  can  be 
prevented  without  this  interference  by  largely  increasing  the 
proportion  of  litharge,  which  keeps  the  interfering  elements  in 
the  form  of  their  oxides,  these  entering  the  slag  by  combination 
with  fluxes.  Copper  oxide  will,  of  course,  combine  with  acid 
fluxes  and  the  proportion  of  borax  glass  or  silica  is,  on  this 
account,  increased.  Oxides  of  arsenic  or  antimony  will  require 
basic  fluxes. 

The  following  charge  may  be  used  for  an  ore  which  is  largely 
chalcopyrite,  Cu2S.Fe2S3. 

Ore 0.5  A.  T. 

Litharge 125  gm 

Sodium  carbonate 30  gm 

Borax  glass 10  gm 

Silica 5  gm 

Salt cover 

It  is  here  assumed  that  sufficient  lead  will  be  reduced  by  the 
sulphides  present  without  the  necessity  for  the  addition  of 
another  reducing  agent.  If  this  is  not  the  case  add  the  required 
amount  of  argols  in  the  next  fusion. 


576  QUANTITATIVE  ANALYSIS 

Copper  in  a  slag  is  indicated  by  a  red  or  green  color,  the  former 
being  due  to  cuprous  silicate  and  borate,  the  latter  to  cupric 
salts  of  the  same  acids. 

Cupellation. — After  the  ore  has  been  decomposed  and  the  alloy 
of  lead,  gold  and  silver  has  been  obtained  the  button  must  be 
treated  in  such  manner  as  to  completely  remove  the  lead.  This 
is  easily  accomplished  because  of  the  readiness  with  which  lead 
oxidizes  when  heated  in  contact  with  air.  The  alloy  is  placed 
in  a  small  vessel  called  a  "cupel,"  which  is  made  of  bone  ash 
and  is  shaped  as  shown  in  Fig.  121.  Bone  ash  is  chiefly  composed 
of  calcium  phosphate  and  this  has  the  very 
valuable  property  of  being  able  to  absorb  lead 
oxide  at  high  temperatures.  In  making  the 
cupel  the  ash  is  moistened  and  pressed 
together.  This  produces  a  mass  which  is 
quite  porous  so  that  the  lead  oxide  is  readily 

12i. cupel,     absorbed.     Cupels  thus  made  are  very  fragile 

and  must  be  handled  carefully.  Laboratories 
in  which  large  numbers  of  assays  are  made  usually  make  their 
own  cupels  by  means  of  an  inexpensive  hand  or  power  press. 
Manufacturers  of  cupels  for  shipping  usually  add  to  the  water 
a  small  amount  of  glue  or  molasses  which  serves  as  a  binding 
material.  This  chars  and  blackens  when  the  cupel  is  heated 
but  it  soon  burns  out. 

The  cupel  must  be  large  enough  to  easily  contain  the  lead 
alloy  after  the  button  is  melted.  It  should  weigh  at  least  as 
much  as  the  button  in  order  to  efficiently  absorb  the  necessary 
quantity  of  lead  oxide. 

The  furnace  in  which  the  cupel  is  heated  is  of  the  muffle  type 
and  it  must  have  a  good  draught  in  order  that  the  lead  may  be 
quickly  oxidized.  The  muffle  is  an  arched  chamber  of  fire  clay, 
varying  from  4  to  12  inches  wide,  8  to  20  inches  long  and  3  to  6 
inches  high.  It  is  heated  by  a  furnace  in  which  the  fuel  may  be 
any  of  those  used  for  the  crucible  furnace. 

As  cupellation  proceeds  the  lead  is  oxidized,  a  part  is  volatilized 

and  drawn  into  the  chimney  and  the  remainder  is  absorbed  by 

the  cupel.     When  this  process  is  finished  the  button  of  gold  and 

silver  is  weighed  and  it  is  then  prepared  for  the  process  of  parting. 

Inquartation. — The  gold  and  silver  of  the  button  are  to  be  sepa- 


THE  FIRE  ASSAY  577 

rated  by  dissolving  the  silver  in  nitric  acid.  If  gold  constitutes' 
more  than  about  one-fourth  of  the  weight  of  the  button  the  silver 
will  dissolve  very  slowly.  In  this  case  it  is  necessary  to  increase 
the  proportion  of  silver  after  the  button  has  been  weighed.  This 
process  is  known  as  "inquartation."  The  button  is  wrapped  in 
the  necessary  quantity  of  pure  silver  foil  and  is  then  placed  in  a 
clean  cupel  and  melted  by  means  of  a  blowpipe.  It  is  kept  in  a 
fused  condition  until  the  added  silver  has  thoroughly  dissolved 
and  it  is  then  allowed  to  cool.  With  some  experience  it  is  not 
difficult  to  determine  whether  it  will  be  necessary  to  inquart  the 
button  before  parting,  the  depth  of  yellow  indicating  the  approxi- 
mate percent  of  gold.  In  the  beginning  it  is  better  to  inquart 
if  the  button  is  appreciably  yellow. 

Parting. — The  addition  of  nitric  acid  (specific  gravity  1.2)  to 
a  button  which  contains  not  more  than  about  25  percent  of  gold 
will  cause  the  solution  of  all  of  the  silver,  leaving  the  gold  in  the 
form  of  a  brown  skeleton  which  later  usually  falls  to  a  coarse 
powder.  The  nitric  acid  must  be  free  from  chlorine  as  otherwise 
gold  will  be  dissolved  to  some  extent. 

Annealing. — Gold  left  from  the  parting  process  has  not  the 
characteristic  yellow  color  of  massive  gold  but  acquires  it  upon 
being  heated.  The  mere  change  of  color  is  of  little  or  no  impor- 
tance but  the  heating  that  is  necessary  for  complete  drying 
causes  the  change.  This  process  is  called  " annealing."  After 
annealing  the  gold  is  brushed  into  the  pan  of  the  button  balance 
and  weighed. 

Determination. — Sample  the  ore  according  to  the  usual  plan  but 
exercise  extraordinary  care  in  the  operations  of  mixing  and  dividing. 
The  last  portion  obtained  should  weigh  at  least  100  gm  and  it  must 
be  ground  to  pass  a  100-mesh  sieve  without  forcing  by  the  brush.  Mix 
the  sifted  sample  by  rolling  and  leave  in  a  flattened  pile  on  the  paper. 
Take  out  the  sample  for  weighing  by  means  of  a  spatula,  dipping  from 
various  parts  of  the  pile  and  taking  the  entire  depth  of  the  pile  at  each 
dipping.  Weigh  in  the  pulp  balance  the  amount  o  ground  ore  that  is 
required  for  the  fusion,  using  one  of  the  charges  already  suggested  or  a 
modification  made  by  the  instructor  according  to  known  variations  in 
the  character  of  the  ore.  (If  the  ore  is  known  to  be  a  very  rich  one  the 
quantity  of  sample  used  will  be  less  than  1  A.  T.  and  the  other  com- 
ponents of  the  charge  will  be  reduced  accordingly.) 

37 


578  QUANTITATIVE  ANALYSIS 

Before  removing  the  weighed  ore  from  the  pulp  balance  weigh, 
on  the  ordinary  laboratory  balance,  the  other  components  of  the  charge 
with  the  exception  of  the  salt,  in  the  order  named  in  the  statement  of  the 
charge,  beginning  with  the  litharge.  All  of  the  reagents  must  be  free 
from  lumps.  Place  these  substances  in  a  flattened  pile  on  a  piece  of 
mixing  paper  or  oilcloth  and  finally  brush  the  ore  sample  onto  the  top 
of  the  pile  and  mix  well  by  rolling.  Empty  the  charge  into  an  assay 
crucible  which  is  6  to  8  inches  in  height,  brush  the  paper  to  remove  the 
last  of  the  mixture  and  tap  the  crucible  slightly  to  settle  the  charge. 
Lastly  cover  the  mixture  with  a  layer  of  salt  about  K  inch  thick  and 
place  the  crucible  in  the  furnace,  which  should  not  be  hot  enough  to 
crack  the  crucible.  Raise  the  temperature  gradually,  using  a  moderate 
temperature  until  violent  effervescence  has  ceased.  After  the  fusion  is 
quiet  heat  to  a  temperature  of  bright  redness  for  a  period  lasting  from  ten 
minutes  to  one  hour,  according  to  the  difficulty  experienced  in  obtain- 
ing complete  decomposition. 

The  pouring  mould,  which  is  made  of  iron  and  has  conical  depressions, 
should  be  warmed  meanwhile  to  prevent  sudden  chilling  of  the  button 
and  slag  when  pouring.  Lift  the  crucible  from  the  furnace,  tap  lightly 
to  settle  globules  of  lead  that  may  be  suspended  in  the  slag  and  pour 
quickly  but  steadily,  into  the  mould.  This  mould  is  never  made  large 
enough  to  contain  all  of  the  slag,  so  that  the  latter  will  always  run  over. 
A  mould  which  would  contain  the  entire  contents  of  the  crucible  would 
require  an  inconveniently  long  time  for  cooling.  The  attention  should 
be  fixed,  not  upon  the  slag  but  upon  the  lead  alloy,  which  appears  as  a 
bright  stream  near  the  end  of  the  pouring.  This  stream  must  be 
directed  toward  the  center  of  the  mould  and  it  must  be  poured  without 
splashing.  The  lead  immediately  sinks  to  the  bottom  where  it  later 
solidifies  as  an  inverted  cone  under  the  slag.  After  pouring,  the  crucible 
must  be  free  from  masses  of  imperfectly  fused  slag  and  from  particles 
of  lead. 

Allow  the  mould  to  stand  quietly  until  the  slag  and  lead  button  are 
perfectly  solid  then  invert  the  mould,  when  the  contents  will  easily  drop 
out.  Carefully  break  the  slag  from  the  button  by  means  of  a  hammer. 
Examine  the  slag  in  order  to  detect  any  particles  of  lead  that  may  have 
been  caught  by  it.  Such  particles  may  be  saved  but  the  assay  is  not 
reliable  in  such  a  case  and  it  is  better  to  begin  again,  changing  the  con- 
ditions as  may  be  necessary  to  obtain  perfect  separation.  The  button 
should  be  quite  malleable  and  it  should  separate  easily  without  leaving 
a  crust  of  lead  on  the  slag.  Carefully  free  the  button  from  all  adhering 
particles  of  slag  by  hammering  on  a  small,  clean  anvil.  This  operation 
should  be  performed  in  such  a  manner  as  to  leave  the  button  in  the  form 
of  a  cube,  finally  truncating  the  corners  to  prevent  later  injury  to  the 


THE  FIRE  ASSAY  579 

cupel  If  detached  particles  of  lead  have  been  recovered  place  these 
on  the  clean  cube  and  weld  into  the  latter  by  a  stroke  of  the 
hammer. 

The  lead  button  should  weigh  from  25  to  30  gm  although  a  button  as 
light  as  20  gm  will  often  contain  all  of  the  gold  and  silver  of  the  ore,  if 
the  fusion  has  been  normal.  If  a  heavier  button  than  30  gm  is  obtained 
do  not  discard  it  but  reduce  its  size  by  scorification,  a  process  to  be 
later  described  (page  580).  If  the  button  is  too  small  or  if  the  slag  does 
not  pour  and  separate  well,  make  another  charge,  properly  modified  in 
accordance  with  the  principles  already  discussed. 

Cupellation. — Place  a  cupel  in  the  already  heated  muffle  furnace  and 
bring  to  a  temperature  of  bright  redness,  then  carefully  drop  in  the 
button  by  means  of  long  tongs  provided  for  the  purpose.  Close  the 
muffle  door  and  raise  the  temperature  to  about  700°  (bright  redness) 
when  the  black  crust  of  lead  suboxide,  Pb20,  changes  to  the  yellow 
monoxide,  PbO,  and  this  begins  to  volatilize.  Remove  the  top  of  the 
muffle  door,  the  lower  half  being  left  in  to  shield  the  cupel  from  the 
entering  current  of  cold  air.  If  the  door  is  in  one  piece  remove  and 
place  two  or  three  empty  cupels  in  front  of  the  one  that  contains  the 
alloy. 

Much  of  the  lead  oxide  is  vaporized  and  is  drawn  into  the  chimney 
while  the  remainder  is  absorbed  by  the  cupel.  If  the  temperature  is 
too  high  the  lead  will  boil,  with  consequent  mechanical  loss.  If  the 
temperature  is  too  low  the  button  will  freeze  and  again  loss  will  occur 
through  "sprouting."  The  latter  action  is  due  to  the  contraction  of  the 
cooled  and  solidified  skin  of  lead,  the  liquid  alloy  from  the  interior 
breaking  through  and  often  being  thrown  out  of  the  cupel.  A  sprouted 
button  should  be  at  once  discarded  as  the  results  obtained  from  it  will 
be  unreliable.  The  correct  temperature  for  cupellation  is  indicated  by 
the  presence  of  a  ring  of  litharge  crystals  upon  the  inner  surface  of  the 
cupel,  just  above  the  liquid  alloy. 

As  cupellation  proceeds  the  button  becomes  smaller.  Toward  the 
end  of  the  process  the  temperature  must  be  raised  somewhat,  on  account 
of  the  rise  in  the  melting  point  of  the  alloy  which  is  now  richer  in  gold 
and  silver,  metals  of  higher  melting  point  than  that  of  lead.  So  long 
as  lead  remains  the  surface  of  the  alloy  is  covered  by  a  thin,  iridescent 
layer  of  oxide  which  is  in  continuous  motion  over  the  surface.  As  the 
last  of  the  lead  is  oxidized  the  iridescence  suddenly  disappears  and  the 
surface  brightens  or  "blicks."  To  insure  the  removal  of  the  last  trace 
of  lead  close  the  door  of  the  muffle  and  raise  the  temperature  for  about 
one  minute.  Remove  the  cupel  from  the  muffle  and  cover  the  former 
in  order  to  prevent  too  rapid  cooling  and  consequent  sprouting  of  the 
button.  When  the  button  is  cold  take  it  out  by  means  of  a  pair  of 


580  QUANTITATIVE  ANALYSIS 

strong,  pointed  pliers  and  brush  with  a  stiff  brush  to  remove  particles 
of  oxide  or  cupel  material.  Weigh  on  the  button  balance  and  record 
the  weight  in  milligrams. 

Inquartation. — If  the  button  is  silver  white  or  only  faintly  yellow  it 
may  be  parted  without  previous  inquartation.  If  the  intensity  of 
yellow  indicates  more  than  about  25  percent  of  gold  cut  a  piece  of  pure 
silver  foil  weighing  from  one  to  three  times  as  much  as  the  button, 
according  -to  the  indicated  composition  of  the  button.  Wrap  the 
button  in  this  foil  and  place  in  a  new,  clean  cupel.  Carefully  fuse  by 
means  of  a  blowpipe  and  keep  in  the  fused  condition  for  one  minute, 
then  allow  to  cool. 

Parting. — Place  the  button  in  a  No.  1  porcelain  crucible  and  nearly 
fill  the  latter  with  nitric  acid  whose  specific  gravity  is  1.2.  Warm 
gently  until  action  begins.  The  silver  should  dissolve  rapidly  enough 
to  cause  a  moderate  evolution  of  gas.  If  it  does  not  do  so.  it  contains 
too  much  gold  and  parting  will  be  imperfect.  In  this  case  remove  the 
button,  wash,  dry  and  fuse  with  more  silver.  When  all  action  of  the 
acid  has  ceased  and  only  a  brown  skeleton  of  gold  remains  carefully 
decant  the  acid  solution  of  silver  nitrate  into  a  porcelain  dish,  allowing 
no  particles  of  gold  to  escape.  Wash  by  decantation,  using  distilled 
water  which  has  been  tested  and  found  to  be  free  from  chlorides.  The 
washing  process  must  be  performed  with  extreme  care  as  small  particles 
of  gold  are  easily  detached  and  lost.  A  white  porcelain  dish  is  used 
for  receiving  the  washings  because  of  the  consequent  ease  in  detecting 
lost  gold  particles.  Wash  until  all  silver  is  removed,  as  shown  by  test- 
ing with  hydrochloric  acid.  After  the  washing  is  completed  dry  the 
crucible  on  the  hot  plate. 

Annealing. — Heat  the  crucible  over  the  ordinary  burner  until  the  gold 
changes  from  brown  to  yellow,  then  allow  to  cool,  brush  into  the  pan  of 
the  button  balance  and  weigh. 

Calculate  the  ounces  per  ton  of  gold  and  silver  upon  the  basis  of 
milligrams  obtained  per  assay  ton  of  ore,  making  the  proper  correction 
in  the  weight  of  the  silver  for  any  silver  that  has  been  found  in  the 
litharge. 

Scorification. — For  the  treatment  of  high-grade  silver  ores, 
and  especially  those  containing  copper,  arsenic,  antimony  or 
zinc,  the  scorification  process  is  simpler  than  the  crucible  process. 
In  the  scorification. process  the  ore  is  mixed  with  granulated  lead, 
usually  with  the  addition  of  a  small  amount  of  borax  glass  or 
silica,  and  is  heated  in  an  oxidizing  atmosphere  in  a  shallow 
vessel  of  fire  clay  called  a  "scorifier."  This  is  shown  in  Fig.  122. 


THE  FIRE  ASSAY  581 


Its  size  varies  between  %  mcn  and  4-J^  inches,  inside  diameter, 
but  the  size  ordinarily  used  is  about  2  inches  in  diameter.    • 

In  the  crucible  process  the  reactions  are  between  the  ore  and 
added  fluxes,  litharge  being  one  of  these,  and  enough  lead  is 
reduced  to  form  a  button  of  the  correct  weight.  Air  plays  little 
or  no  part  in  this  process.  In  the  scorification  process  the  chief 
flux  is  lead  oxide  but  it  is  formed  by  the  oxidation  of  lead  which  is 
added  in  relatively  large  quantities,  the  unused  excess  being 
vaporized  as  oxide.  Because  of  this  possibility  of  expelling  the 
unused  excess  of  flux  there  is  a  rather  large  permissible  latitude  in 
the  quantity  of  lead  that  may  be  taken.  Because  the  process 


FIG.  122. — Scorifier. 

may  be  continued  until  the  required  quantity  of  lead  is  left  for  the 
button,  no  reducing  agent  being  used,  no  preliminary  calculation 
concerning  this  matter  is  necessary.  Because  the  process  is  an 
oxidizing  one  copper,  arsenic,  antimony  and  zinc  are  easily 
driven  into  the  slag  and  do  not  contaminate  the  button.  In 
other  words,  the  adjustment  of  the  charge  involves  little  more 
than  the  addition  of  a  certain  minimum  amount  of  lead,  with 
a  small  quantity  of  borax  glass  or  silica  to  aid  in  the  formation 
of  a  liquid  slag  if  the  ore  contains  little  acid  gangue.  These 
features  give  the  scorification  process  a  decided  advantage  over 
the  crucible  process  with  ores  to  which  it  will  apply.  It  is  not 
suitable  for  low-grade  silver  ores  because  of  the  small  amount 
of  ore  that  must  necessarily  be  used.  Not  more  than  0.2  A.  T. 
can  conveniently  be  scorified  and  0.1  A.T.  will  usually  give  better 
results  if  a  scorifier  of  ordinary  dimensions  is  used.  The  process 
does  not  work  well  with  gold  ores  because  of  appreciable  losses 
of  gold  in  the  slag. 

The  following  charges  are  suitable  for  the  scorification  of  typ- 
ical ores  of  the  classes  named. 


582  QUANTITATIVE  ANALYSIS 

Silicious  ores: 

Ore 0. 1  A.  T. 

Lead 40  gm 

Borax  glass 2  gm 

Ores  containing  arsenic  and  antimony: 

Ore 0.1  A.  T. 

Lead  50  gm 

Borax  glass 5  gm 

Ores  containing  copper: 

Ore 0. 1  A.  T. 

Lead 65  gm 

Borax  glass 1  gm 

Silica 1  gm 

Ores  containing  iron  (pyritic  ores) : 

Ore .'.... 0.1  A.  T. 

Lead 50  gm 

Borax  glass 3  gm 

The  lead  that  is  used  for  this  purpose  (known  as  "test  lead") 
is  a  finely  granular  form  and  it  should  be  as  nearly  as  possible 
free  from  silver.  The  same  difficulty  is  encountered  in  obtaining 
silver-free  lead  as  was  noted  in  the  case  of  litharge  and  for  the 
same  reason.  It  is  therefore  necessary  to  make  a  preliminary 
assay  of  each  lot  of  test  lead  and  to  correct  the  results  of  the 
assay  of  ores  according  to  the  amount  of  silver  found  in  the  lead. 

Determination. — Sample  and  weigh  the  ore  according  to  the  direc- 
tions given  for  the  assay  by  the  crucible  process.  Weigh  the  materials 
for  the  scorifier  charge  according  to  one  of  the  statements  given  above. 
Place  one-half  of  the  lead  in  the  scorifier,  add  the  weighed  sample  of  ore 
and  mix  well  with  the  lead  by  means  of  a  platinum  wire,  then  add  the 
rest  of  the  lead.  Place  the  borax  glass  and  silica  on  the  top  and  then 
place  the  scorifier  in  the  muffle,  which  should  be  hot.  Close  the  door 
of  the  muffle  and  raise  the  temperature  to  the  point  where  the  lead  is 
melted  and  the  black  suboxide  changes  to  the  yellow,  more  volatile 
monoxide.  Open  the  door  of  the  muffle  and  admit  a  full  supply  of  air. 
The  lead  now  rapidly  oxidizes  and  a  part  of  the  oxide  vaporizes  but  most 
of  it  attacks  the  ore  and  decomposes  it  with  the  formation  of  a  liquid 
slag.  This  process  of  oxidation  and  slag  formation  will  now  continue 
until  the  ore  is  completely  decomposed,  a  perfectly  fluid  slag  forming  a 


THE  FIRE  ASSAY  583 

ring  around  the  circumference  of  the  scorifier  leaving  the  lead  exposed 
over  a  large  circle  in  the  center.  As  lead  is  thus  used  in  slag  formation 
and  through  vaporization  of  the  oxide  the  exposed  circle  of  the  metal 
(the  "bull's  eye")  becomes  smaller  on  account  of  the  descent  of  the  sur-~ 
face  toward  the  narrower  part  of  the  scorifier.  When  the  process  is 
finished  the  slag  entirely  covers  the  lead. 

After  the  "bull's  eye"  has  disappeared  close  the  muffle  door  and  raise 
the  temperature  for  a  short  time  in  order  that  the  slag  may  become  so 
thoroughly  liquefied  that  it  will  not  become  viscous  during  pouring, 
then  pour  into  the  mould.  The  mould  that  was  used  in  the  crucible 
process  may  be  used  here  also  but  a  more  shallow  mould  which  has 
hemispherical  depressions  is  preferred.  When  the  slag  and  button  are 
cold,  remove  from  the  mould  and  free  from  slag  exactly  as  was  done  with 
the  button  obtained  from  the  crucible  fusion.  Cupel,  part  and  weigh 
as  already  directed. 


TABLE  OF  LOGARITHMS 


586 


QUANTITATIVE  ANALYSIS 


LOGARITHMS 


Natural 
Numbers 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Parts 

1  2|3|4  5  6  7  8 

9 

10 

0000 

0043 

0086 

0128 

0170 

0212 

0253 

0294 

0334 

0374 

4 

8 

12 

17 

21 

25 

29 

33 

37 

11 

0414 

0453 

0492 

0531 

0569 

0607 

0645 

0682 

0719 

0755 

4 

8 

11 

15 

19 

23 

26 

30 

34 

12 

0792 

0828 

0864 

0899 

0934 

0969 

1004 

1038 

1072  1106 

3 

7 

10 

14 

17 

21 

24 

28 

31 

13 

1139 

1173 

1206 

1239 

1271 

1303 

1335 

1367 

1399  1430 

3 

6 

10 

13 

16 

19 

23 

26 

29 

14 

1461 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732 

3 

6 

9 

12 

15 

18 

21 

24 

27 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

3 

6 

8 

11 

14 

17 

20 

22 

25 

16 

2041 

2068(2095 

2122 

2148 

2175 

2201 

2227 

2253 

2279 

3 

5 

8 

11 

13 

16 

18 

21 

24 

17 

2304 

2330 

2355  2380 

2405 

2430 

2455 

2480 

2504 

2529 

2 

5 

7 

10 

12 

15 

17 

20 

22 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695 

2718 

2742 

2765 

2 

5 

7 

9 

12 

14 

16 

19 

21 

19 

2788 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

2 

4 

7 

9 

11 

13 

16 

18 

20 

20 

3010 

3032 

3054 

3075 

3096 

3118 

3139 

3160 

3181 

3201 

2 

4 

6 

8 

11 

13 

15 

17 

19 

21 

3222 

3243 

3263 

3284 

3304 

3324 

3345 

3365 

3385 

3404 

2 

4 

6 

8 

10 

12 

14 

16 

18 

22 

3424 

3444 

3464 

3483 

3502 

3522 

3541 

3560 

3579 

3598 

2 

4 

6 

8 

10 

12 

14 

15 

17 

23 

3617 

3636 

3655 

3674 

3692 

3711 

3729 

3747 

3766 

3784 

2 

4 

6 

7. 

9 

11 

13 

15 

17 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

3945 

3962 

2 

4 

5 

7 

9 

11 

12 

14 

16 

25 

3979 

3997 

4014 

4031 

4048 

4065 

4082 

4099 

4116 

4133 

2 

3 

5 

7 

9 

10 

12 

14 

15 

26 

4150 

4166 

4183 

4200 

4216 

4232  4249 

4265 

4281 

4298 

2 

3 

5 

7 

8 

10 

11 

13 

15 

27 

28 

4314 

4472 

4330 

4487 

4346 
4502 

4362 
4518 

4378 
4533 

4393  4409 
4548  4564 

4425 
4579 

4440 
4594 

4456 
4609 

2 
2 

3 
3 

5 

5 

6 
6 

8 
8 

9 
9 

11 
11 

13 

12 

14 
14 

29 

4624 

4639 

4654 

4669 

4683 

4698 

4713 

4728 

4742 

4757 

1 

3 

4 

6 

7 

9 

10 

12 

13 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

1 

3 

4 

6 

7 

9 

10 

11 

13 

31 

4914 

4928 

4942 

4955 

4969 

4983 

4997  5011 

5024  5038 

1 

3 

4 

6 

7 

8 

10 

11 

12 

32 

5051 

5065 

5079 

5092 

5105 

5119 

5132  5145 

5159  5172 

1 

3 

4 

5 

7 

8 

9 

11 

12 

33 

5185 

5198 

5211 

5224 

5237 

5250 

5263  5276 

5289 

5302 

1 

3 

4 

5 

6 

8 

9 

10 

12 

34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

1 

3 

4 

5 

6 

8 

9 

10 

11 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551 

1 

2 

4 

5 

6 

7 

9 

10 

11 

36 

5563 

5575 

5587 

5599 

5611 

5623 

5635 

5647 

5658 

5670 

1 

2 

4 

5 

6 

7 

8 

10 

11 

37 

5682 

5694 

5705 

5717 

5729 

5740 

5752 

5763 

5775 

5786 

1 

2 

3 

5 

6 

7 

8 

9 

10 

38 

5798 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5899 

1 

2 

3 

5 

6 

7 

8 

9 

10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

1 

2 

3 

4 

5 

7 

8 

9 

10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

1 

2 

3 

4 

5 

6 

8 

9 

10 

41 

6128 

6138 

6149 

6160 

6170 

6180 

6191 

6201 

6212 

6222 

1 

2 

3 

4 

5 

6 

7 

8 

9 

42 

6232 

6243 

6253 

6263 

6274 

6284 

6294 

6304 

6314 

6325 

1 

2 

3 

4 

5 

6 

7 

8 

9 

43 

6335 

6345 

6355 

6365 

6375 

6385 

6395 

6405 

6415 

6425 

1 

2 

3 

4 

5 

6 

7 

8 

9 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

1 

2 

3 

4 

5 

6 

7 

8 

9 

45 

6532 

6542 

6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

1 

2 

3 

4 

5 

6 

7 

8 

9 

46 

6628 

6637 

6646 

6656 

6665 

6675 

6684 

6693 

6702 

6712 

1 

2 

3 

4 

5 

6 

7 

7 

8 

47 

6721 

6730 

6739 

6749 

6758 

6767 

6776 

6785 

6794 

6803 

1 

2 

3 

4 

5 

5 

6 

7 

8 

48 

6812 

6821 

68306839 

6848 

6857 

6866 

6875 

6884 

6893 

1 

2 

3 

4 

4 

5 

6 

7 

8 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

1 

2 

3 

4 

4 

5 

6 

7 

8 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

1 

2 

3 

3 

4 

5 

6 

7 

8 

51 

7076 

7084 

7093 

7101 

7110 

7118 

7126 

7135 

7143 

7152 

1 

2 

3 

3 

4 

5 

6 

7 

8 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

1 

2 

2 

3 

4 

5 

6 

7 

7 

53 

7243 

7251 

7259 

7267 

7275 

7284 

7292 

7300 

7308 

7316 

1 

2 

2 

3 

4 

5 

6 

6 

7 

54 

7324 

7332 

7340 

7348 

7356 

7364 

7372 

7380 

7388 

7396 

1 

2 

2 

3 

4 

5 

6 

6 

7 

TABLES 


587 


LOGARITHMS 


Natural 
Numbers 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Parts 

1  2  3  4J5  6|7|8  | 

55 

7404 

7412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

2 

2 

3 

4 

5 

5 

6 

56 

7482 

7490 

7497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

2 

2 

8 

4 

5 

5 

6 

57 

7559 

7566 

7574 

7582 

7589 

7597 

7604 

7612 

7619 

7627 

2 

2 

a 

4 

5 

5 

6 

58 

7634 

7642 

7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

1 

2 

a 

4 

4 

5 

6 

59 

7709 

7716 

7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

1 

2 

a 

4 

4 

5 

6 

60 

7782 

7789 

7796 

7803 

7810 

7818 

7825 

7832 

7839 

7846. 

1 

1 

2 

3 

4 

4 

5 

G 

61 

7853 

7860 

7868 

7875 

7882 

7889 

7896 

7903 

7910  7917 

1 

1 

2 

s 

4 

4 

5 

6 

62 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980 

7987 

1 

1 

2 

3 

3 

4 

5 

6 

63 

7993 

8000 

8007 

8014 

8021 

8028 

8035 

8041 

8048 

8055 

1 

1 

2 

3 

3 

4 

5 

5 

64 

8062 

8069 

8075 

8082 

8089 

8096 

8102 

8109 

8116 

8122 

1 

1 

2 

3 

3 

4 

5 

5 

65 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8176 

8182 

8189 

1 

1 

2 

3 

3 

4 

5 

5 

66 

8195 

8202 

8209 

8215 

8222 

8228 

8235 

8241 

8248 

8254 

1 

1 

2 

3 

3 

4 

5 

5 

67 

8261 

8267 

8274 

8280 

8287 

8293 

8299 

8306 

8312 

8319 

1 

1 

2 

3 

3 

4 

5 

5 

68 

8325 

8331 

8338 

8344 

8351 

8357 

8363 

8370 

8376 

8382 

1 

1 

2 

3 

3 

4 

4 

5 

69 

8388 

8395 

8401 

8407 

8414 

8420 

8426 

8432 

8439 

8445 

1 

1 

2 

2 

3 

4 

4 

5 

70 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

8494 

8500 

8506 

1 

1 

2 

2 

3 

4 

4 

5 

71 

8513 

8519 

8525 

8531 

8537 

8543 

8549 

8555 

8561 

8567 

1 

1 

2 

2 

3 

4 

4 

5 

72 

8573 

8579 

8585 

8591 

8597 

8603 

8609 

8615 

8621 

8627 

1 

2 

2 

3 

4 

4 

5 

73 

8633 

8639 

8645 

8651 

8657 

8663 

8669 

8675 

8681 

8686 

1 

2 

2 

3 

4 

4 

5 

74 

8692 

8698 

8704 

8710 

8716 

8722 

8727 

8733 

8739 

8745 

1 

2 

2 

3 

4 

4 

5 

75 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

8797 

8802 

1 

2 

2 

3 

3 

4 

5 

76 

8808 

8814 

8820 

8825  8831 

8837 

8842 

8848 

8854 

8859 

1 

2 

2 

3 

3 

4 

5 

77 

8865 

8871 

8876 

8882  8887 

8893 

8899 

8904 

8910 

8915 

1 

2 

2 

3 

3 

4 

4 

78 

8921 

8927 

8932 

8938  8943 

8949 

8954 

8960 

8965 

8971 

1 

2 

2 

3 

3 

4 

4 

79 

8976 

8982 

8987 

8993 

8998 

9004 

9009 

9015 

9020 

9025 

1 

2 

2 

3 

3 

4 

4 

80 

9031 

9036 

9042 

9047 

9053 

9058 

9063 

9069 

9074 

9079 

1 

2 

2 

3 

3 

4 

4 

81 

9085 

9090 

9096 

9101 

9106 

9112 

9117 

9122 

9128 

9133 

1 

2 

2 

3 

3 

4 

4 

82 

9138 

9143 

9149 

9154 

9159 

9165 

9170 

9175 

9180 

9186 

1 

2 

2 

3 

3 

4 

4 

83 

9191 

9196 

9201 

9206 

9212 

9217 

9222 

9227 

9232 

9238 

1 

2 

2 

3 

3 

4 

4 

84 

9243 

9248 

9253 

9258 

9263 

9269 

9274 

9279 

9284 

9289 

1 

2 

2 

3 

3 

4 

4 

85 

9294 

9299 

9304 

9309 

9315 

9320 

9325 

9330 

9335 

9340 

1 

2 

2 

3 

3 

4 

4 

86 

9345 

9350 

9355 

9360 

9365 

9370 

9375 

9380 

9385 

9390 

1 

2 

2 

3 

3 

4 

4 

87 

9395 

9400 

9405 

9410 

9415 

9420 

9425 

9430 

9435 

9440 

0 

1 

1 

2 

2 

3 

3 

4 

88 

9445 

9450 

9455 

9460 

9465 

9469 

9474 

9479 

9484 

9489 

0 

1 

1 

2 

2 

3 

3 

4 

89 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

9533 

9538 

0 

1 

1 

2 

2 

3 

3 

4 

90 

9542 

9547 

9552 

9557 

9562 

9566 

9571 

9576 

9581 

9586 

0 

1 

1 

2 

2 

3 

3 

4 

91 

9590 

9595 

9600 

9605 

9609 

9614 

9619 

9624 

9628 

9633 

0 

1 

2 

2 

3 

3 

4 

92 

9638 

9643 

9647 

9652 

9657 

9661 

9666 

9671 

9675 

9680 

0 

1 

2 

2 

3 

3 

4 

93 

9685 

9689 

9694 

9699 

9703 

9708 

9713 

9717 

9722 

9727 

0 

1 

2 

2 

3 

3 

4 

94 

9731 

9736 

9741 

9745 

9750 

9754. 

9759 

9763 

9768 

9773 

0 

1 

2 

2 

3 

3 

4 

95 

9777 

9782 

9786 

9791 

9795 

9800 

9805 

9809 

9814 

9818 

0 

1 

2 

2 

3 

3 

4 

96 

9823 

9827 

9832 

9836 

9841 

9845 

9850 

9854 

9859 

9863 

0 

1 

2 

2 

3 

3 

4 

97 

9868 

9872*9877 

9881 

9886 

9890 

9894 

9899 

9903 

9908 

0 

1 

2 

2 

3 

3 

4 

98 

9912 

9917 

9921 

9926 

9930 

9934 

9939 

9943 

9948 

9952 

0 

1 

2 

2 

3 

3 

4 

QQ 

9956 

9961 

9965 

9969 

9974 

9978 

9983 

9987 

9991 

9996 

0 

1 

2 

2 

3 

3 

3 

TABLE  OF  ANTILOGARITHMS 


590 


QUANTITATIVE  ANALYSIS 
ANTILOGAEITHMS 


Logarithms 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Parts 

1  2  3|  4  5  6|7|8 

.00 

1000 

1002 

1005 

1007 

1009 

1012 

1014 

1016 

1019 

1021 

0 

0 

1 

1 

1 

1 

2 

2! 

.01 

1023 

1026 

1028 

1030 

1033 

1035 

1038 

1040 

1042 

1045 

0 

0 

1 

1 

1 

1 

2 

2 

.02 

1047 

1050 

1052 

1054 

1057 

1059 

1062 

1064 

1067 

1069 

0 

0 

1 

1 

1 

1 

2 

2 

.03 

1072 

1074 

1076 

1079 

1081 

1084 

1086 

1089 

1091 

1094 

0 

0 

1 

1 

1 

1 

2 

2 

.04 

1096 

1099 

1102 

1104 

1107 

1109 

1112 

1114 

1117 

1119 

0 

1 

1 

1 

1 

2 

2 

2 

.05 

1122 

1125 

1127 

1130 

1132 

1135 

1138 

1140 

1143 

1146 

0 

1 

1 

1 

1 

2 

2 

2 

.06 

1148 

1151 

1153 

1156 

1159 

1161 

1164 

1167 

1169 

1172 

0 

1 

1 

1 

1 

2 

2 

2 

.07 

1175 

1178 

1180 

1183 

1186 

1189 

1191 

1194 

1197 

1199 

0 

1 

1 

1 

1 

2 

2 

2 

.08 

1202 

1205 

1208 

1211 

1213 

1216 

1219 

1222 

1225 

1227 

0 

1 

1 

1 

1 

2 

2 

2 

.09 

1230 

1233 

1236 

1239 

1242 

1245 

1247 

1250 

1253 

1256 

0 

1 

1 

1 

1 

2 

2 

2 

.10 

1259 

1262 

1265 

1268 

1271 

1274 

1276 

1279 

1282 

1285 

0 

1 

1 

1 

1 

2 

2 

2 

.11 

1288 

1291 

1294 

1297 

1300 

1303 

1306 

1309 

1312 

1315 

0 

1 

1 

1 

2 

2 

2 

2 

.12 

1318 

1321 

1324 

1327 

1330 

1334 

1337 

1340 

1343 

1346 

0 

1 

1 

1 

2 

2 

2 

2 

.13 

1349 

1352 

1355 

1358 

1361 

1365 

1368 

1371 

1374 

1377 

0 

1 

1 

1 

2 

2 

2 

3 

.14 

1380 

1384 

1387 

1390 

1393 

1396 

1400 

1403 

1406 

1409 

0 

1 

1 

1 

? 

2 

2 

3 

.15 

1413 

1416 

1419 

1422 

1426 

1429 

1432 

1435 

1439 

1442 

0 

1 

1 

1 

2 

2 

2 

3 

.16 

1445 

1449 

1452 

1455 

1459 

1462 

1466 

1469 

1472 

1476 

0 

1 

1 

1 

2 

2 

2 

3 

.17 

1479 

1483 

1486 

1489 

1493 

1496 

1500 

1503 

1507 

1510 

0 

1 

1 

1 

2 

2 

2 

3 

.18 

1514 

1517 

1521 

1524 

1528 

1531 

1535 

1538 

1542 

1545 

0 

1 

1 

1 

2 

2 

2 

3 

.19 

1549 

1552 

1556 

1560 

1563 

1567 

1570 

1574 

1578 

1581 

0 

1 

1 

1 

2 

2 

3 

3 

.20 

1585 

1589 

1592 

1596 

1600 

1603 

1607 

1611 

1614 

1618 

0 

1 

1 

1 

2 

2 

3 

3 

.21 

1622 

1626 

1629 

1633 

1637 

1641 

1644 

1648 

1652 

1656 

0 

1 

1 

2 

2 

2 

3 

3 

.22 

1660 

1663 

1667 

1671 

1675 

1679 

1683 

1687 

1690 

1694 

0 

1 

1 

2 

2 

2 

3 

3 

.23 

1698 

1702 

1706 

1710 

1714 

1718 

1722 

1726 

1730 

1734 

0 

1 

1 

2 

2 

2 

3 

3 

.24 

1738 

1742 

1746 

1750 

1754 

1758 

1762 

1766 

1770 

1774 

0 

1 

1 

2 

2 

2 

3 

3 

.25 

1778 

1782 

1786 

1791 

1795 

1799 

1803 

1807 

1811 

1816 

0 

1 

1 

2 

2 

2 

3 

3 

.26 

1820 

1824 

1828 

1832 

1837 

1841 

1845 

1849 

1854 

1858 

0 

1 

1 

2 

2 

3 

3 

3 

.27 

1862 

1866 

1871 

1875 

1879 

1884 

1888 

1892 

1897 

1901 

0 

1 

1 

2 

2 

3 

3 

3 

.28 

1905 

1910 

1914 

1919 

1923 

1928 

1932 

1936 

1941 

1945 

0 

1 

1 

2 

2 

3 

3 

4 

.29 

1950 

1954 

1959 

1963 

1968 

1972 

1977 

1982 

1986 

1991 

0 

1 

1 

2 

2 

3 

3 

4 

.30 

1995 

2000 

2004 

2009 

2014 

2018 

2023 

2028 

2032 

2037 

0 

1 

1 

2 

2 

3 

3 

4 

.31 

2042 

2046 

2051 

2056 

2061 

2065 

2070 

2075 

2080 

2084 

0 

1 

1 

2 

2 

3 

3 

4 

.32 

2089 

2094 

2099 

2104 

2109 

2113 

2118 

2123 

2128 

2133 

0 

1 

1 

2 

2 

3 

3 

4 

.33 

2138 

2143 

2148 

2153 

2158 

2163 

2168 

2173 

2178 

2183 

0 

1 

2 

2 

3 

3 

4 

.34 

2188 

2193 

2198 

2203 

2208 

2213 

2218 

2223 

2228 

2234 

1 

2 

2 

3 

3 

4 

4 

.35 

2239 

2244 

2249 

2254 

2259 

2265 

2270 

2275 

2280 

2286 

1 

2 

2 

3 

3 

4 

4 

.36 

2291 

2296 

2301 

2307 

2312 

2317 

2323 

2328 

2333 

2339 

1 

2 

2 

3 

3 

4 

4 

.37 

2344 

2350 

2355 

2360 

2366 

2371 

2377 

2382 

2388 

2393 

1 

2 

2 

3 

3 

4 

4 

.38 

2399 

2404 

2410 

2415 

2421 

2427 

2432 

2438 

2443 

2449 

1 

1 

2 

2 

3 

3 

4 

4 

.39 

2455 

2460 

2466 

2472 

2477 

2483 

2489 

2495 

2500 

2506 

1 

1 

2 

2 

3 

3 

4 

5 

.40 

2512 

2518 

2523 

2529 

2535 

2541 

2547 

2553 

2559 

2564 

1 

1 

2 

2 

3 

4 

4 

5 

.41 

2570 

2576 

2582 

2588 

2594 

2600  2606 

2612 

2618 

2624 

1 

1 

2 

2 

3 

4 

4 

5 

.42 

2630  2636 

2642 

2649 

2655 

2661  2667 

2673 

2679 

2685 

1 

1 

2 

2 

3 

4 

4 

5 

.43 

2692 

2698 

2704 

2710 

2716 

2723  2729 

2735 

2742 

2748 

1 

1 

2 

3 

3 

4 

4 

5 

.44 

2754 

2761 

2767 

2773 

2780 

2786 

2793 

2799 

2805 

2812 

1 

1 

2 

3 

3 

4 

4 

5 

.45 

2818 

2825 

2831 

2838 

2844 

2851 

2858 

2864 

2871 

2877 

1 

1 

2 

3 

3 

4 

5 

5 

.46 

2884 

2891 

2897 

2904 

2911 

2917 

2924 

2931 

2938 

2944 

1 

1 

2 

3 

3 

4 

5 

5 

.47 

2951 

2958 

2965 

2972 

2979 

2985 

2992 

2999 

3006 

3013 

1 

1 

2 

3 

3 

4 

5 

5 

.48 

3020 

3027 

3034 

3041 

3048 

3055 

3062 

3069 

3076 

3083 

1 

1 

2 

3 

4 

4 

5 

6 

.49 

3090 

3097 

3105 

3112 

3119 

3126 

3133 

3141 

3148 

3155 

1 

1 

2 

3 

4 

4 

5 

6 

TABLES 
ANTILOGARITHMS 


591 


Logarithms 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Farts 

1|  2 

3|  4[  b 

6|  7|  8|  0 

.50 

3162 

3170 

3177 

3184 

3192 

3199 

3206 

3214 

3221 

3228 

1 

1 

2 

3 

4 

4 

5 

6 

7 

.51 

3236 

3243 

3251 

3258 

3266 

3273 

3281 

3289 

3296 

3304 

2 

2 

3 

4 

5 

5 

6 

7 

.52 

3311 

3319 

3327 

3334 

3342 

3350 

3357 

3365 

3373 

3381 

2 

2 

3 

4 

5 

5 

6 

7 

.53 

3388 

3396 

3404 

3412 

3420 

3428 

3436 

3443 

3451 

3459 

2 

2 

3 

4 

5 

6 

6 

7 

.54 

3467 

3475 

3483 

3491 

3499 

3508 

3516 

3524 

3532 

3540 

2 

2 

3 

4 

5 

6 

6 

7 

.55 

3548 

3556 

3565 

3573 

3581 

3589 

3597 

3606 

3614 

3622 

2 

2 

3 

4 

5 

6 

7 

7 

.56 

3631 

3639 

3648 

3656 

3664 

3673 

3681 

3690 

3698 

3707 

2 

3 

3 

4 

5 

6 

7 

8 

.57 

3715 

3724 

3733 

3741 

3750 

3758 

3767 

3776 

3784 

3793 

2 

3 

3 

4 

5 

6 

7 

8 

.58 

3802 

3811 

3819 

3828 

3837 

3846 

3855 

3864 

3873 

3882 

2 

3 

4 

4 

5 

6 

7 

8 

.59 

3890 

3899 

3908 

3917 

3926 

3936 

3945 

3954 

3963 

3972 

1 

2 

3 

4 

5 

5 

6 

7 

8 

.60 

3981 

3990 

3999 

4009 

4018 

4027 

4036 

4046 

4055 

4064 

1 

2 

3 

4 

5 

6 

6 

7 

8 

.61 

4074 

4083 

4093 

4102 

4111 

4121 

4130 

4140 

4150 

4159 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.62 

4169 

4178 

4188 

4198 

4207 

4217 

4227 

4236 

4246 

4256 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.63 

4266 

4276 

4285 

4295 

4305 

4315 

4325 

4335 

4345 

4355 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.64 

4365 

4375 

4385 

4395 

4406 

4416 

4426 

4436 

4446 

4457 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.65 

4467 

4477 

4487 

4498 

4508 

4519 

4529 

4539 

4550 

4560 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.66 

4571 

4581 

4592 

4603 

4613 

4624 

4634 

4645 

4656 

4667 

1 

2 

3 

4 

5 

6 

7 

9 

10 

.67 

4677 

4688 

4699 

4710 

4721 

4732 

4742 

4753 

4764 

4775 

1 

2 

3 

4 

5 

7 

8 

9 

10 

.68 

4786 

4797 

48084819 

4831 

4842 

4853 

4864 

4875 

4887 

1 

2 

3 

4 

6 

7 

8 

9 

10 

.69 

4898 

4909 

4920 

4932 

4943 

4955 

4966 

4977 

4989 

5000 

1 

2 

3 

5 

6 

7 

8 

9 

10 

.70 

5012 

5023 

5035 

5047 

5058 

5070 

5082 

5093 

5105 

5117 

1 

2 

4 

5 

6 

7 

8 

9 

11 

.71 

5129 

5140 

5152 

5164 

5176 

5188 

5200 

5212 

5224 

5236 

1 

2 

4 

5 

6 

7 

8 

10 

11 

.72 

5248 

5260 

5272 

5284 

5297 

5309 

5321 

5333 

5346 

5358 

1 

2 

4 

5 

6 

8 

9 

10 

11 

.73 

5370 

5383 

5395 

5408 

5420 

5433 

5445 

5458 

5470 

5483 

1 

3 

4 

5 

6 

8 

9 

10 

11 

.74 

5495 

5508 

5521 

5534 

5546 

5559 

5572 

5585 

5598 

5610 

1 

3 

4 

5 

6 

8 

9 

10 

12 

.75 

5623 

5636 

5649 

5662 

5675 

5689 

5702 

5715 

5728 

5741 

1 

3 

4 

5 

7 

8 

9 

10 

12 

.76 

5754 

5768 

5781 

5794 

5808 

5821 

5834 

5848 

5861 

5875 

1 

3 

4 

5 

7 

8 

9 

11 

12 

.77 

5888 

5902 

5916 

5929 

5943 

5957 

5970 

5984 

5998 

6012 

1 

3 

4 

5 

7 

8 

10 

11 

12 

.78 

6026 

6039 

6053 

6067 

6081 

6095 

6109 

6124 

6138 

6152 

1 

3 

4 

6 

7 

8 

10 

11 

13 

.79 

6166 

6180 

6194 

6209 

6223 

6237 

6252 

6266 

6281 

6295 

1 

3 

4 

G 

7 

9 

10 

11 

13 

.80 

6310 

6324 

6339 

6353 

6368 

6383 

6397 

6412 

6427 

6442 

1 

3 

4 

6 

7 

Q 

10 

12 

13 

.81 

6457 

6471 

64866501 

6516 

6531 

6546 

6561 

6577 

6592 

2 

3 

5 

6 

8 

9 

11 

12 

14 

.82 

6607 

6622 

6637 

6653 

6668 

6683 

6699 

6714 

6730 

6745 

2 

3 

5 

6 

8 

9 

11 

13 

14 

.83 

6761 

6776 

6792 

6808 

6823 

6839 

6855 

6871 

6887 

6902 

2 

3 

5 

G 

8 

9 

11 

13 

14 

.84 

6918 

6934 

6950 

6966 

6982 

. 

6998 

7015 

7031 

7047 

7063 

2 

3 

5 

6 

8 

10 

11 

13 

15 

.85 

7079 

7096 

7112 

7129 

7145 

7161 

7178 

7194 

7211 

7228 

2 

3 

5 

7 

8 

10 

12 

13 

15 

.86 

7244 

7261 

7278 

7295 

7311 

7328 

7345 

7362 

7379 

7396 

2 

3 

5 

7 

8 

10 

12 

13 

15 

.87 

7413 

7430 

7447 

7464 

7482 

7499 

7516 

7534 

7551 

7568 

2 

3 

5 

7 

9 

10 

12 

14 

16 

.88 

7586 

7603 

7621 

7638 

7656 

7674 

7691 

7709 

7727 

7745 

2 

4 

5 

7 

9 

11 

12 

14 

16 

.89 

7762 

7780 

7798 

7816 

7834 

7852 

7870 

7889 

7907 

7925 

2 

4 

5 

7 

9 

11 

13 

14 

16 

.90 

7943 

7962  7980 

7998 

8017 

8035 

8054 

8072 

8091 

8110 

2 

4 

6 

7 

9 

11 

13 

15 

17 

.91 

8128 

8147 

8166 

8185 

8204 

8222 

8241 

8260 

8279 

8299 

2 

4 

6 

8 

9 

11 

13 

15 

17 

.92 

8318 

8337 

8356 

8375 

8395 

8414 

8433 

8453 

8472 

8492 

2 

4 

6 

8 

10 

12 

14 

15 

17 

.93 

8511 

8531 

8551 

8570 

8590 

8610 

8630 

8650 

8670 

8690 

2 

4 

6 

8 

10 

12 

14 

16 

18 

.94 

8710 

8730 

8750 

8770 

8790 

8810 

8831 

S851 

8872 

8892 

2 

4 

6 

8 

10 

12 

14 

16 

18 

.95 

8913 

8933 

8954 

8974 

8995 

9016 

9036 

9057 

9078 

9099 

2 

4 

b 

8 

10 

12 

15 

17 

19 

.96 

9120 

9141 

9162 

9183 

9204 

9226  9247 

9268 

9290 

9311 

2 

4 

6 

8 

11 

13 

15 

17 

19 

.97 

9333 

9354  9376 

9397 

9419 

9441  9462 

9484 

9506 

9528 

2 

4 

7 

9 

11 

13 

15 

17 

20 

.98 

9550 

9572  9594 

9616 

9638 

9661 

9683 

9705 

9727 

9750 

2 

4 

7 

9 

11 

13 

16 

18 

20 

.99 

9772 

9795  9817 

9840 

9863 

9886 

9908 

9931 

9954 

9977 

2 

5 

7 

9 

11 

14 

16 

18 

20 

INDEX 


Abbe"  refractometer,  359 
Absorbents  for  carbon  dioxide,  133 

for  hydrochloric  acid,  131 
Accuracy,  limit  of,  2 
Acetate  method  for  manganese,  465 

for  phosphorus,  452 
Acetic  acid  in  vinegar,  236 
Acetyl  value,  376 
Acid  fluxes,  50,  568 

hydrochloric,  standard  solution, 
224 

value  for  oils,  368 
Acidity  of  oils,  352 
Acids,    gravimetric    standardization 
of,  222 

laboratory,  concentration,  236 

standard,  221 

primary  standards  for,  217 
Adjustment  of  balance,  58 

of  standard  solutions,  205 
Adsorption,  27 

by  aluminium  hydroxide,  89 
Adulteration  of  milk,  536 
Agricultural  limestone,  239 

materials,  510 

Albumenoid  nitrogen,  419,  422 
Albumin  of  milk,  541 
Aliquot  parts,  194 
Alkali  metals  in  silicates,  291,  294 
Alkalinity  of  limestone,  239 

of  water,  229 
Allotropism  of  iron,  481 
Alloys,  anti-friction  metals,  508 

antimony  in,  508 

brass  and  bronze,  157,  505 

copper  in,  157,  161,  506,  509 

lead  in,  506,  509 

nickel-chromium,  for  triangles, 
44 


Alloys,  nickel  in,  163 

silver  in,  161,  276 

tin  in,  505,  508 

zinc  in,  507 
Aluminium,  87 

hydroxide,  adsorption  by  89 
solubility,  88 

in  minerals,  288,  294 
Alundum,  42 

Ammonia  in  water,  417,  422 
Ammonium,  101 

citrate  solution  for  phosphorus, 

529 

Analyzed  chemicals,  70 
Annealing  of  gold,  577,  580 

of  steel,  488,  504 
Anti-friction  metals,  508 
Anti-logarithms,  table,  590,  591 
Antimony    in    anti-friction    metals, 
508 

in  the  fire  assay,  575 
Apparatus  for  electro-analysis,  148 

general,  77 

Apparent  valence,  241 
Arachis  oil,  383 
Argols,  570 
Arm  ratio,  65 
Arsenical  insecticides,  272 
Arsenic  in  fire  assay,  575 

in  insecticides,  272 

in  Paris  green,  273,  274 

oxidation  by  iodine,  260 

reduction  by  hydriodic  acid,  261 
Arsenous  chloride,  distillation,  274 
Ash,  fusing  point,  302 

of  coal,  300,  305 

of  condensed  milk,  558 

of  cream,  557 

of  milk,  540 
Assay,  fire,  564 

preliminary,  572 


38 


593 


594 


INDEX 


Assay  ton,  566 

Atomic  weights,  8 

Austenite,  485 

Availability  of  nitrogen,  521 

Available     chlorine     in     bleaching 

powder,  270 
oxygen,  Bunsen's  method,  265 

in  peroxides,  255,  264 
Averages,  rule  of,  4 


B 


Babcock  method  for  fat,  552,  557 
Balance,  51 

adjustment,  58 

arm  ratio,  65 

arms,  relative  length,  61,  65 

assay,  566 

beam  rests,  52 
.  chainomatic,  57 
.  pulp,  565 

rests,  63 

sensibility,  53,  60 

to  set  in  motion,  58 

zero  point,  59,  63 
Barium,  91 

hydroxide,    standard    solution, 
438,  442 

salts,  solubility,  91,  92 

sulphate,  decomposition,  93 

occlusion  of  salts,  93 
Bases  and  carbonates,  mixtures,  227 
Bases,  primary  standards  for,  235 

standard  solutions,  233 
Basic  fluxes,  50,  569 
Basicity  of  platinum  after  ignition, 

40 

Baudouin  test  for  sesame  oil,  385 
Baum6  scale,  345 

Beam,  balance,  to  set  in  motion,  58 
Beeswax,  369 

Bicarbonates,  titration,  229 
Bismuthate  method  for  manganese, 

461 

Blast  lamps,  47 
Bleaching  powder,  270 
Blistering  of  platinum,  40 


Boiler  compounds,  403 
Borax,  238,  568 
Borda  method  for  weighing,  65 
Boric  acid,  237 
Brass,  157,  505 
British  thermal  unit,  314,  327 
Bromine,  115 
Bronze,  505 

Bulbs  for  standardizing,  185 
Bumping,  515 

Bunsen's  method  for  available  oxy- 
gen, 265 
Burette,  gas,  328 

Schellbach,  171,  176 
Burettes,  170 

calibration,  188,  189 

outflow  time,  176   * 
Burners,  45 

blast  lamps,  47 

E.  and  A.,  46 

Meker,  47 

Teclu,  46 
Burning  oils,  345 

point  of  oils,  348,  350 
Butter,  558 

casein  in,  560 

colors,  562 

fat  in,  559 

moisture  in,  559 

salt  in,  560 

substitutes,  558 
Butyrin,  369 
Butyro-refractometer,  361 


Cain  method  for  vanadium,  475 
Calcium,  78 

chloride  as  drying  agent,  33,  132 
determination     by     permanga- 
nate, 252 

in  minerals,  288,  294 
oxalate,  decomposition,  79 

solubility,  78 
salts,  solubility,  78 
sulphate,  80 

Calculations  of  volumetric  analysis, 
191 


INDEX 


595 


Caldwell  crucible,  25 

Calibration  by  standard  bulbs,  184, 

187,  189 

of  burettes,  188,  189 
of  flasks,  187,  188 
of  pipettes,  189 
of  standard  bulbs,  186 
of    volumetric     apparatus     by 

weighing,  179,  188 
of  weights,  66,  67 
temperature  for,  182 
Calorie,  314,  327 
Calorimeter,  317,  319 
Calorimetry,     radiation    correction, 

322,  326 
Capacities,    absolute    and    relative, 

172 

Caprin,  369 
Caproin,  369 
Caprylin,  369 
Carbonate  minerals,  284 
Carbonates  and  bases,  227 
and  bicarbonates,  228 
titration,  226,  228,  229 
Carbon,  combustion,  in  steel,  445 
dioxide,  127 

absorbents  for,  133 
absorption  by  standard  bases, 

234 

in  carbonate  minerals,  287 
in  gas  mixtures,  333,  341 
monoxide,  in  gas  mixtures,  334, 

341 

total,  in  coal,  311 
Carbonic  acid,  127 
in  water,  404 
Care  of  platinum,  40 
Carius  method  for  halogens,  126 
Case  hardening,  497,  504 
Casein  in  butter,  560 

in  milk,  541 
Cathode,  mercury,  166 
Caustic  potash,  228 

soda,  228 

Cementite,  483,  485 
Chain  rider,  57 
Chainomatic  balance,  57 


Chemical  glassware,  71 

porcelain,  37 
Chemicals,  analyzed,  70 
Chemists'  slide  rule,  7 
Chill  test,  353 
Chimney  gases,  343 
Chlorides,  87 

reduction     of     permanganates, 

244 
Chlorine,  115 

in  water,  415 

available,  in  bleaching  powder, 

270 

Chlorplatinate  method    for    potas- 
sium, 97 

Chlorplatinic  acid,  103 
Chromite,  259 
Chromium,  259 

and  nickel  alloys,  44 

in  steel,  468 
Chromophors,  208 
Circuit  fo'r  electro-analysis,  150 
Circular  action  of  beam  rests,  52 
Citrate  insoluble  phosphorus,  528 
Citric  acid,  236 

Clark's  method  for  hardness,  230 
Classes  of  methods,  4 
Classification  of  indicators,  209 
Cleaning  solution,  186 
Cleanliness,  1 

of  balance,  58 
Clinker  of  coal,  302 
Clupanodonic  acid,  364 
Coal,  297 

ash,  300,  305 

clinker,  302 

combustion  apparatus,  312 

fixed  carbon,  299,  306 

fuel  value,  314,  318,  324 

fusing  point  of  ash,  302 

hydrogen  in,  311 

moisture  in,  299,  305 

nitric  acid  formation,  321 

oxygen  in,  309 

sampling,  298,  303 

sulphur  in,  307,  310 

sulphuric  acid  formation,  321 


596 


INDEX 


Coal,  total  carbon  in,  311 

volatile  combustible  matter  in, 

299,  306 

Cochineal,  211,  214 
Coefficient  of  fineness,  413 
Coke,  299 
Cold  test,  354 
Colloids,  19 
Color  change  of  indicators,  207 

of  iron  solutions,  245,  248 

of  water,  413 

reactions  for  oils,  386 
Colors  in  butter,  562 
Combined  carbon  in  steel,  445 
Combustion  apparatus,  312,  436 
Condensed  milk,  557 
Copper,  156,  161,  163,  166,  167 

determination  by  thiosulphate, 
267 

in  alloys,  161,  269 

in  anti-friction  metals,  509 

in  brass,  157,  506 

in  fire  assay,  575 

in  ores,  269 

in  Paris  green,  273 

iodide  method,  267 
Corallin,  213 
Corrosives  in  water,  392 
Cotton  seed  oil,  383 
Cracking  of  coal  products,  300 
Cream,  557 

Critical  points  of  steel,  485,  487 
Crucible  charge  for  fire  assay,  571 

Gooch,  24,  86 

process  for  fire  assay,  567 
Crucibles,  platinum,  38 

porcelain,  37 
Cupellation,  576,  579 
Current  density,  145 
Cyanides,  278 


Decimal  system.  202 
Decomposition  in  a  crucible,  37 
Decomposition  voltage,  140 


Density,  current,  145 

of  water,  181 
Desiccators,  30 

Direct  combustion  of  carbon,  436 
Distilled  water,  72 
Double  layer,  142 
Drying  agents,  33,  132 

of  oils,  363 

precipitates,  29 


E 


Edible  fats  and  oils,  354 
Elaidin  test,  386 
Electro-analysis,  138 

apparatus  for,  148    <, 

circuit  for,  150 

electrodes,  146,  164,  166 

electrolytes  for,  138 

electromagnetic  stirring,  165 

laboratory,  150 

records,  155 

solvents,  140 

temperature  of  electrolyte,  140 
Electrodes  for  electro-analysis,  146, 
164,  166 

mercury  cathode,  166 

moving,  164 

Electrolytes  for  electro-analysis,  139 
Electromagnetic  stirring,  165 
Elliott  tester,  349 
End  point,  192 

with  permanganate,  245 
Enlargement  of  particles,  20 
Equilibrium,  in  reactions,  16 
Equivalent  weight,  195 
Erythrosine,  211,  214 
Etching  metals,  501 
Ether  method  for  iron  separations, 

472 

Ethylene  in  gas  mixtures,  335,  341 
Ethyl  orange,  211,  214 
Evaporation  of  steel  solutions,  434 
Evolution  method  for  sulphur,  450 
Explosion  of  gas  mixtures,  336,  342 
Expression     of     results     in     water 
analysis,  393 


INDEX 


597 


Factors,  6,  9 
Factor  weights,  6 
Fatigue  of  steel,  498 
Fat  in  butter,  559 

in  condensed  milk,  558 
in  cream,  557 
in  milk,  547 

Fats,  melting  point,  362 
Felspars,  290 
Ferrite,  482,  485 
Ferrous     ammonium     sulphate     as 

primary  standard,  247 
Fertilizers,  510 

moisture  in,  511 
nitrates  in,  513,  518 
nitrogen  in,  511 
phosphorus  in,  523 
potassium  in,  533 
Filtering  by  Munro's  method,  25 
Filter  paper,  22 
Filters,  inorganic,  24 
Filtration,  22 
Fire  assay,  564 

crucible  process,  567 
cupellation,  576,  579 
fluxes,  567 

inquartation,  576,  580 
oxidizing  agents,  570 
parting,  577,  580 
reducing  agents,  569 
scorification,  580 
Fire  test  for  oils,  348,  350 
Fish  oils,  385 

Fixed  carbon  in  coal,  299,  306 
Flash  point  of  oils,  346,  349 
Flasks,  volumetric,  169,  175 

calibration,  187,  188 
Fleming  tube,  438 
Fluxes,  49,  567 
acid,  50,  568 
basic,  50,  569 
for  fire  assay,  567 
Foaming  of  water,  393 
Ford  method  for  manganese,  463 
Ford-Williams    method    for    mang- 
anese, 463 


Formaldehyde  in  milk,  556 

Fractional  distillation  of  oils,  350 

Fuel  gases,  333 

Fuel  value,  314,  318,  324 

from  proximate  analysis,  316 
from  ultimate  analysis,  315 

Fuels,  297 

Fusing  point  of  ash,  302 

Fusion,  48 


G 


Gas  burette,  328 

illuminating,  333,  340 
mixtures,  328 

carbon  dioxide  in,  333,  341 
ethylene  in,  335,  341 
hydrocarbon  vapors  in,  336, 

341 

hydrogen  in,  336,  341,  342 
methane  in,  339,  341,  342 
nitrogen  in,  334,  341 
oxygen  in,  334,  341 
pipette,  330,  335 

explosion,  336,  342 
Gases,  chimney,  343 
fuel,  333 

solubility  in  reagents,  332 
Gauss  method  for  weighing,  65 
Gay-Lussac  method  for  silver,  276 
General  operations,  9 

principles,  1 

Gladding  method  for  potassium,  100 
Glass,  solubility,  71 
Glassware,  chemical,  71 
Glycerine,     compound    with    boric 

acid,  237 

Glyoxime  method  for  nickel,  470 
Gold  for  crucibles,  42 

ores,  564 

Gooch  crucible,  24,  86 
Graded  oxidation,  118 
Granulation  of  steel,  493,  499,  504 
Graphitic  carbon  in  steel,  445 
Gravimetric  analysis,  5 

standardization  of  acids,  222 
Gunning  method  for  nitrogen,  519 


598 


INDEX 


Halogen  compounds,  organic,  125 

oxy  acids,  125 
Halogens,  Volhard  method  for,  278 

free,  125 

indirect  method,  115 
Hardened  oils,  389 
Hardening  of  steel,  488,  504 
Hardness  of  water,  229 

Clark  method  for,  230 

non-carbonate,  232 

permanent,  229 

temporary,  229 
Heat  units,  314,  327 
Hehner  value  for  oils,  370,  372 
High-speed  steels,  467 
Hydriodic  acid,  reduction  of  arsenic, 

261 
Hydrocarbon  vapors  in  gas  mixtures, 

336,  341 

Hydrochloric    acid,    absorbent    for, 
131 

standard  solution,  224 
Hydrogenation  of  oils,  389 
Hydrogen  in  coal,  311 

in  gas  mixtures,  336,  341,  342 

peroxide,    with    hypobromites, 
123 

sulphide,  in  water,  405 

reduction  of  iron,  249 
Hydrolysis  of  iron  salts,  246 
Hydrosols,  19 

Hypobromites,  with  hydrogen  per- 
oxide, 123 

Hypothetical,  compounds  in  water, 
394 


Ignition  of  precipitates,  33 
Illuminating  gas,  333,  340 
Immersion  refractometer,  555 
Incrustants  in  water,  392 
Index  of  refraction,  358 
Indicators,  193,  207 

chromophors  in,  208 


Indicators,  classification,  209 

cochineal,  211,  214 

color  change,  207 

corallin,  213 

description  of,  212 

erythrosine,  211,  214 

ethyl  orange,  211,  214 

iodeosine,  214 

lacmoid,  211,  214 

litmus,  211,  213 

methyl  orange,   208,   209,   211, 

214 
red,  211,  215 

phenolphthalein,  207,  208,  210, 
211,  213 

p-nitrophenol,  211,  213 

potassium  ferricyanide,  257 

rosolic  acid,  211,  213 

starch,  261,  263 
Indirect  method,  115 
Industrial  analysis  of  water,  399 

products,  analysis,  283 
Ingot  iron,  482 
Ingotism,  495 
Inquartation,  576,  580 
Insoluble  acids,  370,  372 

matter  in  minerals,  287 
Interpretation   of   results   in   water 

analysis,  408 
Inorganic  filters,  24 
Insecticides,  272 
Iodeosine,  214 

Iodide  method  for  copper,  267 
Iodine,  115 

absorption  number,  363 

oxidation  of  arsenic,  260 

standard    solution,     260,     261, 

271 
Iron,  161,  163,  167 

allotropism,  481 

in  minerals,  288,  294 

in  ores,  244,  250,  259 

in  water,  405 

primary  standard,  246 

reduction  of,  248 

salts,  hydrolysis,  246 

solutions,  color  of,  245,  248 


INDEX 


599 


K 


Kaolin,  290 

Kerosene,  348 

Kjeldahl  method  for  nitrogen,  513 

Kottstorfer  number,  368,  371 


Laboratory  for  electro-analysis,  150 

Lacmoid,  211,  214 

Lactose  in  condensed  milk,  558 

in  milk,  542 
Lead,  162 

in  anti-friction  metals,  509 

in  brass,  506 

Limestone,  alkalinity,  239 
Limit  of  accuracy,  2 
Lindo-Gladding     method     for     po- 
tassium, 102,  534 

Lindo  method  for  potassium,  99,  102 
Linolenic  acid,  364 
Linolic  acid,  364 
Litmus,  211,  213 
Logarithms,  7 

table  of,  586,  587 
Low  method  for  copper,  267 
Lubricating  oils,  350 

M 

Magnesium,  106 

ammonium  phosphate,  107,  108, 

111 
decomposition,  108,  111 

in  minerals,  289,  294 

salts,  solubility,  106 
Manganese,  114 

by     potassium     permanganate, 
253 

in  minerals,  288,  294 

in  steel,  460 

Volhard  method,  277 
Manganous  acid,  254 

sulphate  in  iron  titration,  244 
Marine  animal  oils,  385 
Martensite,  487 
Mass  law,  15 


Maumene  number,  381 
Maximum  size  of  particles,  12 
Meke*  burner,  47 
Melting  point  of  fats,  362 
Mercury  cathode,  166 
Metallography,  479,  500 
Methane  in  gas  mixtures,  339,  341, 

342 
Methyl  orange,  208,  209,  211,  214 

red,  211,  215 
Meyer  bulbs,  439 
Milk,  536 

added  water,  555 

adulteration,  536 

albumin  in,  541 

ash,  540 

casein  in,  541 

composition,  537 

condensed,  557 

fat  in,  547 

formaldehyde  in,  556 

lactose  in,  542 

nitrogen  in,  540 

specific  gravity,  539 

total  solids,  539 
Mixing  and  dividing,  10 
Mohr's  salt  as  primary  standard,  247 
Moisture  in  butter,  559 

in  coal,  299,  305 

in  fertilizers,  511 

in  silicates,  293 
Molybdate  method  for  phosphorus, 

453 

Moments,  principle  of,  54 
Morse  and  Blalock  bulbs,  185 
Moving  electrodes,  164 
Munroe's  method  for  filtering,  25 


N 


Nesslerization,  418 
Nessler's  reagent,  417,  419 
Nickel,  163 

alloys  with  chromium,  44 

in  steel,  468 
Nitrates  in  fertilizers,  513,  518 

in  water,  425 


600 


INDEX 


Nitric  acid  from  coal  combustion, 

321 

Nitrites  in  water,  424 
Nitrogen,  availability  of,  521 

Gunning  method,  519 

in  condensed  milk,  558 

in  fertilizers,  511 

in  gas  mixtures,  341,  342 

in  milk,  540 

in  water,  415 

Kjeldahl  method,  513 
Non-carbonate   hardness   of   water, 

232 
Normal  solution,  199,  201 


Occlusion  by  barium  sulphate,  93 
Odor  of  water,  413 
Oils,  acetyl  value,  376 

acidity  of,  352 

acid  value,  368 

burning,  345 
point,  348,  350 

chill  test,  353 

cold  test,  354 

color  reactions,  386 

constants,  table,  388 

"drying"  of,  363 

edible,  345 

elaidin  test,  386 

fats  and  waxes,  345 

fire  test,  348,  350 

fish,  385 

flash  point,  346,  349 

fractional  distillation,  350 

hardened,  389 

Hehner  value,  370,  372 

hydrogenation,  389 

index  of  refraction,  358 

insoluble  acids  in,  370,  372 

iodine  absorption  number,  363 

lubricating,  350 

marine  animal,  385 

Maumene  number,  381 

mineral  from  saponifiable,  sepa- 
ration, 352 


Oils,  Polenske  value,  375 

qualitative  reactions,  382 

Reichert-Meissl  number,  373 

Reichert  number,  373 

saponification  number,  368,  371 

soluble  acids,  370,  372,  377 

specific  gravity,  345,  352,  356 
temperature  reaction,  381 

titer  test,  363 

viscosity,  350 

volatile  acids,  377 
Oleic  acid,  363 
Olein,  354,  369 

Optical  methods  for  lactose,  542 
Ores,  copper,  269 

gold,  564 

iron,  250,  259 

silver,  564 

zinc,  280 

Organic  nitrogen  in  water,  423 
Orthoclase,  290 
Outflow     time     for     pipettes     and 

burettes,  176 
Ovens,  29 

Overheating  of  steel,  494 
Oxalic  acid  as  primary  standard,  248 
Oxidation,  graded,  118 

in  a  crucible,  36 

potential,  119 

reactions  of,  240 

selective,  119 

Oxidizing  agents  for  fire  assay,  570 
Oxy acids  of  halogens,  125 
Oxygen  in  coal,  309 

in  gas  mixtures,  334,  341 

in  steel,  476 


Palau,  41 

Palladium  tube,  338,  342 

Palmitin,  354,  369 

Paper  coil  method  for  fat,  549 

filter,  22 

reduction  of  precipitates,  22,  34, 

85 
Paris  green,  arsenic  in,  273,  274 

copper  in,  273 


INDEX 


601 


Parting,  577,  580 

Peanut  oil,  383 

Pearlite,  484,  485 

Pemberton  method  for  phosphorus, 

454 

Percent  by  direct  reading,  198 
Perchlorate  method  for  potassium, 

104 

Permanent  hardness,  229 
Permanganate,  end  point,  245 

standard  solution,  243,  250 
Peroxides,  available  oxygen. in,  255, 

264 
Persulphate  method  for  manganese, 

465 
Phenolphthalein,  207,  208,  210,  211, 

213 
Phenolsulphonic    acid    method    for 

nitrates,  427  ^ 
Phosphate,  magnesium  ammonium, 

107,  108,  111 
Phosphorus  in  fertilizers,  523 

in  steel,  4'52 

pentoxide,  33,  132 
Physical  examination  of  water,  410 

tests  of  steel,  505 
Pipette,  gas,  330,  335,  336,  342 
Pipettes,  169,  175 

calibration,  189 

outflow  time,  176 
Platinum,  102 

basicity  after  heating,  40 

blistering,  40 

care  of,  40 

crucibles,  38 

deterioration,  39 

melting  point,  38 

recovery  from  waste,  103 

substitutes,  41 
p-nitrophenol,  211,  213 
Polarimeter,  542 
Polenske  value,  375 
Polishing  machines,  500 
Porcelain  crucibles,  37 

for  chemical  uses,  37 
Potability  of  water,  407 
Potassium,  96 


Potassium,  chlorplatinate,  98 

dichromate,  242 

standard  solution,  257 

ferricyanide,  indicator,  257 

in  fertilizers,  533 

in  silicates,  291,  294 

Lindo-Gladding    method,    102, 
534 

perchlorate,  104 

permanganate,  240 

primary  standards  for,  246 
reduction  by  chlorides,  244 
standard  solution,  243,  250 
thiocyanate,    standard    solu- 
tion, 277 
Potential  differences,  143 

oxidation,  119 
Precipitates,  33 

drying  of,  29 

enlargement  of  particles,  20 

washing,  26 
Precipitation,  15 

in  volumetric  analysis,  276 
Preliminary  assay,  572 
Preparation  of  samples,  9 
Pressure  reduction  for  filtration,  23 
Primary  standards,  217 

for  permanganate,  246 

for  standard  acids,  217 

for  standard  bases,  235 

oxalic  acid,  248 

sodium  carbonate,  222,  224 

sodium  oxalate,  247 
Proximate  analysis,  285,  297,   299, 
305 

fuel  value  from,  316 
Pulp  balance,  565 
Pyrex  glass,  72 
Pyrolusite,  255,  265 
Pyrosulphates,  formation,  101 


Qualitative  reactions,  oils,  382 

Quartering,  11 

Quenching  of  steel,  482,  485,  490,  504 


602 


INDEX 


R 

Radiation   corrections,    calorimeter, 

322,  326 

Rapid  methods,  432 
Reactions,  velocity,  16 
Reagents,  69 
Records,  73,  155 

Reducing  agents  for  fire  assay,  569 
Reduction  method  for  nitrates,  428 

methods  for  lactose,  545 

of  iron,  248 

of  precipitates  by  paper,  22,  34, 
85 

reactions  of,  240 
Refractometer,  359 

immersion,  555 
Reichert-Meissl  number,  373 
Reichert  number,  373 
Relative  length  of  balance  arms,  61, 

65 

Renard  test  for  arachis  oil,  383 
Required  oxygen,  429 
Resin  oil,  383 
Rests,  balance,  use  of,  63 
Reversible  colloids,  19 
Rhotanium,  41 
Ricinoleic  acid,  363 
Rider,  chain,  57 
Riffle,  14 

Rock  analysis,  284 
Rose-Gottlieb  method  for  fat,  551 
Rosolic  acid,  211,  213 
Rule  of  averages,  4 


S 


Salt  in  butter,  560 
Sampler,  riffle,  14 
Samples,  preparation,  9 
Sampling  coal,  298,  303 

gold  and  silver  ores,  564 

quartering,  11 

size  of  particles,  12 

steel,  434 

water,  409 
Sanitary  examination  of  water,  404 


Saponifiable    oils,    separation    from 

mineral  oils,  352 
Saponification  number,  368,  371 
Schellbach  burette,  171,  176 
Scorification,  580 
Seger  cones,  303 

Segregation  of  carbon  in  steel,  495 
Selective  oxidation,  119 
Sensibility  of  balance,  53,  60 
Sesame  oil,  385 
Silver  in  alloys,  276 

in  ores,  564 
Silica  in  carbonate  minerals,  287 

in  silicate  minerals,  293 

in  steel,  446 

ware,  42 

Silicate  minerals,  289,  292* 
Silicates,  alkali  metals  in,  291,  294 

moisture  in,  293 

potassium  in,  291,  294 

sodium  in,  291,  294 
Silicic  acids,  290 
Silver,  83,  159,  161,  167 

Gay-Lussac  method,  276 

ores,  564 

salts,  solubility,  83 

Volhard  method,  277 

volumetric  determination,  276 
Slag  in  steel,  500 
Slide  rules,  chemists',  7 
Snelling  method  for  filtering,  25 
Soda  ash,  225 

lime,  132 
Sodium,  96 

bicarbonate,  229 

carbonate,    primary    standard, 

222,  224 
tiration,  226,  228,  229 

in  silicates,  291,  294 

oxalate,  primary  standard,  247 

thiosulphate,  standard  solution, 

260,  262,  263,  268 
Solid  solutions,  486 
Sols,  19 

Solubility  of  aluminium  hydroxide, 
88 

of  barium  salts,  91,  92 


INDEX 


603 


Solubility  of  calcium  salts,  78 
of  gases  in  reagents,  332 
of  glass,  71 

of  magnesium  salts,  106 
of  silver  salts,  83 
product,  16 

Soluble  acids,  370,  372,  377 
Solution,  15 

of  steel,  434 

tension,  141 

Solutions,  solid,  486 

standardization,  216 
Solvents  for  electroanalysis,  140 
Sorbite,  490 

Specifications  for  apparatus,  174 
Specific  gravity,  Baume  scale,  345 
of  burning  oils,  345 
of  edible  oils  and  fats,  356 
of  lubricating  oils,  352 
of  milk,  539 

temperature  reaction,  381 
Specified  weight,  to  obtain,  64 
Spermaceti,  369 
Standard  acids,  221 
bases,  233 

absorption  of  carbon  dioxide 

by,  234 

bulbs,  calibration  of,  186 
methods  for  steel  analysis,  433 
samples,  435 

solutions,  adjustment,  205 
decimal  system,  202 
normal  system,  199 
standardization,  216 
temperature  corrections,  204 
temperature  for  calibration,  182 
Standardization  by  direct  weighing, 

223 

of  solutions,  216 
Standards,  primary,  217 

use  of  two,  238 
Stannous  chloride,  reduction  of  iron, 

249 

Starch  indicator,  261,  263 
Stearin,  354,  369 
Steel,  432 

annealing,  488,  504 


Steel,  carbon  in,  435 

case  hardening,  497,  504 

chromium  in,  468 

combined  carbon  in,  445 

critical  points,  485,  487 

effect  of  work,  498 

etching,  501 

evaporation  of  solutions,  434 

fatigue  of,  498 

granulation,  493,  499,  504 

graphitic  carbon  in,  445 

hardening,  488,  504 

high-speed,  467 
.    manganese  in,  460 

metallography,  479,  500 

overheating,  494 

oxygen  in,  476 

physical  tests,  505 

phosphorus  in,  452 

quenching,  482,  485,  490,  504 

rapid  methods  for  analysis,  432 

sampling,  434 

segregation  of  carbon  in,  495 

silicon  in,  446 

slag  in,  500 

solution  of,  434 

standard  methods  for  analysis, 
433 

streaks  in,  495 

sulphur  in,  448 
prints,  496 

tempering,  491,  504 

thermal  changes,  480,  485,  487 

titanium  in,  457,  458 

treatment,  478 

tungsten  in,  467 

vanadium  in,  475 
Streaks  in  steel,  495 
Strontium,  96 

Substitutes  for  platinum,  41 
Sucrose  in  condensed  milk,  558 
Sulphates,  95 
Sulphur  in  coal,  307,  310 

in  steel,  448 

prints,  496 

Sulphuric   acid  from   coal  combus- 
tion, 321 


604 


INDEX 


Sulphurous  acid,  reduction  of  iron, 

249 

Supersaturation,  20 
Suspended  matter  in  water,  415 
Systems  of  volumetric  analysis,  198, 

204 


Teclu  burner,  46 

Temperature  correction  for  standard 
solutions,  204 

for  electrolysis,  140 
Tempering  of  steel,  491,  504 
Temporary  hardness,  229 
Tension,  solution,  141 
Thermal  changes  in  steel,  480,  485, 

487 

Thiocyanate,  standard  solution,  277 
Time-temperature  curves,  323 
Tin  in  anti-friction  metals,  508 

in  brass,  505 

Titanium  in  steel,  457,  458 
Titer  test  for  oils,  363 
Tolerance,  173,  178 
Total  solids  in  condensed  milk,  558 

in  milk,  539 

in  water,  415 
Transfer  of  liquids,  73 
Treatment  of  steel,  478 

of  water,  401 
Trial  of  weights,  63 
Triangles,  42 
Troostite,  489 
Tungsten  in  steel,  467 
Turbidity  of  water,  410 


U 


Ultimate  analysis,  285,  297,  307,  310 

fuel  value  from,  315 
Units  of  volume,  172 
Use  of  logarithms,  7 


Valence,  apparent,  241 
Vanadium  in  steel,  475 


Velocity  of  reactions,  16 
Vertical  action  of  beam,  52 
Villavecchia  test  for  sesame  oil,  385 
Vinegar,  236 
Viscosity  of  oils,  350 
Volatile  acids,  377 

combustible  matter,  299,  306 
Volhard  method  for  halogens,  278 
for  manganese,  253 
for  silver,  277 

Voltage,  decomposition,  140 
Volumetric  analysis,  168 

apparatus,    absolute    and   rela- 
tive capacity,  172 
calibration,  179 
specifications,  174 
tolerance,  173,  178 
flasks,  169,  175 

calibration,  187,  188 
Volume,  units  of,  172 


W 


Wash  bottles,  28 
Washing  precipitates,  26 
Water,  added,  in  milk,  555 

alkalinity,  229 

ammonia  in,  417,  422 

analysis,  391 

carbonic  acid  in,  404 

chlorine  in,  415 

color  of,  413 

corrosives  in,  392 

density,  181 

distilled,  72 

expression  of  results  of  analysis, 
393 

foaming  of,  393 

hydrogen  sulphide  in,  405 

hypothetical  compounds  in,  394 

incrustants  in,  392 

interpretation     of     results     of 
analysis,  408 

iron  in,  405 

nitrates  in,  425 

nitrites  in,  424 

nitrogen  in,  415 


INDEX 


605 


Water,  non-carbonate  hardness,  232 
odor,  413 

organic  nitrogen  in,  423 
permanent  hardness,  229 
physical  examination,  410 
potability,  407 
required  oxygen  in,  429 
sampling,  409 
sanitary  examination,  404 
suspended  matter  in,  415 
temporary  hardness,  229 
total  solids  of,  415 
treatment,  401 
turbidity,  410 

Waxes,  355 

Weighing,  50,  61 

Borda  method,  65 


Weighing,  Gauss  method,  65 

gold  and  silver  ores,  565 
Weight,  equivalent,  195 
Weights,  56 

calibration,  66,  67 

specified,  64 

trial  of,  63 
Working  steel,  498 


Zero  point,  59 

estimation,  63 
Zimmermann-Reinhardt      solution, 

246 
Zinc,  ferrocyanide  method,  279 

in  brass,  507 

in  ores,  280 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


RENEWED  BOOKS  ARE  SUBJECT  TO  IMMEDIATE 
RECALL 


RET.  FEB  2  4  1956 


REf/0 


LIBRARY,  UNIVERSITY  OF  CALIFORNIA,  DAVIS 

Book  Slip-70m-9,'ti.:J(F7151.s4)4.')S 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


